Reading (ADC) negative current

[sougata]

Hi,

I need to measure variable positive current/voltage and negative current/voltage through a single PIC. Measuring the high side is done. Measuring the negative side voltage is also done but I am stuck at the current monitoring for the negative voltage. Cannot use LT/Maxim opamps or current sensors due to unavailability. Tried referencing it with by modifying the classical high side current monitor. Failed. Using OP07. Any ideas ???

[mat janssen]

By putting a low ohmic resistor in the negative wire and a differential opamp with the right connection to the measuring resistor. You will get a positive value of your negative current. And you know what input that voltage is connected to, so you know now your negative value.

[sougata]

Dear mat,

Thanks for the reply. I tried this too. It is basically a variable power supply with a low budget. Since the input is more than 40 volts I have to level shift the supply for the op-amp (a resistor connected to supply Ground and clamped by a zener, that is the negative supply pin of the opamp is connected to the negative unregulated supply). So using a difference amplifier did not work , level shift, high common mode voltage. May be you can help with some schematics.

[mat janssen]

I will draw something today and post it here.
Here is a drawing. The 10K and 3K3 resistors placed near the currentmeasuring resistor is for bringing the common mode voltage under the supply voltage.
If this is still too high, then lower the 3K3 resistors in value and rase the value of the feetback resistors of the oppamp (47K).

[sougata]

Hi,

Thanks mat for the schematic. But as you know that scaling down a common mode signal does introduce error even if 1% resistors are used.I do not have the option to use 0.1% tolerance resistors. Op-Amp offset and drift also plays an important role. I have already solved the problem by using two different supplies. A current to voltage converter running from the negative supply itself followed by an inverting amplifier to get a ground referenced voltage signal. Anyway thank you very much for shedding some light and helping. I may use it in a different application.

[mat janssen]

You'r welcome!