At 04:25 PM 3/12/2004 -0500, you wrote:
>To give some back ground I am a technician working on the code for this
>beast and I didn't design the circuit. So my questions may seem simple.

Okay, fair enough.

>Spehro Pefhany Wrote:
>
> >Are you trying to measure the voltage across the 1K resistor? That's
> >REALLY not feasible, your whole signal will be lost by a 100ppm
>mismatch
> >between the 100M resistors, and if you wanted 10% accuracy, you'd need
> >them to match and track within +/-10ppm. That's just not going to
>happen
> >at that resistance level.
>
>
>So the mismatch in resistance is drowning out my measurement, and it
>can't be accounted for as an offset?

Yes. The offset is large and unstable.

>So what type of tolerance would that be to get it to work?

Forget it. You can't get stable enough resistors at that resistance
level and with that much DC voltage across them. You'll be lucky
if they don't shift a percent or more in time and with temperature.
To put it bluntly, it's a really silly circuit.

Work it out in detail if you'd like, it's only Ohm's law and you
should be able to do it easily.

Roughly, you should be able to see that the voltage change
across the 1K due to the load current is Ix*1K. That results in
a change in current through the 100K load resistor of

-Ix * 1K/100M

IOW, a +1mA current change results in a -10nA change

Now suppose you want 5% accuracy at 1mA. The changes due to the
resistors should be less than 5% of this, or +/- 0.5nA

A small change in the 100M resistor results in a change in current of
roughly -a * Vhv /100M, where a is the fractional change in the 100M
resistor. IOW, a +1% change, if Vhv is 2kV, results in a -200nA change.

(if you don't see this, then try calculating dI/dR (differentiating
the resistor value) and make approximations for small changes)

I think it's obvious that |200nA| >> |0.5nA|

> >Howzabout you put the 10K in the TP10 to ground net and measure the
> >voltage across that? Add some protection in case the output gets
> >shorted you don't lose the op-amp etc. every time.
>
>So it will not work as it is done now? My boss designed this baby, if
>his design won't work how should I tell him and convince him it won't
>work?

No, it won't work, if the goal is to measure the voltage across the
1k. If your boss is an engineer, ask him what happens when the
resistors are 1% off (keep in mind that they WON'T stay put, they
will vary with time, temperature, the voltage across them, the length
of time the voltage has been across them, physical stress, humidity
and probably more things than that). High meg resistors are unstable
beasts at the best of times, and all the more so under your conditions.

>P.S This is my first job out of college. And thanks for all the help.

No problem. Time to chuck that circuit and stop wasting time on it.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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