The ammeter has proved to be the trickiest part so far, because of the huge common mode voltages involved in measuring say the plate current on a tube under test. I've looked at floating the A/D converter and optoisolating in digital, isolation amps (extremely pricey), ground referencing with a Norton opamp (doesn't work well if the differential voltage is less than one volt, which is more disturbance than I wanted to see.), and so forth... I only put this here because it's so simple that it should have been obvious, in all my searches, it took me a while to find this.
This circuit works by sweeping the bottom leg of a voltage divider, so that the middle rests at zero volts common mode. The values used in the voltage divider are what determines the common mode voltage limit. I've breadboarded this at low voltages, and I'm working on the high voltage prototype at the moment. I intend to stick this inside the feedback loop on the voltage regulator.
This circuit spits out the differential voltage across the sampling resistor,
Rs, provides a bit of gain, and spits out a nicely scaled 0 - 5v
voltage proportional to the current passing through Rs. The final
version will have diode clamps on the output. I only use a 1458 in the picture
to emphasize that the opamp doesn't have to be a high voltage opamp. You might
could use a higher performance opamp for noise, depending on your application.
Not specified in this circuit is the supply voltage of the opamp. You will need a negative supply to cancel a positive common mode voltage, In this circuit, the opamp will have to be able to put out -10V at the bottom of the divider to cancel +600V CMV, so watch your rails and your ranges.
Remember to watch resistor voltage limits on high voltage circuits, hence the series R1 & R2, R3 & R4 pairs. Also, the resistors forming the top leg of the voltage divider do have a power concern - they run from CMV + Signal to ground.
Depending on your application, you may not need extreme precision resistors in certain places. Certainly R5 and R6, and R7 and R8 need to be matched (within each pair) or the amp will incorrectly adjust for CMV. Also, the ratio of R1&R2 to R5 needs to be matched to the ratio of R3&R4 to R6. If the sampling resistor or the feedback resistor in the second opamp are off, this can be compensated to some degree at the computer.
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