How a precision DMM have a high input impedance in the Gohm Rang?

[ali_asadzadeh]

Hi,
I always wanted to know how a precision DMM have a high input impedance? don't they use 10M ohm resistor dividers?

[Gyro]

Try this thread as a starting point 

https://www.eevblog.com/forum/beginners/input-protection-for-high-impedance-voltmeter/msg992838/

[David Hess]

If the input signal is within the common mode range of the input buffer, typically up to about +/-5 volts maximum, then nothing special needs to be done and this is why many high performance voltmeters support gigohm input resistance at low input voltages.

For a larger input voltage range, the input buffer is bootstrapped to follow the input voltage.  This can provide a gigohm input resistance over an input range of hundreds of volts which is common with electrometers.  Bootstrapping also reducing errors caused by changes in operating point just like operational amplifiers can be bootstrapped to improve common mode rejection ratio which would otherwise limit performance of non-inverting amplifier configurations.

[zlymex]

I made myself a front-end buffer allowing 5V to 1000V DC signal to be measured at ultra-high input resistance of multi-Tero Ohms.
The error(the in-out difference) is mainly duo to the Vos of the opamp, and the leakage current is mainly duo to the protection diodes and Ib of the opamp.
If this buffer is built into a desktop multimeter and supply with -1015V and +1015V, the input resistance of all the DCV ranges will be very high too.

[ali_asadzadeh]

Thanks for sharing,I can not see your schematics completely, would you attach the full circuit, and Also it would be very appropriated if you explain your circuit.
Thanks in advance.

[behzad63]

HI
I had read about bootstrap for DMM high impedance.do you have any article or schematic to explain that? Do you know any solution for designing DMM high impedance?
Thanks

[zlymex]

Here is the complete schematics. This is the one ideally working at 1000V to 2500V. If the input is 100V to 800V, it can be simplified. I have an old thread at 38hot discussing the high voltage related topic: http://bbs.38hot.net/thread-11133-1-1.html

There are two tricks involved, one is the boostrape supplies of opamp that make them float. This idea is known for sometimes and can be found in some existing circuit such as Linear AN67 page 58. But here I use transistors instead of MOSFET because of the low dropout. Also, I use a current source(consisting of D1, R21 and Q21) to provide constant working current and high dynamic for the float.

The second trick is the cascade of transistors allowing high working voltage than single transistor. There are three current paths, one is the supply path for the opamp(D1, Q26-Q29), two is the constant current(R21, Q21-Q24,D2,,,), three is the start-up and balancing voltage distribution among the cascade transistors(R22-R29).

Q5, Q6 and R10 is a bidirectional current limiter(1.5mA).

I actually use 600V rated transistor complimentary pair of 2SC3632 and A1413 in my trial PCB. Another option would be 900V rated 2SC4030 and 2SA1967 allowing 2500V to be buffered. In theory, the number of series connected pairs may extend as many as you like, to increase the max. working voltage.

Replace the opamp with low-Ib one to achieve higher input impedance.

[bktemp]

Here is the complete schematics.

Thanks for the schematic. It is an interesting solution using the opamp supply current for biasing the current source.

I built a similar circuit a while ago. Basically it worked, but I experienced a couple of problems:
The circuit was quite unstable. The culprit was the input protection diode. The parasitic capacitance fed the output signal back into the high impedance input, making the circuit oscillate somewhere between 100-500kHz.
I simulated your circuit using different transistors and a generic opamp model and it also shows a significant spike (+30dB) in the ac response at about 200kHz. Depending on the diodes used I can see the circuit oscillating after a voltage step. Without the 4 input protection diodes there is almost no spike in the ac response and no overshoot/ringing after a voltage step.

Did you see a similar behaviour in your circuit or do you have any idea how to solve this problem (maybe the opamp is critical for stability)?

[Gyro]

HI
I had read about bootstrap for DMM high impedance.do you have any article or schematic to explain that? Do you know any solution for designing DMM high impedance?
Thanks

Hi behzad63, Welcome.

You'll find some references in the thread I linked in reply #1...

https://www.eevblog.com/forum/beginners/input-protection-for-high-impedance-voltmeter/msg992838/

[David Hess]

Here is the complete schematics.

Thanks for the schematic. It is an interesting solution using the opamp supply current for biasing the current source.

