A discussion on Precision OpAmps

[Ole]

Hiho,
since I am planning a project that requires some pretty precise OpAmps, I have been on the lookout for them.
The OpAmp that I´m mostly going to use (as of now) is the LTC2057, as the xDevs Article on the KX-Board mentions them
and they do have decently low noise statistics.
The reason I am interested in the LTC2057 is it´s capability to utilize a +/-15V supply safely.

But my little project is not the only reason why I want to start this topic. It´s also because I could not find another similar thread.
So if anyone has a recomendation or has noticed that my choices are "suboptimal" please tell me.

Cheers
Ole

[Andreas]

Hello,

I also use often the LTC2057 but also the ADA4522-1.

Depends on what you are doing.

What I have observed with the LTC2057 is that the chopper frequency may have a interference with my ADC (LTC2400) + 2:1 Divider (LTC1043 based ~400Hz).
At certain temperatures I get excessive noise at the ADC output.

Ok with a LTC1050 it is even worse.

The ADA4522 I did not test up today. But since it has internally 2 different chopper frequencies it is likely that there is less interference with my ADCs.

with best regards

Andreas

[Kleinstein]

With AZ OPs one has to be aware of possible interference from spikes in the input current. This can effect ADCs or in some cases other OPs.
The supply should also have good decoupling and ideally filtering as they are possible EMI sources.

Also take care if the source is high impedance, as the AZ OPs can have quite some current noise and the data for the current noise are not all valid. The LTC2057 and ADA4522 data seem to be valid, but others like AD8628 and MCP6V51 are too optimistic.

With a +-15 V supply the power consumption and thus heating can become an issue if highest precision is needed.

[MiDi]

Overview of modern AZ op-amps for +-15V supply:
LTC2057 (1 Ch)
LTC2058 (2 Ch)
ADA4522 (1, 2, 4 Ch)
ADA4523 (1 Ch)
OPA189 (1, 2, 4 Ch, MUX-Friendly)
MCP6V51  (1 Ch)



Edit: Added MCP6V51 & corrected ADA4522 current noise

[Kleinstein]

A nice table.

If cost and availabiltiy is an issue, the MCP6V51 would be an option too: somewhat comparable voltage noise to the LTC2057, but lower power. The DS value for the current noise is wrong (more like the shot noise from the input current). My crude measurement of the noise current is at around 250 fA/sqrt(Hz) and this somewhat higher than for the LTC2057, but still OK.
The other problem with the Microchip data-sheets is the slightly odd definition of the bias current as the average including the sign. So the the comparable value would bias plus 1/2 the offset current. So expect more like 160 pA typ and 750 pA max input bias with the normal definition.

The ADA4522 current noise specs are 800 fA/sqrt(Hz) , not 80.

[KT88]

The precision that an opamp can provide depends a lot on the application and the purpose it serves in it:
Are you interested in time domain or frequency domain signals?
AC or DC?
Bandwidth?
The frequency range.
Source characteristincs - high or low impedance, noise...
Level of precision?
Load? ADC, cable...
Temperature range?
Power constraints?
Budget?
....

Cheers

Andreas

[Ole]

First of all, thanks for the input.

Second, KT88, I am more interested in the DC side or up to maybe 1kHz. The Load that the OP´s would need to drive are 5kOhm Resistors at worst and another OP at unity gain (Av=1) at best.
I intend to operate the entire instrument at an internal temperature of ~35-40°C. As for Power constraints, the instrument is not intended to run via battery power, so a very low power consumption is not essential.

Generally the lower the ouput voltage noise is the better for my usecase.

Cheers
Ole

[macaba]

I think we'd have to see the circuit to give the best recommendations. For example, there are some circuits where AZ isn't ideal and something like OPA140 is better.

[KT88]

@Ole:
Still the source impedance, input signal level --> gain, and accuracy (16bit?) are required...

Cheers
Andreas

[Ole]

source impedance, input signal level --> gain, and accuracy (16bit?)

Well as you may or may not have guessed I intent to build a multimeter,
so the source impedance can vary between near zero, if I was to measure a Power Supply
and near infinite, if I was to measure a specific resistor in a device.
But only the OP´s directly on the frontend are in charge of the very high impedance

The maximum input signal lever should be a 10V difference between two cooperating OP´s
As for the gain and accuracy, the gain will most likely not exceed 1000 and the accuracy should be as high as possible.

