Chuck,

As pointed out by several others, try to arrange your current sense
resistor in the ground side of the load.  Many problems will go away if you
do that.

If you cannot do that for some good reason, you will have to solve each of
the various problems.  Large resistors tend to not be as stable as low
value ones unless you pay *lots* for them. So you will need to calculate
drift effects over the intended operating range.

Your circuit shows no offset adjustment; it will almost certainly need one
just to dial out initial tolerance differences.

Something not shown is protection circuitry.  Your 100Meg resistors should
either be big physically and potted -or- comprised of several discrete
parts to reduce the voltage across each one.  Resistors have a voltage
coefficient - you must find out what it is for the parts you plan on using.
 Also, multiple resistors offer more protection against arcing.

A common approach at the bottom of the divider is 2 series resistors to
your amp inputs with clamp diodes in between.  Then if the top 100meg
resistor arcs, the first series resistor limits the current, the diodes
clamp it, the second resistor in series makes sure the diodes clamp and not
the internal amplifier diodes.

Finally, highvoltage modules often exhibit highvoltage, high frequency
noise.  With 100meg resistors and lets say 100khz converter noise, you get
a corner with 16 femtofarads - easily seen across such resistors.  So keep
in mind that you may need filter caps across the bottoms of the dividers.

If possible, really, really try to move your sense resistor! ;)
HTH,
Tom

Who used to work in highvoltage power supplies a long time ago in a galaxy
far far away.

At 03:29 PM 3/12/04 -0500, you wrote:
>Hello All
>
>I am working on a PIC based High Voltage module. The schematic of the
>current meter portion of the module is attached. I am measuring the
>voltage drop across a knowm resistor, but to do it I had to use two
>voltage dividers to bring the levels to something I can read. I measure
>about the same current with or without a load. What could be going on?
>Loooking into the devider there are 50Meg Ohms to Ground, Is this what
>is messing up my reading?
>
>Thanks in Advance
>
>Chuck Roberts

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