7.5digit diy voltmeter?

[MisterDiodes]

Another way:  if you're going to chase ppm, just go big and OWN it:  Throw on your Rubidium GPSDO and call that "pretty darn good" for clean, low drift timing.

Barring that, like Kleinstein noted: We use good quality shielded canned oscillators (a few $$ each unheated, or perhaps $30~$60 for OCXO, stability down to ppb) on thousands of industrial boards, and we've never had to replace one.  They can work very well, and we've never seen any reliability issue.  Handy tip:  Look at the low temp range and stick to the units -guaranteed- to start at -40°C.  Then test some samples to make sure they weren't making that part up.  The lesser quality units will have real trouble with a cold start.

[floobydust]

For the integrator switches, I see discrete JFETs or MOSFETS or CMOS switch IC's used.
Is this just a wild datasheet hunt for low charge injection with reasonable cost? I wasn't sure of each ones merits. In other applications, switch leakage current verses temperature was an issue I ran into.

74HC4053 (34401A)
VN0605T (K2000)
DG411 family

[Andreas]

Hello,

I throw in the MAX4053A for low charge injection, leakage current and flat on resistance over voltage.
But works only for +/-8V. (similar to the HC, but for the HC-device you will need level shifters).

See also comparison of INL (in mV) for a  0-5V PWM based calibrator.

with best regards

Andreas

[Kleinstein]

There are many options for the switching. Usually the more important switches are those for the reference current(s). So in the Keithley 2000 and 2002 these are SD5400 mosfets. The choice of the switch also depends on where you are switching:

The switch at the integrator side of the resistor is likely more sensitive to charge injection, but have the advantage of operating all at the same voltage level (near ground). The switches also only have to work at a low voltage (this is why the 74HC4053 is possible even with  a +-10 V reference.

The switches at the reference side should be less critical with charge injection, but have to work with a higher voltage (e.g. the full +- ref voltage), but the switches at the high and low side work at a different voltage level. This makes matching of the on resistance more difficult. So the tendency is to use lower resistance switches in this case.

Finding a good switch is more than just a datasheet hunt - there are additional parameters important usually not found in the datasheets. Another problem is that with parameters like leakage current the spread from typical parameters to tested limits is often very large.
An important parameter can be the speed of switching: the more often the switches are operated, the more important jitter gets. So variants with fast switching, like the 34401 need low jitter levels (preferably below 100 ps). Low jitter usually is easier with faster switches. Even the rather slow modulating Solatrons and the circuit shown by Mickle T below take quite some effort for fast switching of the gates.

In principle JFETs make very good switches with low leakage, but I have a slight problem in switching them fast. Also the choice of p channel Jfets is very limited and the on resistance is kind of high. So they are mainly an option for switching at the integrator level.

CMOS switches integrate MOSFET switches with gate drivers.  The extra P-MOSFET part may not be needed and add some extra charge injection, but could also do some compensation of the charge injection. Using separate MOSFETs like the Keithley meters (2000,2002) do might get slightly better parameters, but the extra circuit can also add capacitive coupling and supply noise.

The Keithley 2002 and what it looks like the ADvantest/Japanese T6481 use current sources instead of just resistors for the reference. This kind of eliminates the on-resistance (any might this way allow for very small switches) of the reference switch and thus needs a low on_resistance switch for the signal path, as there is no more compensation.

In the old times there where also meters that used diode current steering (HP3455) and just 4000 series CMOS gates powered by the reference.

My personal favorite for switching at the integrator side is the 74HC4053 or as a possible upgrade option the ADG633. They are very simple to use and low voltage MOSFETs do have a good value for the figure of merit R_on * C_gate. Despite there really low price they could be still good performance. The low price of the HC4053 comes at the price of less testing - so one might have to do extra leakage testing. The 34401 shows that those switches seem to work quite well, even in a scheme with very frequent switching, where charge injection and jitter might be critical.
Having 3 SPDT switches in one chip is also nice, as it helps with compensation of the on resistance.

[David Hess]

It is too bad that small signal 4 lead MOSFETs are not more common.  I wonder what the pricing and availability of the ones from Linear Systems are.  Their web site is horrible and the purchase links are broken.

FETs can often be paired up to cancel charge injection, leakage, and on resistance; I know some of the HP designs do this.

My first choice would be current switching using diodes (actually diode connected transistors) if feasible.  My last choice would be integrated analog switches.  Complex level shifting is something I would just accept in a high performance design.

[zhtoor]

It is too bad that small signal 4 lead MOSFETs are not more common.  I wonder what the pricing and availability of the ones from Linear Systems are.  Their web site is horrible and the purchase links are broken.

hello,

check here.

https://shop.micross.com/shop-default.aspx

-zia

[MisterDiodes]

It is too bad that small signal 4 lead MOSFETs are not more common.  I wonder what the pricing and availability of the ones from Linear Systems are.  Their web site is horrible and the purchase links are broken.

We always order direct from Linear Systems without any problem, and generally they have several hundred of each part number on hand.  If you need more than that the lead time is usually just a few weeks.  At least when we've ordered, they don't seem to have a problem selling just a few or a few hundred of any part number.  Order on Monday, parts in hand by Weds.  Great service and good tech support.