Also I´m reworking the circuit so that other people can understand it without headaches (hopefully)

Cheers
 Ole

[David Hess]

source impedance, input signal level --> gain, and accuracy (16bit?)

Well as you may or may not have guessed I intent to build a multimeter,
so the source impedance can vary between near zero, if I was to measure a Power Supply
and near infinite, if I was to measure a specific resistor in a device.
But only the OP´s directly on the frontend are in charge of the very high impedance

Multimeters and other high input impedance instruments do not use chopper stabilized or automatic zero operational amplifiers because of the high current noise; they are simply not suitable for high input impedance applications.

The usual solution is to use a voltage follower, and then *if necessary* correct the offset of the voltage follower as part of the integration cycle, so essentially make an automatic zero amplifier that operates at a low frequency synchronously with the conversion cycle.

Additionally, if the offset is corrected at the input voltage, then the common mode rejection is corrected as well.  This is why chopper stabilized and automatic zero operational amplifiers have high common mode rejection, and why CMOS voltage followers which have atrocious common mode rejection can be used with automatic zeroing.

I think we'd have to see the circuit to give the best recommendations. For example, there are some circuits where AZ isn't ideal and something like OPA140 is better.

The OPA140 would be my first choice.  It has the right combination of input bias current, offset drift, noise, and common mode rejection ratio.  In the past the LT1012 low input bias current bipolar part might have been used in the highest precision applications.

[macaba]

Agree with David - if talking about DMM ADC input buffer; OPA140 for up to 6.5 digits, bootstrapped (to follow input signal) OPA140 for 7.5 digit upwards. It is a wonderful device for lowering part count here.

[Kleinstein]

There are DMMs that use a zero drift ADC at the input (e.g. Keithley 2000, 2002, likely DMM6500, Sigilent SDM30x5, Prema5000), thought often no on its own. The input bias current is a problem and thus the choice of amplifier is somewhat tricky and may need selection. The maximum limits for the input bias are often not good enough, but typical or selected slightly better one can be OK.
Even though the input impedance of a DMM is high, they are not really made for high impedance sources. So the >10 Gohms input impedance is more like made to get less than 1 ppm error for a 10 Kohms source.

Using a JFET amplifier (like OPA140 as a modern chip) and seprate switching the input is an option, but also not that easy. There are similar or possibly even worse switching spikes from doing the choper type offset correction under software control.

The choice of amplifier also depends on the ADC used.

[SoundTech-LG]

David,

Would LT1008 also have been a good choice in the past, as the main difference (to LT1012) seems to be internal/external compensation?

Thanks!

Martin

[Kleinstein]

There were a few DMMs (e.g. Datron106x, Fluke 8505 and related) with a BJT based input amplifier, though more like discrete transistor pair.
The LT1008 / LT1012 would perform somewhat similar. For todays standards the bias current is a bit on the high side, but still not way off.

[Ole]

The choice of amplifier also depends on the ADC used

One of the two ADC´s is planned to be a AD7177, and the other being a ADS1235-Q1.
Yes, the multimeter has a role model or two.
I do intend to perform HI and LO sensing, so for each function I would need two* OP´s with a very low bias current if possible.
(The exeption is the low current ranges where I intend to use a inverting I/V converter)

The HPM7177 uses OPA189 for the input stage, most likely for the very low noise I/O,
so those might be worth remembering for the internal amplifiers.

[KT88]

except in low-cost applications it is never a good idea to use the input amplifier as the ADC driver. If one is already concerned about the current noise a zero-drift amplifier produces, the noise injected by the ADC is orders of manitude higher than that - considering the internal buffers are bypassed for accuracy reasons.
An opamp output appears as an inductor to a signal (or noise-) source attached to the output. That means the noise will be pushed into the signal source only slightly attanuated at best.
The only proper way to mitigate these issues is to have a dedicated ADC driver and appropriate filtering in between the amps.
This problem gets even worse if the input amplifier has a gain (much) greater than A=1.

