Re: eliminating CMRR problems -- was, 3 dB bandwidth

Win wrote:

>>> A common technique in the old days was to create a set of
>>> floating supplies for the input amplifier. Run your LMC6082 or
>>> LMC6062 op amps (these are better choices) on 5V regulators,
>>> etc., whose common reference terminal operates from the opamp's
>>> output. This way there won't be any common-mode input error.

>> Here's roughly one way that trick can be done,

>>. +/-20V +30V
>>. input _________________| ____
>>. A1 _| +5F | | R/10
>>. o-/\/\-+----|+ \ A2 _| 5V ,---/\/---,
>>. | | >-------------|+ \ | R | A3__ |
+/-2V
>>. prot ,-|-_/ | >---+-+--+--/\/-+--|- \ |
output
>>. | | | -5F ,-/\/-+--|-_/ | | |
>--+--A/D
>>. guard | | | | | | | 5V ,--|+_/
>>. o-----+ | | | '-||-| --/\/-+ | |
>>. '--+---| -----+----------| ------' | gnd
>>. infinite |_________________| _________|
>>. Zin input |
>>. -30V

>> This circuit features a high-voltage infinite-input-impedance
>> buffer followed by a low-voltage low-error attenuator for the A/D
>> signal.

>> Note that the critical low-voltage opamps A1 and A3 are operated
>> with no common-mode voltage, hence their CMRR isn't important.
>> The high- voltage opamp A2 is inside A1's feedback loop, so A2's
>> CMRR and other specs aren't important either. A2 does have to
>> operate with 60V total supply, but in exchange, it allows you to
>> have a Terra-ohm Zin, common for a +/-2V on a multimeter, etc.,
>> on 20V or even 200V fs input ranges. If A2 is one of my +/-1100V
>> amplifiers, then you can go to +/-1100V with infinite Zin. This
>> is the basic approach used in Electrometers.

>> The 5V floating supplies are often made using separate AC
>> transformer secondaries, but you can do it with current sources
>> and active zeners, powered from the higher-voltage output-amp
>> supply.

> There's a substantial simplification that's possible, with a
> simple class-A emitter follower A2 to the high-voltage rails. A1
> supplies all the gain, but is relieved from having a good CMRR
> spec. The EF only has to supply the current to drive A3 stage's
> input resistor.

>> +/-200V +230V
>> input _________________| ____
>> A1 _| +5F | | R/100
>> o-/\/\-+----|+ \ A2 _| 5V ,---/\/---,
>> | | >-------------| \ | R | A3__ |
+/-2V
>> prot ,-|-_/ biased | >---+-+--+--/\/-+--|- \ |
output
>> | | | -5F emitter |__/ | | | >--+--
A/D
>> guard | | | followers | | 5V ,--|+_/
>> o-----+--+---| ----------------| ------' | |
>> infinite |_________________| _________| gnd
>> Zin input |
>> -230V

> I expanded the operating range to +/-210 volts.

> Thanks,
> - Win

Thanks, Win. Sorry for the delay in replying - I thought I had the
mold toxins under control. But then the weather changed and I had to
figure out where they were coming from and how to get rid of them.

Your suggestions are very good. It looks like floating the op amp
supply and using an inverter at the input to the ADC solves the cmrr
problem. Thanks!

I would like to allow much higher input voltages, perhaps up to
+/-1kV. You have mentioned your 1100V amplifier several times
before, but I haven't been able to find any circuits in google. I
hope one day you might let us know how it works.

I tried stacking NPN's and PNP's in a complimentary emitter follower
with a crossover reducing resistor between the bases and emitters.
It looks like this may work. It could be extended to any desired
voltage with very low power consumption and reasonable bandwidth.

One problem was trying to model it. The version of SPICE I use does
not like having the op amp supply voltages track the input signal,
and it gave very wierd output signals. I tried several op amps with
similar results and finally found the UA741 barely works in this
circuit. However, SPICE claims the system bandwidth is over 1MHz,
which I do not believe at all.

As usual, one big problem is protecting the inputs. Using a pair of
fets in series with the input to limit the current only works to
500V or so.

How do HP, Fluke and the others get above this limit?

Mike Monett

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