[time-nuts] Precision voltage to current conversion

Bruce Griffiths bruce.griffiths at xtra.co.nz
Mon Jan 26 17:10:48 EST 2009

Mark Sims wrote:
> I recently picked up a Fluke 515A meter calibrator.  It puts out 0-10V AC/DC (and 100V fixed) suitable for 4.5 - 5.5 digit meters.  The DC voltage source impedance is around 300 ohms.  I would like to build a (reasonably simple) converter to allow current ranges to be calibrated to around the same level of accuracy.  A simple single op-amp floating load converter might do for low currents (I have some 0.001% 1K resistors that would make a nice 1ma/V converter).
> Can anybody recommend any other circuits?  Also circuits for higher load currents?

I presume you mean that the floating load constitutes the feedback
impedance in an opamp inverter with the presision resistor forming the
input resistor?

If a floating load is OK, one can use a non inverting opamp with the
precision shunt connected between the load and common.
The other end of the load is driven by the opamp output (can include a
high current output booster within the loop) and the inverting opamp
input is connected to the upper sense terminal of the precision
resistor. The source is connected to the opamp non inverting input.

An inverting equivalent is also possible (may be more practical with a
high voltage booster connected to the opamp output).
Cancellation of the loading of the feedback resistor on the current
sensing resistor is possible if an additional pamp is used.


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