Author Topic: Sense resistor offset  (Read 875 times)

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Offline rulleeeeeTopic starter

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Sense resistor offset
« on: September 25, 2018, 06:49:41 pm »
Hi,

I have a question that seems like it would be simple, but that simple solution eludes me. I'm doing some testing with a DAC controlled current limiter in what seems to be the typical way. (See schematic). I'm using the current shunt amplifier to give the sensed voltage a range from 0-4V as current range from 0-10A, and in the actual implementation, the potentiometer is replaced by a 16-Bit DAC. The issue I was having is that while setting low current values, 10-20mA, the zero code offset of the DAC of up to 10mV was causing a positive current offset of 20-30mA.

The solution I have been trying so far was to use the additional op amp as a summing amplifier, and bias the sense voltage by ~50mV, and then adjusting the DAC in software to the same offset in order to avoid the troublesome zero scale. Is this a valid approach, or is there a simpler way that I overlooked? The reason I'm asking is that with my attempted solution, I end up with an input voltage bias on the summing amplifier of ~4mV (inv input is 4mA when non-inv input is 0), which sort of takes me back to square 1. Any suggestions would be appreciated.

Edit, figured out what was happening, the input is almost rail to rail, so very low values were not well behaved, with a split supply however the issue is resolved.
« Last Edit: September 26, 2018, 12:24:33 am by rulleeeee »
 

Offline StillTrying

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Re: Sense resistor offset
« Reply #1 on: September 26, 2018, 02:24:54 pm »
Could you use the op amp's -INB to sense the resistors 0-200mV directly, and divide the DAC's output to +INB down to 0-200mV.
.  That took much longer than I thought it would.
 


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