[Kleinstein]

The SD ADC chips have a differential input and a range of some +-2.5 to +-5 V.  To use the full range and get best INL one would need to drive the input differentially, so more than just a simple amplifier. For a 10 V range one would also need some attenuation, not just gain

[RoGeorge]

MT-054: Precision Op Amps - Analog Devices
MT-055: Chopper Stabilized (Auto-Zero) Precision Op Amps

[Ole]

I´ve looked at the input amplifiers of a few other DMM´s, and found a few interesting OP´s that could be used:
AD795 (from the HM8012, 1-2pA Ib but -100µV Vos and 1-3µVpp Noise)
AD706 (from the 34401A, 50-200pA Ib and +30µV Vos and 0.5µVpp Noise)

An Idea that I had was to attach a Peltier Element to the Input Amps and alter their temperature to the point, that their offset voltage is nearly zero.
The primary problem with that, besides the thermal EMF caused by the leads, would be that the offset drift and the offset are rarely of the same polarity.

Edit:
Additionally I had a look at the LT6018, which is nearly pefect (0.03µVpp Noise and +-8µV Vos) exept for one thing: 40nA typ Ib.
I might Edit this post again if I find some other interesting infos.

[Kleinstein]

For an DMM input amplifier (unless using an old chip like ICL7106) one would not care about the absolute offset voltage, as this can be subtracted in software anyway. So critical parameters are more like the input bias current (usuallaly < 50 pA are aimed for, ideally < 10 pA) and the voltage dirft.

The HP34401 (old version) uses an AD706 in the input amplifier, but this is only for some current sources and the main amplifier is still old style with a dual JFET. The drift compensation is with switching at the input with most of it inside the Hybrid. Today one could consider an OPA140 or similar, though there the CMRR and linearity is limited.

The OPs like AD706 may need an extra adjustment for the bias current and the offset drift can still be borderline.

[KT88]

You will run into a trade-off at some point of time...
Bipolar amps offer a very low voltage noise but pretty high Ib - they are only viable for low impedance sources.
JFET (and MOS-FET-) amps offer excellent Ib and current noise.
AZ-amps - there are many different architectures - offer excellent voltage noise without 1/f noise and low voltage TC at the cost of moderate current noise.
A way to extend the usability you could roll your own AZ scheme - it would allow to sync the frequency with your ADC and also reduce the AZ frequency which would reduce the average charge injection into your source.
I would look into an AZ amp like the ADA4522 with some RC filtering to smooth the noise from the input as well as BW limiting the input signal. As AZ amps have some sampling happening, too high frequencies will cause some interference (Intermodulation distortion, IMD) https://www.analog.com/en/analog-dialogue/articles/demystifying-auto-zero-amplifiers-part-1.html
Bandwidth limiting the output is recommended as well.
Also: the longer you integrate the signal the better SNR gets (oversampling, process gain).

[RoGeorge]

...critical parameters are more like the input bias current (usuallaly < 50 pA are aimed for, ideally < 10 pA)

In terms of bias current, some are specified for the fA range. 
LMP7721 3-Femtoampere Input Bias Current Precision Amplifier

[Ole]

some are specified for the fA range

So similar to the ADA4530 and the LMC6044.

Well, I´ve looked into the datasheet of the LMP7721 and it might be the right canidate for the input,
though I do need to find a way to reduce the LF noise on the output, or increase the integration time or sample number.

Cheers
Ole

[KT88]

@Ole: What is the max. input impedance you expect and what voltage noise level do you need referred to the input?

[David Hess]

Would LT1008 also have been a good choice in the past, as the main difference (to LT1012) seems to be internal/external compensation?

The LT1008 is a fine choice and in some cases better because the external compensation allows its dynamic performance to be optimized.  Before I found the OPA140, I was going to use the LT1008 to replace the damaged input buffer on my Tektronix 4.5 digit DM501A multimeter.

Because of the flawed design of the ADC in the DM501A, the input buffer must have high CMRR so even the best older precision JFET operational amplifiers are barely good enough unless tested and selected for high CMRR.