I built a similar circuit a while ago. Basically it worked, but I experienced a couple of problems:
The circuit was quite unstable. The culprit was the input protection diode. The parasitic capacitance fed the output signal back into the high impedance input, making the circuit oscillate somewhere between 100-500kHz.
I simulated your circuit using different transistors and a generic opamp model and it also shows a significant spike (+30dB) in the ac response at about 200kHz. Depending on the diodes used I can see the circuit oscillating after a voltage step. Without the 4 input protection diodes there is almost no spike in the ac response and no overshoot/ringing after a voltage step.

Did you see a similar behaviour in your circuit or do you have any idea how to solve this problem (maybe the opamp is critical for stability)?

Increase C2 maybe?

Maybe but the coupling to the non-inverting input can be halved by doubling up on the input diodes by using pairs in series which will halve the capacitance.  The LT1012 shown has internal back to back diodes across the inputs anyway.

[zlymex]

Here is the complete schematics.

Thanks for the schematic. It is an interesting solution using the opamp supply current for biasing the current source.

I built a similar circuit a while ago. Basically it worked, but I experienced a couple of problems:
The circuit was quite unstable. The culprit was the input protection diode. The parasitic capacitance fed the output signal back into the high impedance input, making the circuit oscillate somewhere between 100-500kHz.
I simulated your circuit using different transistors and a generic opamp model and it also shows a significant spike (+30dB) in the ac response at about 200kHz. Depending on the diodes used I can see the circuit oscillating after a voltage step. Without the 4 input protection diodes there is almost no spike in the ac response and no overshoot/ringing after a voltage step.

Did you see a similar behaviour in your circuit or do you have any idea how to solve this problem (maybe the opamp is critical for stability)?

I did experience unstable problem when I first assembled the trial PCB and use LMC6062 for the opamp(no oscillation for LT1012 though), and then I realized that the feedback capacitor(C2) is missing(not shown in my partial diagram) so I soldered an 1nF capacitor at the back of the PCB.
I also found out that large current(of the opamp, and of the constant current) helps to stabilize so I modified R11 and R21 to 5.1k on the PCB(and to 2.2k in the new circuit).

Q1 and Q2 may be omitted for LT1012 and reduce the capacitance by more than half because they are zero voltage biased. I used these because I'm trying other opamps as well that may not have the internal protection. These protection diodes are the bc junctions of small signal transistor which have small capacitor of about 4pF when reverse biased at 2V.

Attached is a new circuit for 7V-500V. If up to 1000V is required, a cascade has to be used(add two pair of transistors).

[bktemp]

Using a LT1012 the overshoot is much lower than when using a generic opamp. Looking deeper into the datasheet, LT1012 seems to have a large phase margin of 70°. This seems to be really important here.
Increasing the current seems not to help with unstable opamps. Also increasing or shorting C2 has no effect, because it does almost nothing in the unity gain configuration (except helping with driving capacitive loads, but that is not the problem here).
Except from changing the opamp and selecting one with a large phase margin or using a heavily overcompensated one, the only other working solution I have found is adding a small capacitor at the input to ground. 1pF seems so be a low value that is always there due to parasitic capacitance to ground, but using guard traces reduces this capacitance to almost zero. Using guard traces actually increases the positive feedback capacitance, making the circuit even more unstable.
The capacitance to ground can also be dangerous (especially when using a shielded cable with some 100pF of capacitance), because if left floating the opamp may drift to one of the supply rails charging this capacitance to some 100 or 1000V and delivering quite some charge when connected to a target at a different voltage.

[ali_asadzadeh]

zlymex why did you use big transistors? couldn't you use small transistors like MMBTA92LT3G or similar?

[zlymex]

zlymex why did you use big transistors? couldn't you use small transistors like MMBTA92LT3G or similar?

MMBTA92LT3G only withstand 300V, big transistors usually rate more such as 600V for 2SC3632-Z, 900V for 2SC4030 and 2000V for 2SC4913.  It takes space/gap for high voltage anyway. I was aiming at 1kV to 2.5kV for a start at that time and I had already got many 2SC3632-Z and 2SA1413 at hand too.

Of course, if only 200V is required(like Keithley 6514), MMBTA92LT3G or similar can be used.

Another factor in choosing these transistors is not too small DC gain(hFE) at low Ic from which is often suffers for high voltage power devices.

[ali_asadzadeh]

How do you generate the High voltage for example 1.1kV for the circuit? Also can we apply AC voltage to the input? what's the maximum frequency limit of the circuit? how can we calculate it?

