Programmable DC Electronic Load - First Look

I have made a little progress...

Received these parts which more or less match the 2nd schematic drawn above:

• Bourns PWR220T-35-1R00FBourns PWR220T-35-1R00F TO-220 style 1% 1 ohm resistors
• IRLZ34NPBFIRLZ34NPBF Infineon N-Ch MOSFET
• MCP4725A0T-E/CHMCP4725A0T-E/CH Microchip 12 Bit DAC
• OPA192IDBVROPA192IDBVR Texas Instruments Op Amp
• 10C035410C0354 Bourns 2 Ch Encoder

I also recently purchased an Adafruit Feather M4 Express which has the ATSAMD51J19 microcontroller with 12 bit ADCs and 12 BIT DACs so I did some experimentation with that.  The DAC was programmed in the Arduino IDE to output various voltages and then measured with via ADC (2 pins away) and a digital multimeter.  The first ADC reading was thrown out and then the next 5 averaged. The following plot shows the deviation of the programmed DAC output and ADC readings from the meter.

The DAC output in blue is very linear and could be easily corrected to a large extent in software.  There is a lot of scatter in the orange ADC readings.  One ADC reading at around 1800 mV is in error by approximately 35 mV which is outside what the datasheet gives as maximum error - not sure what caused that.  I seem to remember the MSP432 giving much better results and had a look at the TI datasheet.  It reports error differently but does seem to be both more accurate and precise.  It might be interesting to do a real world comparison to the Atmel chip.  It is a shame there isn't a MSP432 variant with a DAC.

The following plot shows the percent error for the ADC and DAC on the Feather M4 Express.  The title is a bit misleading - the X axis is the meter reading in mV and the Y axis is the percent error for the DAC in blue and the ADC in orange.

Percent error in the DAC would improve if the software correction described above was implemented.

In summary, DAC output seems good on the Atmel SAMD boards so far.  The ADC is probably only good for 20 mV or so.  This may be good enough for reading the input voltage of the DUT but not good enough for reading current.  For that, it would be better to just report the DAC setting.

The Bourns 1 ohm resistors were measured with the milliohm meter and are within 0.1% of 1 ohm.

Hello Frank,

Your ADC seems to be very noisy.

Could you try two experiments:

1) 1000 samples with maybe 50mV input (the 50mV must be low noise - ideally source from a battery and divider with a big low esr cap across it.

2) 1000 samples close to full scale

The first measurement will tell you about the ADC noise, the second will tell you if the ADC reference voltage is noisy

From your results so far it looks as if you have both problems.

How has the ADC been set up. (in terms of acqusition time, sampling speed averaging etc) ?

If you don't need to sample very fast you should be able to get a decent noise performance out of this chip by oversampling.

MK

Hello Michael,

Great insight and suggestions and I set up experiments as you suggested.  Power is from two alkaline batteries with a voltage divider of ~ 100k and 2.2k resistor for the lower voltage (66 mV) and batteries in series for the higher voltage (3.08 V).

I ran 1000 tests on both the higher and lower voltages with the following Arduino code.

/*
 * Test ADC on Adafruit Feather M4 Express
 * Reads raw analog input on ADC_PIN, calculates voltage, and outputs results numTests times...
 */
const int DAC_RES = 12;                    // Set resolution
const int ADC_RES = 12;                    // Set resolution
const int MAX_DAC = pow(2,DAC_RES);
const int MAX_ADC = pow(2,ADC_RES);
const float V3 = 3.309;                    // 3V Feather Reference Voltage per DMM
const float VIN = 3.07;                    // Measured input voltage per DMM
const int ADC_PIN = A2;
const int numTests = 1000;
void setup() {
  Serial.begin(115200);
  Serial.setTimeout(8000);
  while (!Serial) {
    ;                                     // wait for serial to start
  }
  
  Serial.println("i,RAW_ADC,ADC_V,V_ERR");      // Output labels
  analogReference(AR_DEFAULT);            // use V3 (3V3) Reference Voltage
  analogReadResolution(ADC_RES);          // ADC input resolution
  analogWriteResolution(DAC_RES);         // DAC output resolutio
  int i = 0;
  for (i=0; i< numTests; i++){
    unsigned int rawAnalogInput = analogRead(ADC_PIN);
    float voltInput = V3 * (float)rawAnalogInput / MAX_ADC;
    float voltErr = voltInput - VIN;
    Serial.print(i);              Serial.print(",");
    Serial.print(rawAnalogInput); Serial.print(",");
    Serial.print(voltInput,4);    Serial.print(",");
    Serial.println(voltErr,4);
  }
}
void loop() {
  // do nothing
}   

Below is the summary output run through an Excel spreadsheet for the 66 mV test.  The raw analog input readings have been plotted.  Shown in the table below it are the raw readings, the calculated voltage, and the error when calculated voltage is subtracted from the voltage as read by a DMM.

Below is the same data for a 3.08 V test.

So we might interpret this as meaning there is high ADC noise?  The readings at 3.08 V are quite tight.

A friend pointed out that the Adafruit Feather M4 Express does not have a ferrite bead to prevent digital VDD noise interfering with VDDANA on the SAM D5 as recommended in the datasheet.  I hooked up the oscilloscope and found the following on the 3V3 pin which appears to also be feeding VDDANA.