They supply parts in SMT all the way to shielded TH cans.  That's nice to have a choice. 

They don't offer much of a pricing discount to distributors so might as well go right to the source. There's not a lot of profit incentive for distributors to stock these parts, and most distributors haven't got a clue of what they're selling in the first place.

What a lot of people don't realize is that LS will also custom characterize parts for you (at added cost of course and MOQ) - that's a great service if you're running a critical project.

Their fet  switches work very well - and depending on what you're building you can control / compensate for charge injection on your matched switch pairs.  Also watch your dV/dt on your gate signals, and don't over-drive the gates!.

[Inverted18650]

FETs can often be paired up to cancel charge injection, leakage, and on resistance; I know some of the HP designs do this.

David,

Can you recommend app note or other resource on charge injection?

thank you

[HalFET]

FETs can often be paired up to cancel charge injection, leakage, and on resistance; I know some of the HP designs do this.

David,

Can you recommend app note or other resource on charge injection?

thank you

If you have "The Art of Electronics" it's mentioned in there a few times, though I wouldn't really know where to find much about it in detail in one spot. Most of what I've learned about measuring it comes from academic papers on IEEEXplorer.

[David Hess]

FETs can often be paired up to cancel charge injection, leakage, and on resistance; I know some of the HP designs do this.

David,

Can you recommend app note or other resource on charge injection?

thank you

Usually I see the subject of charge injection come up with sample and hold circuits, even diode based ones, which have various ways to counteract it but I do not have any references which are more generic than that.

[MisterDiodes]

...Can you recommend app note or other resource on charge injection?

Besides Art of Electronics, other relevant books you might find helpful are Circuit Designer's Companion, and Designing with Analog Switches (Older but still applies).  Take a look at how MUX508 / 509 parts work from TI (and AD508G from ADI, etc.).  Then take a look at the DMOS switches from Linear Systems.  LS makes a lot of of the excellent parts from Intersil, Siliconix, etc. (when those companies got dissolved or bought out, LS took over the fet and some of the BJT parts) , and then LS made new and improved parts along the way.  A lot of designers don't even realize the value of these parts anymore, but they can be extremely useful in precision analog designs:

http://linearsystems.com/lsdata/others/DMOSAnalogSwitchIntroduction.pdf

If you aren't aware, the founder of Linear Integrated Systems was John Hall - who was also a co-founder of Intersil.  He knew what he was doing when it came to building performance FETs.

Notice that if you're looking at datasheets or app notes from Vishay or ON Semi, they are probably warmed over versions from the 70's thru 90's from Siliconix, Fairchild and sometimes Motorola sheets will pop up.  Etc.  These were the big players.

After that, look at older test equipment designs, and then by all means get your hands dirty testing your own circuits and make some discoveries - best way to learn.

Here's some inspiration on how to measure charge injection for yourself - see figure 4:

http://www.vishay.com/docs/70606/70606.pdf

HINT: You can't get rid of charge injection completely, but you can do a good job canceling (at least some of) the effects out if you're careful. 




 

[HalFET]

I'd say http://www.analog.com/en/analog-dialogue/articles/ask-the-applications-engineer-26.html explains it slightly better.

[floobydust]

That Analog Devices note left me a bit sceptical. Their approach is to add dummy capacitance to match the NMOS to PMOS parasitics. Only works if the source is grounded.

ADG1211,1212,1213 datasheet claiming "0pC" charge injection. The "iCMOS" is industrial HV 33V process.

I do like the DMOS parts such as SD5400. I haven't seen anything with such low capacitance and high speed. Price is reasonable, compared to quad JFETs.

[HalFET]

That Analog Devices note left me a bit sceptical. Their approach is to add dummy capacitance to match the NMOS to PMOS parasitics. Only works if the source is grounded.

ADG1211,1212,1213 datasheet claiming "0pC" charge injection. The "iCMOS" is industrial HV 33V process.

I do like the DMOS parts such as SD5400. I haven't seen anything with such low capacitance and high speed. Price is reasonable, compared to quad JFETs.

It makes sense though, I wouldn't be amazed if half the "proprietary charge injection reduction" techniques are actually just adding that capacitor there.

[zhtoor]

how about using H11F1 style opto-mos switches to avoid charge injection?

like the one with attached datasheet.

regards.

-zia

[David Hess]

how about using H11F1 style opto-mos switches to avoid charge injection?

Some designs use them but they are expensive and slow.

[Kleinstein]

The switches for switching the reference currents need to be low jitter which usually means they have to be relatively fast. The opto-mos are rather slow, so it would get difficult to get them to below ns jitter.  The direct effect of charge injection is an added offset that would not be a real problem. It is only the variations in charge injection (with temperature, time or voltage) that causes trouble.

There are D-MOS chips with integrated driver available, like DG613. They could be a good candidate too (for switching at the integrator).