I´ve looked at the input amplifiers of a few other DMM´s, and found a few interesting OP´s that could be used:
AD795 (from the HM8012, 1-2pA Ib but -100µV Vos and 1-3µVpp Noise)
AD706 (from the 34401A, 50-200pA Ib and +30µV Vos and 0.5µVpp Noise)

In the past if a low input bias current bipolar part was not used, then the highest precision JFET input part was.  The AD542 (25pA Ib, 250uV Vos, 1uV/C Vos Drift, <100uV/V CMRR, and 2uVpp Noise) was an early precision BIFET part recommended for this application.  I am sure Burr-Brown had a bunch.  Another recommended low input bias current bipolar part was the National LM11 (25pA Ib, 100uV Vos, 1 uV/C Drift, 0.3uV/V CMRR, >5uVpp Noise)  which is an improved 308.  In the past I used a lot of LM11s.

The limit to performance with JFET parts was usually their common mode rejection ratio.  I assume that high resolution meters which used them without an automatic zero implementation like the ICL7106 bootstrapped them?

An Idea that I had was to attach a Peltier Element to the Input Amps and alter their temperature to the point, that their offset voltage is nearly zero.
The primary problem with that, besides the thermal EMF caused by the leads, would be that the offset drift and the offset are rarely of the same polarity.

They used to make constant temperature oven "hats" for the metal TO-99 package.

Bipolar precision operational amplifiers since before the OP-07 are designed so that the offset null adjustment also minimizes  offset voltage drift.  I do not know of any JFET parts with this property but if the temperature is regulated, then the offset null adjustment can be used to trim the offset.

For an DMM input amplifier (unless using an old chip like ICL7106) one would not care about the absolute offset voltage, as this can be subtracted in software anyway. So critical parameters are more like the input bias current (usuallaly < 50 pA are aimed for, ideally < 10 pA) and the voltage dirft.

The ICL7106 has its buffer built in with offset corrected as part of its automatic zero process, but there were variations of that idea intended to be used with an external buffer.

except in low-cost applications it is never a good idea to use the input amplifier as the ADC driver. If one is already concerned about the current noise a zero-drift amplifier produces, the noise injected by the ADC is orders of manitude higher than that - considering the internal buffers are bypassed for accuracy reasons.
An opamp output appears as an inductor to a signal (or noise-) source attached to the output. That means the noise will be pushed into the signal source only slightly attanuated at best.
The only proper way to mitigate these issues is to have a dedicated ADC driver and appropriate filtering in between the amps.
This problem gets even worse if the input amplifier has a gain (much) greater than A=1.

I am not sure what you are saying here.  Some precision operational amplifiers will have problems driving against the charge injection of the ADC which requires fast settling time but that will not show up at their input.  It is easily solved by adding a low impedance output buffer *within the feedback loop* and capacitive decoupling of the signal at the ADC input.

Using a JFET amplifier (like OPA140 as a modern chip) and seprate switching the input is an option, but also not that easy. There are similar or possibly even worse switching spikes from doing the choper type offset correction under software control.

Potentially it is no worse than the ICL7106.  I have been working on this problem for a while and suitable input multiplexers are few.  Did any production multimeters use an input multiplexer made from discrete transistors like the 3N series small signal MOSFETs, or even JFETs?

[KT88]

I am not sure what you are saying here.  Some precision operational amplifiers will have problems driving against the charge injection of the ADC which requires fast settling time but that will not show up at their input.  It is easily solved by adding a low impedance output buffer *within the feedback loop* and capacitive decoupling of the signal at the ADC input.

I'm referring to attempts to use a single amplifier for both input and driving the ADC.

[Kleinstein]

Did any production multimeters use an input multiplexer made from discrete transistors like the 3N series small signal MOSFETs, or even JFETs?

The older DMMs (e.g. HP3456, 3458, Keithley 19x, 2000) use mainly JFETs for the input switching. It still looks like the DMM6500 still uses JFETs at least for some parts.  Discrete MOSFETs are rare, due to the parasitic diode. So it would need 2 in series and maybe an isolated drive - still an option for higher voltage.  One can also use CMOS switch chips (like DG508, ADG....) and some type have good specs. Some DMMs like Sigilent SDM3055, SDM3065, KS3446x seem to use such CMOS switches. At least for later stages where leakage and charge injection is less of an issue CMOS switches are very common.

When switching directly at the input one should use a kind of precharge: So there is an extra buffer to provide an auxiliary signal for guarding (and maybe gate drive for JFETs) and the sequency is so that before switching to the main input path the auxiliary buffered signal is read. This can reduce the switching spike quite a lot. This is really help when doing classic auto zero switching all the way to the input.

