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

[zlymex]

Dear zlymex thanks for sharing your time and knowledge with us, I have some basic questions regarding your bootstrap circuit.
First of all can I power your circuit with +1100V and -1100V power supply so that I could measure AC voltages?

2- what is the purpose of Q1-Q4? I think they are acting like normal diodes, why didn’t you use a normal diode instead?

3-what are R6-R9 doing in your circuit? Doesn’t they generate some voltage drop for us, because we should connect the output of this circuit to the 10Mohm voltage divider, also for current limitation you have used Q5,Q6 as a J-FET can we replace them with simple BJT or MOSFET.

4- why did you use R1-R5 in series with the op-amp? What are they doing?
5- you mentioned the input should be 50V-1000V, so what happens to lower input voltages? For example, the mV and uV input voltages? Also what would affect the circuit performance, I mean which part are critical for achieving the Best lowest temp co-efficient?

Thanks in advance

Kleinstein explained almost everything, here are something I'd like to add.

1 - Yes, the circuit is designed to use at low frequency AC as well, 60Hz is fine, probably can achieve good result at even higher frequency.

2 - Those transistors only have pA leakage, but normal diodes such as 1N4148 have nA leakage.

3 - R6-R9 also function as short time short circuit protection.

4 - It works fine for small voltages like -10V to +10V, provided it is powered by +15V and -15V.
As for the best performance, it depends on whether you need a very small voltage error or very high input impedance. For former, use a precision opamp and small R1 to R7. For later, use opamp with very small bias current and also pay attention to leakage of Q1 to Q4 and guarding.

[wine+dine]

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.

Not at all - you can use a 10 GOhm input resistor if that is all you need, and measure the current.
For 1 kV that is 100 nA, plenty to work with.  Your nanoamperemeter can be a simple transimpedance amplifier. For 1 V out that's a 10 M feedback resistor around a low-bias, low-offset opamp.

Downside is the voltage coefficient of the 10G resistor.

1. >10G means exclude 10G
2. 100nA is still consider large for many high impedance 1kV source
3. not only the voltage coefficient, but the T.C. and aging rate of 10G resistors is worse than a 10M.

What a really helpful reply, thanks. 

[alireza7]

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).

can you explain the function of q1 q2 q3 and q4 in this circuit?

[d-smes]

can you explain the function of q1 q2 q3 and q4 in this circuit?

Reread replies 51 and 53:
    Q1-Q4 are used as diodes and one could use low leakage diodes as well.
    Those transistors only have pA leakage, but normal diodes such as 1N4148 have nA leakage.

[David Hess]

Q1-Q4 are used as diodes and one could use low leakage diodes as well.

Those transistors only have pA leakage, but normal diodes such as 1N4148 have nA leakage.

I wonder why they included Q1 and Q2 though since the LT1012 shown has these built in.

[d-smes]

Q1-Q4 are used as diodes and one could use low leakage diodes as well.

Those transistors only have pA leakage, but normal diodes such as 1N4148 have nA leakage.

I wonder why they included Q1 and Q2 though since the LT1012 shown has these built in.

In an earlier post, zlymex explained that he tried a few different op-amps and not all had built-in protection diodes.  Another post said that if protection diodes are built-in, eliminate Q1 and Q2 to lower input capacitance and yield improved phase margin.

[agaelema]

Where I can find the values of leakage (B-E and B-C) of this bjt and others like 2n3904, 2n2222, etc. Normally datasheet not show this parameters.

[Kleinstein]

Only very few datasheets show leakage for BJTs used as a diode. One can get a first idea from looking at the C-E Leakage with open base, that is more often in the DS: this is higher by about the transistors gain. The pA range currents mentioned are measured values - so not guarantied but typical numbers measured on sample transistors by others. Leakage current is also scattering between samples - so they might need special selected / checked parts.

In the better DMMs the protection diodes are usually operated at a very low voltage (e.g. mV range) and thus the leakage is low even for quite normal diodes.

[David Hess]

In the better DMMs the protection diodes are usually operated at a very low voltage (e.g. mV range) and thus the leakage is low even for quite normal diodes.

Just make sure your "normal" diode is not a gold doped switching diode which is almost all of the small signal ones.  They have considerable leakage even at millivolts and present a low impedance at zero volts which just gets worse at higher temperatures.  A 2N3904 or 2N2222 is a great way to avoid this problem and cheaper than a real non-switching small signal diode.

[floobydust]

What about the BAV199?  Dual diode and 3pA typical  but ugh 5,000pA max.  terrible spread
Is a diode-connected transistor's spread better for leakage?

[Kleinstein]

The spread is the difference form testes value (5 nA) to typical value ( 3 pA).  Testing for very low leakage currents is expensive as it takes time to settle. Only very few parts would come close to the 5 nA limit (e.g. more than 500 pA).

With transistors there are often not even tested specs for leakage, thus typical values only. Not all transistors are equal, especially fast ones can show higher leakage. The actual spread can be similar.

In many circuits the critical diodes would be used with a very small voltage across, not the rather high voltage for the leakage specs.

The BAV199 is a good choice. The expected leakage at comparable conditions is expected to be lower for the diodes.

[MisterDiodes]

...Or instead of trying to make transistors work with unknown specs - just use a real purpose-built sub-pA leakage (at low voltage) dual protection diode - now you've got a spec for leakage current.

These are a based on Jfets, work very well & the cans are light-tight:

http://www.linearsystems.com/product-search-result.html?type=products&partnumber=id101

Another handy tip - if you're needing a low leakage diode and trying to use something encapsulated in glass (or some plastics) be aware that light hitting the junction will be converted into higher leakage current flow.

[David Hess]

What about the BAV199?  Dual diode and 3pA typical  but ugh 5,000pA max.  terrible spread
Is a diode-connected transistor's spread better for leakage?

The BAV199 has the same problem with requiring a test for low leakage and costs more than a transistor.  The cheapest way to get a tested low leakage specifications is to use a JFET.  It is too bad that Linear Systems' parts do not have better availability.

I always found sample testing sufficient for transistors from the same lot.

[MisterDiodes]

 The cheapest way to get a tested low leakage specifications is to use a JFET.  It is too bad that Linear Systems' parts do not have better availability.

Huh?  We've never had a problem ordering - it's easiest to order direct from LS and generally they have many hundreds on hand - at least that's been the case when we order.  It's a popular part.

[floobydust]

I've had no problems ordering from Linear Systems, aside from a lighter wallet. You can order directly from Linear Systems California sales or their distributors such as Trendsetter Electronics in Texas, it was painless.

I don't know how you'd make a 10Gohm input resistance multimeter and not have clamp leakage current in the way.
I did not see protective clamps in the Keithley 2000, 2001 multimeters (but don't have official schematics) and wondered if this is why JFETS are failing in them, from (test lead) ESD?

There's no ESD or impulse rating on those LS ID101 lovely gold legs diodes, just a "20mA max."

[Kleinstein]

The meters do have clamping inside.

In the Keithley2000 they use LEDs inside an optocoupler towards bootstrapped zener diodes. A problem however might be that the current limiting relies on MOSFETs and a rather small resistor.  So over-voltage with enough power might go through to sensitive parts. For too high a voltage at the input, there is a spark gap, but this might be slow sometimes.  It's also parasitic effects (series inductance) that can cause ESD to reach point one does not expect.

[Alex Nikitin]

I've measured a number of BAV199 diodes and some makes of these are the best choice for low leakage protection IMHO. The spread from good makes is surprisingly small. You need to keep things very clean though.

Cheers

Alex