The HP34401 shows that even the cheap 74HC4053 works reasonably well, even with frequent switching. Frequent switching makes the ADC more sensitive to the switch properties. It is used to allow good performance for short integration times too - it's less important for long integration time. The overall ADC construction in the 34401 is not made for low noise: It starts with dividing the input signal by 3  , and the second limiting point is that the limited resolution ADC in the µC is used on the fly with a rather steep ramp. A third weak point is the rather high current noise of the OP27.  So I don't think the performance of the 34401 is limited by the switches. Reducing the switching frequency could reduce the importance of the switches if needed - at the cost of slightly limited performance at very short integration time (e.g. < 5 ms).

[pigrew]

This discussion about using an ADC has made me wonder if the comparator could be completely removed, and if that would be a good idea. My idea is to use an ADC to measure the integrator output, and set the run up/down time based on the integrator output voltage to slightly overshoot zero. This should reduce jitter from the comparator/edge time capture circuits.

Thoughts? Or is this a standard technique?

[lukier]

This discussion about using an ADC has made me wonder if the comparator could be completely removed, and if that would be a good idea. My idea is to use an ADC to measure the integrator output, and set the run up/down time based on the integrator output voltage to slightly overshoot zero. This should reduce jitter from the comparator/edge time capture circuits.

Thoughts? Or is this a standard technique?

34410/34411A and descendants (34465A etc) use this architecture. High res ADC for the residual, low-res high speed ADC (80 MSPS AFAIR) instead of a comparator.

Also, it has some PWM based DAC feeding of the reference currents that I don't fully understand.

More details here:
https://www.google.co.uk/patents/US7176819
https://www.google.ch/patents/US6876241

[RandallMcRee]

OK, back to trying to build a, ahem, modern 7.5 digit voltmeter.

My idea (not claiming originality, just not sure who else to blame) is to use a precision dac to come close to the unknown input voltage and then use a 24 or 32 bit ADC to measure the residual. The DAC vref input would be a precision 10V source like a KX/Px/Rx/ LTZ1000. This vref would also, in 5v form, used for the ADC.

I see that our own TiN has investigated the ADS1262 here:
https://xdevs.com/review/ti_ads1262_p1/

In particular, he measured the zero voltage for ADS1262 and came up with this table (attached png), which I found encouraging. It seems that an ADC like the ADS1262 (or AD7177-2?) would be good for the task mentioned above--measuring the differential between the DAC and input voltage.

If the DAC is 10 bits and the ADC ENOB is, say, 22 bits then it would seem that we could get to 7.5 digit precision. There are a lot of details, of course, that need to be correct.

Using the LTC2400 for the ADC would require one of the circuits here in AN78 http://cds.linear.com/docs/en/application-note/an78fs.pdf

Thoughts on this approach?

[David Hess]

Where are you going to get a 10 bit DAC which is linear to 24 bits?

[RandallMcRee]

Where are you going to get a 10 bit DAC which is linear to 24 bits?

Uh, I'm going to make one?

But really--10 bits is only 1024 points. I think we could, as a calibration step, input the known voltages for each and every one measured using a 3458a. (It's modern so a microcontroller is obviously tying things together). Then linearity would not be an issue, right? Temperature dependence would be the big variable, I guess.

I am actually constructing a 10bit dac using S102K resistors, to see what issues arise. Its like a hamon divider, but binary.

[e61_phil]

I build some something like that (AD5791BRUZ with LTZ1000A + LTC2410 with LTC6655) for High Voltage drift measurements. It works really nice and stable. The INL is limited by the AD5791 of course, but still very good. If you build your own DAC it could be very interesting.

[Magnificent Bastard]

Where are you going to get a 10 bit DAC which is linear to 24 bits?

The only way that I have been able to do it in the past is a PWM DAC-- made very similar to the Keysight patent above ^^^^ , but not with the high-speed op-amps.  I used a different scheme to remove the switch resistance variation.  I've been able to get 1ns edges, with very small TC (0.007ppm/K).  0.1ppm to 0.02ppm INL is possible in practice, depending on how much money you want to spend on it.  I had no idea that Keysight patented this idea-- I was using it years before they filed; and I thought that it would not have passed the new "non-obviousness" rules...

[Kleinstein]

Super high precision resistors and PWM are not the only options to get a very stable DAC. Another option would be an AC divider with a special precision transformers and synchronous rectification. However high quality PWM is a kind of obvious way.

Depending on the quality of the ADC chip used, one might not even need 10 Bits for the voltage to subtract. There may be also the possibility to use the ADC chip to check the linearity of the coarse DAC. So even if the DAC is not that linear, one could in principle use a numerical correction, as long as the DAC is stable.

For just 7.5 digits of resolution one does not really need super fast OPs in the integrator like in the HP3458. The Keithley 2000,2001 and HP34401 and also the R6581 use rather normal speed OPs (e.g. 4-8 MHz) GBW for the integrators.
For a DIY solution one would not go the way of the 3446x with two auxiliary ADCs - this is mainly used to get good performance at fast sampling rates (e.g. 10000 SPS). This can be important for use in automated test  systems, but of not much use for reading from the display. When it comes to fast readings at moderate resolutions there are ready made chips as an option - though some are expensive.