Programmable Electronic Load - ADC and DAC BoosterPack test

Hi Jan,

I notice on your table of measurements for the DAC, that channel B was outputting +5mV for an input of 0; max for this channel was +6mV; channels C and D were -1mV at max.  I was reading through your calibration post to see how you dealt with this but it didn't cover DAC calibration.  Are you able to share how you calibrated this output: in particular, I'd be interested in understanding how (if) you dealt with the +5mV offset error.

Use with caution, because I haven' been able to define a stable and linear solution yet.

I'll include links to the source code in gitub....

I use two calibration functions. One that deals with offset, one with scale (I use a corrective multiplier):

My calibration functions are in this API: https://github.com/jancumps/msp432/tree/master/MSP432_SCPI_ElectronicLoad/calibration_impl

The function that uses the calibration corrections: https://github.com/jancumps/msp432/blob/master/MSP432_SCPI_ElectronicLoad/control_impl/control_strategy_constantcurrent.…

I have several of those offset + multiplier correction pairs. I'll focus on the DAC:

// current write calibration


bool calibrationSetCurrentWriteMultiplier(float value) {
    if(_bCalibrationActive) {
        _CalibrationData.current_write_multiplier = value;
    }
    return _bCalibrationActive;
}


float calibrationGetCurrentWriteMultiplier() {
    // The DAC drives the FET todo in my case the DAC setting multiplied by 0.000116 gives current
    float fRetval = 0.0;
    fRetval = _CalibrationData.current_write_multiplier;
    return fRetval;
}


bool calibrationSetCurrentWriteOffset(float value) {
    if(_bCalibrationActive) {
        _CalibrationData.current_write_offset = value;
    }
    return _bCalibrationActive;
}


float calibrationGetCurrentWriteOffset() {
    // The DAC drives the FET todo in my case the DAC runs 0.008 A when not driven
    float fRetval = 0.0;
    fRetval = _CalibrationData.current_write_offset;
    return fRetval;
}

Then in the current set function, I use these (stored in EEPROM) values to correct the DAC seting:

void ConstantCurrentStrategyControlFunction () {
    if (bConstantCurrentChanged) {
        uint32_t value = 0U;
        value = ((fConstantCurrent/calibrationGetCurrentWriteMultiplier())-calibrationGetCurrentWriteOffset()); // todo: double check if it is - or plus offset
        eloadRawSetDac(0, value);
        bConstantCurrentChanged = false;
    }
    return;
}

I've tried to explain the calibration approach:

Programmable Electronic Load - Calibration Steps (focus on Configure the Input Current Set Correction)

Programmable Electronic Load - Calibration Data in Flash

Perhaps I missed it when I read through the calibration steps - but I understand what you are doing above and I am trying to do the same, coincidentally!  It's effectively compensating for the gain (multiplier) and offset (addition).   I can see with my DAC that it isn't linear so I was expecting that I'd need to keep a look up table, which, again, seems to be the approach you have taken.  What I was hoping to understand is how you drove the DAC output to 0 when it has an offset of 5mV or to 2.040 when it has an offset of -1mV.  My DAC has a 0 offset of +7mV and a max offset of -2.5mV so it can't get a full range through a simple software adjustment - I think, I was hoping to get an insight from the work you have done here.

Thanks

I was expecting that I'd need to keep a look up table, which, again, seems to be the approach you have taken.

I haven't. I just store the offset and the correction factor. 2 values. But I haven't been able to turn this in a precise result.

My DAC has a 0 offset of +7mV and a max offset of -2.5mV so it can't get a full range through a simple software adjustment - I think, I was hoping to get an insight from the work you have done here.

I have tried to use an external additional OpAmp to deal with that offset. On the output of my DAC, I wired up  that opamp as analog adder, and provided the offset error as input.

So that if the DAC outputs 0.0001 V at 0, I would feed - 0.0001 into the opamp, and the offset would be erased.

It didn't work in my case. I blogged about it, but couldn't find it back right now. I'll search for it ...

I was expecting that I'd need to keep a look up table, which, again, seems to be the approach you have taken.

I haven't. I just store the offset and the correction factor. 2 values. But I haven't been able to turn this in a precise result.

My DAC has a 0 offset of +7mV and a max offset of -2.5mV so it can't get a full range through a simple software adjustment - I think, I was hoping to get an insight from the work you have done here.

I have tried to use an external additional OpAmp to deal with that offset. On the output of my DAC, I wired up  that opamp as analog adder, and provided the offset error as input.

So that if the DAC outputs 0.0001 V at 0, I would feed - 0.0001 into the opamp, and the offset would be erased.

It didn't work in my case. I blogged about it, but couldn't find it back right now. I'll search for it ...