Found a few suitable ADC with built in microcontrollers frm T.I...

Problems:

1. What kind of op-amp circuitry shld I use for conversion of current readings (10uA-65mA) to voltage to input into the ADC?

2. Anything to look out for like noise etc...

thanks a million

 

Your biggest problem will be common mode voltage. Most hand-held ammeters don't have this problem because their ground is floating and can be tied to one end of the current sense resistor. But if you want to interface to RS-232 to a computer, then you are going to have to deal with a ground reference that might be far from the common mode voltage of the current sense point. Look for op-amp circuits with exceptionally high CMMR. Also, the low current range that you mention makes matters even worse, because the sense voltage is probably going to be very low too. Just don't tell me you need to measure 10 uA through a 10 Ohm shunt at a point that is 600 volts from ground!

Another option for the common mode problem is to run an isolated power supply referenced to the current sense resistor. Then you will need to use optical isolation somewhere in the RS-232 link.

-Robert Scott
Real-Time Specialties
Embedded Systems Consulting

 

A 1 Ohm precision resistor in series and a standard differential amplifier with a gain of 100 measuring the voltage across it would give you 0-6.5 V output. Increasing the gain to 150 would give you 10 V full scale for 65 ma. Measuring 10 ua. accurately on the same scale might be a little optimistic as this is < .1% of full scale. If you used 1k resistors for the inputs and 150k for the feedback and bias resistors you would be operating at impedance levels that minimize noise. The Op Amp GBW should limit the bandwidth to a practical level at AV-150, but if noise is a problem you could go with a .1 Ohm shunt and an AV of 1500 to narrow it a bit more. Error starts creeping in with contact and wire resistances needing to be accounted for at the lower impedance, but it's practical with good design. This is a pretty vanilla solution.

Regards

cww