To get really good CMRR (and also more loop gain and higher input resistance) it is quite common to use a bootstrapped supply for the main input supply. With this one can also use an OP for a lower supply (like 5 V) for the input, espeically also 5 V AZ OPs like the AD8628 or max4238 as lower bias AZ OPs.
Compared to a bootstrapped OP the classical discrete JFET amplifier is also not that much more complicated.

[Ole]

So, first of all
I am not expecting any input impedances higher than about 1MOhm and the voltage noise level should be as low as possible

and secondly I do have to try and wrap my head around the concept of bootstrapping.

[Kleinstein]

The input stage of the Keithley 2000 is a very simple case of bootstrapping the supply. This is not bootstrapping in the stricter sense, but more in a feed forward sense. Bootstrapping in the stricter sense would derive the supply from the feedback network and not directly from the input. The extra delay makes this case a little more tricky to get stable and usually slower (slowing down the driven supply is one way to stabilize).

The idea is to have the supply of the critical OP to follow the input voltage, so that this OP allways sees an essentially constant input relatively to it's supply. So it only has to correct the errors of the circuit (e.g. OP) to drive the supply.
For just a buffer, like in the K2000 bootstrapping is still relatively easy. It gets more tricky with gain, as the output voltage range of the floating OP is limited. This needs an extra amplifier stage to get the full output range.
An amplifier with bootstrapped supply can indeed be mind bogling.  Getting it stable, well protected and latch-up free can also be a slight challange.

[RoGeorge]

I do have to try and wrap my head around the concept of bootstrapping.

Fig. 1 from AnalogDialogue Vol 53 No 3 August 2019 makes the main idea of bootstrapping easier to understand.  It is exactly like the tale that gave birth to the bootstrapping word: lifting yourself by pulling up from your own shoelaces. 



The whole opamp is floating relative to its own output voltage (which is the same as the input voltage). 

[KT88]

As Kleinstein mentioned, bootstrapping avoids errors due to CMRR. The LTC1050 has 125dB which is not bad. A more recent one like the ADA4522 is specified with at least 140dB at room temperature.
A non-AZ amp like the LMP7721 has only 83dB in comparison - boostrapping is a must with such an amp...
Another important reason for bootstrapping is the limited supply voltage of older AZ amps which mostly stays well below 20V thus prohibiting a true +/- 10V-range.
The ADA4522 can operate up to around 55V (+/-27V).

[RoGeorge]

It looks like a big PSRR (Power Supply Rejection Ratio) will also be desirable, but I have no hands-on experience and can not tell how much the PSRR influences the end results in practice.

[KT88]

PSRR refers to supply noise. A very clean supply voltage is of course mandatory.

[martinr33]

A discrete FET input switch in an early TR 6.5 digit multimeter, the predecessor of the Advantest 6581.

The transistors are 2N4117A devices, now about $10 each from Interfet.

In the 6581, this system is updated to a hybrid module.

[Ole]

I think I might have an idea how to get the Vo+5V and Vo-5V or in the case of the LMP7721 +/-2.5V:
Simple linear regulators, a metaphoric ton of capacitance and a overvolt protection zener diode and a minimum load resistor
The latter would only be added if required.

With a supply of +/- 15V, closely connected linear regulators and hopefully low dropout voltages, I could get +/-12V through the input.

[Kleinstein]

The simple schematics from AD for the bootstrapping is missing a point that is usually needed for stability: the drive for the floating OP supply needs to be somewhat slower than that OP.  Another point is that the output should not be loaded very much, especially capacitive load can be a problem.

Attached is an example for a relative simple bootstrapped buffer I use for a high impedance (500 K ohms at lower side) voltage divider. The 2 zener diodes are there to limit the swing of the supply, especially to avoid going too positive so that the supply for floating OP breaks down. The TL031 is not the best choice, but was what I had at hand.

Getting a full +-12 V range with a +-15 V supply is challanging, but not impossible.

A minimum load for the OP can be a good idea (e.g. to avoid the output stage cross over error, that can be at around 1 µV for a non Az precision OP).  A resistor to the floating OPs supply gives an essentially constant current.

[macaba]

Attached is an example of feedforward bootstrap to give you an idea. U1 and U2 do not need to be expensive OPA140, but perhaps some advantage to having everything be the same speed.