[zlymex]

How do you generate the High voltage for example 1.1kV for the circuit? Also can we apply AC voltage to the input? what's the maximum frequency limit of the circuit? how can we calculate it?

I have three high voltage power supplies. As for DIY, a high voltage transformer plus a regular will do, the current consumption is only a few mA making things easier.
From the simulation, the follower can handle AC voltage up to a reasonably high frequency, and with high input impedance as well because of the bootstrap, provided the follower is powered by negative voltage as well. The maximum frequency limit of the circuit is determined IMO by the phase shift and gain variation that you can tolerate, that can both be seen from the simulation. However, the frequency limit is also bounded by the slew rate which may be obtained by large signal step-response test.

[VintageNut]

For the OP, I recently read a Fluke discussion of using a DMM for a null meter. The DMM can be used if the input bias current is 50pA or less for a 40k ohm source resistance. ( If I recall correctly)

To measure this, place a short into the DMM and measure voltage on the most sensitive range. Perform a REL or record the voltage if REL is not supported. Then replace the short with a 40k resistor and note the voltage measured by the DMM. If you performed a REL the input bias current is Vmeasured/40k. I did this for my Keithley 2000 and the bias current is such that the input resistance is >10G as specified in the datasheet.

If you do not have a REL, you have to subtract the initial measurement with the short in place from the 40K voltage measurement. Delta V / 40k is your input bias current.

[Kleinstein]

A 40 K resistor might be rather low to measure the input bias current. The more usual value would be 1 M or 10 M.

[David Hess]

A 40 K resistor might be rather low to measure the input bias current. The more usual value would be 1 M or 10 M.

Or higher if your meter is that good.

I have occasionally used my meters which have a standard 10 megohm input resistance to measure leakage of diodes and such with the meter measuring DC volts and the 10 megohm input resistance acting as the current shunt.

[Gyro]

Datron specify (or specified) 10M in parallel with low leakage 100nF for bias current evaluation / adjustment.

[VintageNut]

Ok, I can see using a larger resistor to use more of the voltage range. I just tried a 10M on a DMM7510. -2mV measured yields 20pA yielding about 50G input impedance. The error is going to be on the order of 0.2% (10M / 50G). Not any kind of error to care about if all you are looking for is 2 significant digits to characterize the input impedance.

[Kleinstein]

The input is not that well described with just a simple resistor. There definitely is a capacitance at the input too and the bias current is often more important than the the "resistance". It is more like having a small bias current (e.g. in the 10-100 pA range) that slightly depends on the voltage. The voltage dependence of the bias current is what is describe by input resistance. However this does not have to be constant slope and could very well get nonlinear (especially higher currents) when you get close to the min or max voltage.

The bias can also very much depend on the temperature of the DMM. Higher temperatures increase semiconductor leakage and on the other side might reduce surface leakage. There are usually currents in both directions that compensate, so it can change in both directions.

[ali_asadzadeh]

Having a 1.1K high voltage in a DMM is a practical thing? or is there some other way of achieving high input impedance  for higher voltages? if the 1.1KV supply is the way to go doesn't it affect the precision of the circuit? I mean coupling the high voltages to the front end or reference path?

[Kleinstein]

There are no real alternatives to having a kind of buffer amplifier for the high voltage if you really want high impedance at high voltages. There is one variant of driving the negative side terminal. But this is rather similar and also needs the high supply voltage, though it could save on the current.

The main backdraw is, that the high voltage amplifier needs a significant amount of power. It could also be a safety concern. Otherwise a high voltage is not a big deal for the input stage. There are a few electrometer amplifiers that do it, but general purpose DMMs have high impedance only for lower voltages like +-12 V or +-20 V.

If one does not need high resolution, there are static electrometer type solutions, like a field mill. However noise and drift is much higher - but it can work to 10 kV or more.

[zlymex]

Having a 1.1K high voltage in a DMM .....

That seems to me is the only option if one wants to measure 1kVDC precisely and draw very little current(like >10G Ohm). I don't think a 9.9G:100M resistive divider is practical in this case. Further more, if 1000VAC is to be measured in this manner, a dual supply of +-1500V is needed.

Having a 1000V in a meter is not a hard thing if the power consumption is low. I have two meters(one is Chinese hand held, the other is HP 4329A) both with internal 1000V source.