EDIT:  The following screenshot was taken while the Feather was powered from an Ikea USB on a power strip.  The data above was all taken while the Feather was powered from the USB on a laptop.

Regarding the way the ADC been set up in terms of acquisition time, sampling speed, averaging etc. I would have to dig into the way Adafruit has implemented the Arduino functions and have not done that yet.  In my first trials above I did average 5 samples.  Based on what is seen above I could take more samples and maybe throw out fliers.

Hello Michael,

Great insight and suggestions and I set up experiments as you suggested.  Power is from two alkaline batteries with a voltage divider of ~ 100k and 2.2k resistor for the lower voltage (66 mV) and batteries in series for the higher voltage (3.08 V).

I ran 1000 tests on both the higher and lower voltages with the following Arduino code.

/*
 * Test ADC on Adafruit Feather M4 Express
 * Reads raw analog input on ADC_PIN, calculates voltage, and outputs results numTests times...
 */
const int DAC_RES = 12;                    // Set resolution
const int ADC_RES = 12;                    // Set resolution
const int MAX_DAC = pow(2,DAC_RES);
const int MAX_ADC = pow(2,ADC_RES);
const float V3 = 3.309;                    // 3V Feather Reference Voltage per DMM
const float VIN = 3.07;                    // Measured input voltage per DMM
const int ADC_PIN = A2;
const int numTests = 1000;
void setup() {
  Serial.begin(115200);
  Serial.setTimeout(8000);
  while (!Serial) {
    ;                                     // wait for serial to start
  }
  
  Serial.println("i,RAW_ADC,ADC_V,V_ERR");      // Output labels
  analogReference(AR_DEFAULT);            // use V3 (3V3) Reference Voltage
  analogReadResolution(ADC_RES);          // ADC input resolution
  analogWriteResolution(DAC_RES);         // DAC output resolutio
  int i = 0;
  for (i=0; i< numTests; i++){
    unsigned int rawAnalogInput = analogRead(ADC_PIN);
    float voltInput = V3 * (float)rawAnalogInput / MAX_ADC;
    float voltErr = voltInput - VIN;
    Serial.print(i);              Serial.print(",");
    Serial.print(rawAnalogInput); Serial.print(",");
    Serial.print(voltInput,4);    Serial.print(",");
    Serial.println(voltErr,4);
  }
}
void loop() {
  // do nothing
}   

Below is the summary output run through an Excel spreadsheet for the 66 mV test.  The raw analog input readings have been plotted.  Shown in the table below it are the raw readings, the calculated voltage, and the error when calculated voltage is subtracted from the voltage as read by a DMM.

Below is the same data for a 3.08 V test.

So we might interpret this as meaning there is high ADC noise?  The readings at 3.08 V are quite tight.

A friend pointed out that the Adafruit Feather M4 Express does not have a ferrite bead to prevent digital VDD noise interfering with VDDANA on the SAM D5 as recommended in the datasheet.  I hooked up the oscilloscope and found the following on the 3V3 pin which appears to also be feeding VDDANA.

EDIT:  The following screenshot was taken while the Feather was powered from an Ikea USB on a power strip.  The data above was all taken while the Feather was powered from the USB on a laptop.

Regarding the way the ADC been set up in terms of acquisition time, sampling speed, averaging etc. I would have to dig into the way Adafruit has implemented the Arduino functions and have not done that yet.  In my first trials above I did average 5 samples.  Based on what is seen above I could take more samples and maybe throw out fliers.

I need to think about this one.

The amazing thing is that the noise is way lower with the input at 3V than when at 100mV.

If the ADC noise were constant that would give you either the same std dev at both or possibly higher at 3v due to reference noise.

The other difference is that you are driving the input from the batteries (nice low source impedance) or the divider (2k2 source impedance.)

Well worth repeating the low voltage experiment but with a 10uF low esr cap// 100nF ceramic across the input as well.

Unfortunately the Arduino code hides most of the ADC set up.

Trying a low noise power supply and the ADC internal reference might be interesting as well.

MK

Feeling like a dull boy... New trials were set up using the microcontroller's internal reference source and capacitance on the input voltage at 66 mV separately and together with slight advantage to adding capacitance.  Then the original setup at 66 mV with no capacitance and the analog reference set to the "default" to try and replicate yesterday's results was set up and almost identical good results were obtained.  In other words, I cannot replicate yesterday's noisy results.

Default analog reference, Capacitance added

Raw Avg:  90.0

Raw Median: 90

Std Dev: 0.9

Internal 2V23 analog reference, no capacitance

Raw Avg: 90

Raw Median: 90.0

Std Dev: 1.3

Internal 2V23 analog reference, with capacitance

Raw Avg: 90.1

Raw Median: 90

Std Dev: 1.0

Repeat original test, Default analog reference, no capacitance

Raw Avg:  90.3

Median: 90

Std Dev: 1.3 (yesterday was 11.5)

Looking back at the photo taken above the differences in the setup are limited to moving the resistor divider to fit the capacitors but something in the original tests gave noisy results at low input voltages that no longer seems to be there.   I am going to move off of the breadboard, solder something up, and repeat the experiments with care over the weekend.  The DAC also needs to be tested for noise.

Well, the good news is that the problem is fixable. I've not used pluggable breadboard for a very long time -  so I'm all for soldering.

Good luck.

MK