[3roomlab]

Attached is an example of feedforward bootstrap to give you an idea. U1 and U2 do not need to be expensive OPA140, but perhaps some advantage to having everything be the same speed.

i remember the k2000 version
i think opa141 is enough?
Qn : is there CMRR improvement?

[Gyro]

Attached is an example of feedforward bootstrap to give you an idea. U1 and U2 do not need to be expensive OPA140, but perhaps some advantage to having everything be the same speed.

U5 doesn't need to be expensive either if you connect it to the output of U6 rather than the input. When connected to U6 input, it will also adversely affect input bias and offset currents.


P.S. The Datron approach to input bootstrapping... https://www.eevblog.com/forum/projects/very-low-bias-current-op-amp-to-buffer-a-kelvin-varley-divider/msg694586/#msg694586

[macaba]

U5 doesn't need to be expensive either if you connect it to the output of U6 rather than the input.

This is then not feedforward bootstrapping...

[Gyro]

Sure, I realise that. I'm just not sure what it buys you (apart from maybe speed? U6 supplies will still be lagging anyway due to 2 opamp delays in the bootstrap path), when it compromises input characteristics by having a non-bootstrapped opamp connected to the input.

[Kleinstein]

Attached is an example of feedforward bootstrap to give you an idea. U1 and U2 do not need to be expensive OPA140, but perhaps some advantage to having everything be the same speed.

i remember the k2000 version
i think opa141 is enough?
Qn : is there CMRR improvement?

The bootstrapping is improving the CMRR and thus the linearity quite a lot. Ideally it adds the accuracy of the supply drive, so something like 40-100 dB to the CMRR. Even the OPA141 is already overkill to drive the supply, unless one needs a very fast response in the feed forward version. The TL031, OPA990 or TL072H would be reasonable candidates to drive the supply. The main point is low bias and a sufficient output range.

Classic bootstrapping from the output is a bit slow (it needs the slow down to avoid oscillation), but avoids extra bias.
Feedforward driven supply from the input side gives a little more bias current (thus the need for a low bias amplifier), but can be higher speed / faster settling.
So both versions have there justification.

[RoGeorge]

Another example, with a simpler implementation than the one from the AnalogDialog pdf:
https://www.pi.infn.it/~federico/Immagini/bootstrap.pdf

[Ole]

Getting a full +-12 V range with a +-15 V supply is challanging, but not impossible.

I noticed that the +/-12V are the differential voltage between the HI and LO inputs.
So the maximum Voltage on either input would be about 6V from GND.
macaba posted a concept which, I must admit, I do like it for the simplicity.
Although I´m not sure if the second Amp, the one that handles the bootstrapped voltages needs to be in parallel to the bootstrapped OP
Alternatively my Idea would be to put resistors in paralell to the constant curent J-FETs (say 50-100kOhm) so that the Bootstrapped Supply Voltages Vb+/- center around GND, if nothing is connected to the input.

Cheers
Ole

[Kleinstein]

Resistors are the JFET current sources would not effect the behaviour with an open input: this input voltage would just float and with the high impedance and low bias would slowly dirft up or down and pic up hum. This is OK for a DMM mit high impedance input.

With a +-15 V supply and moden OP with a near rail to rail output one can get +-12 V for the voltage at either side / input.

It is still a decision to make if the input low side terminal is fixed to the supply ground, which is the more conventional design, or the negative side terminal also moving to get a symetric signal. The second way can get a bit confusing but allows the larger voltage range. E.g. the Sigilent SDM3065 this way gets a +-20 V range without a divider at the input with only a +-15 V supply. With an ADC with a differential input the vesion with a driven low side would definitely make sense as it is a way to generate a differential signal for the ADC even in a mode with no extra gain.

This choice of amplifiers and the general DMM input setup are somewhat seprate topics. The available amplifiers effect what configuration is practical.

[David Hess]

Did any production multimeters use an input multiplexer made from discrete transistors like the 3N series small signal MOSFETs, or even JFETs?

The older DMMs (e.g. HP3456, 3458, Keithley 19x, 2000) use mainly JFETs for the input switching. It still looks like the DMM6500 still uses JFETs at least for some parts.  Discrete MOSFETs are rare, due to the parasitic diode. So it would need 2 in series and maybe an isolated drive - still an option for higher voltage.

Discrete MOSFETs like the 3N series that I mentioned are 4 wire parts with a separate substrate connection, so they have no body diode between the source and drain.  Signetics and Siliconix used to sell them but now they are available from companies like Linear Integrated Systems and Calogic, at least in theory.

One can also use CMOS switch chips (like DG508, ADG....) and some type have good specs. Some DMMs like Sigilent SDM3055, SDM3065, KS3446x seem to use such CMOS switches. At least for later stages where leakage and charge injection is less of an issue CMOS switches are very common.

I have had problems finding integrated CMOS switches with low enough guaranteed leakage specifications.  The DG419 is on my order list for my delayed DM501A repair project.  I have been assuming that precision applications for these parts are selecting them for low leakage.

When switching directly at the input one should use a kind of precharge: So there is an extra buffer to provide an auxiliary signal for guarding (and maybe gate drive for JFETs) and the sequency is so that before switching to the main input path the auxiliary buffered signal is read. This can reduce the switching spike quite a lot. This is really help when doing classic auto zero switching all the way to the input.

Precision sample and holds invert the gate drive and apply it to the output through a capacitance to cancel charge injection.  The same could be done to suppress charge injection at the input, and I suspect parts like the LTC1043 do this.

It looks like a big PSRR (Power Supply Rejection Ratio) will also be desirable, but I have no hands-on experience and can not tell how much the PSRR influences the end results in practice.

Power supply rejection applies to symmetrical voltage variation in the supplies, including noise.  It is only relevant if the supply voltage variation is large which will not be the case with regulated supplies.

The same applies to common mode rejection in the inverting configuration since then the common mode voltage does not vary.  If the operational amplifier is bootstrapped, then the common mode voltage also does not vary so common mode rejection is no longer important.

I believe in a discussion on the forums about the OPA140, it was stated that the OP140 series uses internal bootstrapping of its input stage and input protection network to achieve its outstanding specifications.

[Kleinstein]

For the charge spike when switching there are 2 parts: one is the charge injection in the chip itself, which is normally measured when both sides are at the same voltage and usually for the turn off case. The other part is the extra charge spike when a connection is made to a different votlage. This is from the capacitance of the switch part and also from the input capacitance of the amplifier and possibly extra input current when exceeding slow rate limits, so that amplifier internal parts can not follow and may increase the charge beyound what is expected from small signal input capacitance.

The 4 pin MOSFETs with separate substrate are quite often only low voltage. The Keithley2000 and similar meter use such FETs as switches in the ADC.

The guarantied leakage specs on the CMOS switches are usually not that low - testing to that level is quite expensive.
There are a few Maxim parts (e.g. max327), but with relatively high on resistance.
Typical leakage at least can be quite low with quite a few more afforable parts too (e.g. DG211B, TMUX6111). It still gets tricky when the meter gets hot.
Separate screening for low leakage is very likely in some cases. This also applies to JFETs used as switches - the guarantied specs are often not sufficient and selcted parts are quite often specified.

[RoGeorge]

LMC662 of former NS now TI, CMOS dual op amp, single supply 15V, RRO, 22nV/sqrtHz, bias 2fA typical, input R > 1Tera\$\Omega\$ and in stock at Mouser for about $2-3/1pcs.

Voltage offset is 3mV and drift 1.3uV/*C, but you'll probably chop it in software anyway.

[David Hess]

LMC662 of former NS now TI, CMOS dual op amp, single supply 15V, RRO, 22nV/sqrtHz, bias 2fA typical, input R > 1Tera\$\Omega\$ and in stock at Mouser for about $2-3/1pcs.

Voltage offset is 3mV and drift 1.3uV/*C, but you'll probably chop it in software anyway.

Or use the better LMC6081 or LMC6082, but these parts are limited in follower applications by low common mode rejection.  They are also pretty noisy.

[Kleinstein]

Using some kind of chopping under SW control with extra swtiched is a possible way, but this usually is rather slow chopping, less than 25/30 Hz to get at least 1 power line cycle for each configuration. With a SD ADC with extra settling time, or to reduce the number of switching spikes one may chose an even lower frequency. So one would still care about the noise a relatively low frequencies. So those CMOS OP with super low bias are not really the right choice. The switches anyway tend to have a leakage current of a few pA and thus no need for the amplifier to be much better. So good JFET OPs like the OPA140 or ADA4622 are the more suitable choice.

The very low bias CMOS OPs may be good for a kind of electrometer input with switching via low leakage relays, not so much for a more normal voltmeter input.

[Ole]

Fros the input I do need both the signal OP-Amps and the Bottstrapping OpAmps, the latter being either AD549L or OPA104. But I a m not completely sure about the Signal OpAmp.
The two contestants are the LMP7721 (2fA typ and 20fA max @25°C with 2µVp-p Noise and 50µV typical and 180µV max Vos)
and the ADA4530 (<1fA typ and 20fA max @25°C with 4µVp-p Noise and 9µV typical and 300µV max Vos)

[Kleinstein]

The LMP7721 and ADA4530 are very low bias OPs, suitable for a rather special electrometer type input. This is very low bias ( < 1 pA), but quite some noise and drift (e.g. > 2µV_pp,  > 2 µV/K).
These would normally not be combined with a feed forward driven supply directly from the input, but more with classical bootstrapping with direving the supply from the feedback path. For the feedback path there is no need to have very low bias / good precision like the old and expenside AD549, OPA104 - there a more moderate TL031 or TL072H should be sufficient.
Such a very high impedane input would have relay switching at the input. It also needs care with guard traces / cut outs.

For a more normal voltmeter input I would more look at something like OPA145, max4238, ICL7652 or AD8628. They have more bias current, but also much lower DC noise and drift. If switching at the input is done with JFETs, one can consider extra low bias auxiliary buffers (for the gate signal and maybe the protection). Here the AD549 is more like overkill and the TL031 may be good enough.
A good more normal voltmeter input is moderately low bias (e.g. < 50 pA), but low noise (e.g. < 1 µV_pp). Electronic input swithing may be used to compensate for input drift, but adds input bias. An zero drift OP could avoid the repeated input switching.

[Ole]

I think with an OP-Amp like the AD8655 (1pA typ. Ib, 50µV Vos typ, 0,4-2,3µV/dK dVos and 1,2µVp-p) as the Signal carrying Amplifier and the OPA828 (1pA typ. 50µV Vos typ, 0,3-1,5µV/dK dVos and 0,34µVp-p) as the bootstrapping Amplifier in a parralel structure and a low noise OpAmp to supply the AD8655 (those could be the OPAx140)

[Kleinstein]

I think with an OP-Amp like the AD8655 (1pA typ. Ib, 50µV Vos typ, 0,4-2,3µV/dK dVos and 1,2µVp-p) as the Signal carrying Amplifier and the OPA828 (1pA typ. 50µV Vos typ, 0,3-1,5µV/dK dVos and 0,34µVp-p) as the bootstrapping Amplifier in a parralel structure and a low noise OpAmp to supply the AD8655 (those could be the OPAx140)

This makes not sense for a normal voltmeter. These are nice fast OPs with good higher frequency performance and low noise there, but more mediocre in the low frequency range. 1.2 µV 0.1 - 10 Hz noise is still more on the poors side for DC precision, especially if most of the noise is 1/f noise.

One still has to decide if one want's auto zero switching before the amplifier (need resonable good speed and good noise of the 1-10 Hz range) or not (needs very low offset dirft and good reasonable noise for the 0.01 - 1 Hz range). The speed for the AZ switching also depends on the ADC used. Some are relatively slow and some loose quite some time on switching (higher order fitlers with SD ADCs).

[Ole]

I´ve continued looking for low noise, low Bias Current OpAmps.
What I have found is: ADA4625 (15pA Ib with 15µV Vos, 0,2-2,1µV/dK and 0,15µVp-p) and the OPAx140 (0,5-10pA Ib 30µV Vos and 0,25µVp-p)
These are both fitting in noise characteristics and in typical bias current.
The ADA4610 is in a similar realm in terms of noise (0,45µVp-p), but has a much higher offset (0,2-1,8mV) and a higher drift (0,5-12µV/dK) but still a respectable bias current of 5-25pA.
Other OpAmps that I am going to look into are the ADA4625, LTC6240 and ADA4637.