ADuC7061. ADC Offset Calibration Register (ADC0OF, ADC1OF)

I checked that you could write manually to the offset calibration register.

Is it possible to perform the calibration, then record the value of the offset calibration register and write it manually later in the user code?

At the manufacturing stage, such calibration can be performed. But in the operating mode, it is impossible to calibrate the offset, because this will require an external zero. And in the temperature range -40..+85 C will have to do this periodically. Therefore, zero-scale calibration with the connection of an internal zero is used. For ADC2/ADC3 pins, it does not work correctly (see above). You can evaluate the error only by connecting an external zero.

I tried to replicate your setup with the code provided.

IEXCON  = BIT7 | BIT2 | BIT1; // IEXC1=1, 600uA
ADCMDE = BIT1|BIT0; // ADC IDLE MODE
ADCFLT  = BIT15|(0x07<<0)|(0x3B<<8);						    
// ADC0 Gain=1, ext.ref (VREF+,VREF-), ADC0/ADC1
ADC0CON = BIT15|BIT4;
// ADC1 Gain=1, ext.ref (VREF+,VREF-), ADC2/ADC3	    
ADC1CON = BIT15|BIT4;
	    
ADCMDE  = BIT7|BIT0;			// Enable Continuous conversion mode
while (!(ADCSTA & (BIT1|BIT0))) {} 
while (!(ADCSTA & (BIT1|BIT0))) {}  
while (!(ADCSTA & (BIT1|BIT0))) {}

slADC1Result = ADC1DAT;
slADC0Result = ADC0DAT;

// ADC self-gain calibration
ADCMDE = BIT7|BIT2;
while(!(ADCSTA & BIT15)) {} // Wait ADC calibration status

ADCMDE  = BIT7|BIT0;			// Enable Continuous conversion mode
while (!(ADCSTA & (BIT1|BIT0))) {} 
while (!(ADCSTA & (BIT1|BIT0))) {}  
while (!(ADCSTA & (BIT1|BIT0))) {}

slADC1Result = ADC1DAT;
slADC0Result = ADC0DAT;

For the external reference, I assumed that REF- = GND and REF+ = 180mV.

For ADC0 = (ADC0/ADC1) and ADC1 = (ADC2/ADC3),

Before Calibration: ADC0 = -3391, ADC1 = -1686
After Calibration: ADC0 = -2, ADC1 = 14

For ADC0 = (ADC2/ADC3) and ADC1 = (ADC2/ADC3),

Before Calibration: ADC0 = -3402, ADC1 = -1640
After Calibration: ADC0 = 11, ADC1 = 14

The ADC input pins are shorted during the testing.

The self-offset calibration is working on my setup. Could you let me know if I missed anything in the ADC configuration or the external reference?

Have you tried using another unit in your testing?

According to the datasheet "External Reference Input Range"  should be within = 0.1V .. AVDD. In my case, VREF−(pin 18) = 1.144V and VREF−(pin 17) = 1.324V. Diff VRef=180mV. The results are described above.

Now I want to redo REF- = GND and REF+ = 180mV. This will require reworking of finished products. Let's see what happens.

The "External Reference Input Range" refers to the REF+ and REF- pins voltage difference.

I tried replicating the schematic shown. I measured different voltage readings in the REF+ and REF- pins. REF+ = 0.73V and REF- = 0.55V. The Diff Vref is still 180mV.

Before Calibration: ADC0 = -3451, ADC1 = -1737
After Calibration: ADC0 = -66, ADC1 = -13

I changed 8 channels to REF- = GND and REF+ = 180mV and 300mV. The ADC1 offset remained the same bad. No changes before self-offset calibration and after :(( On average 1500.

On channels assembled on chips from another distributor - about 230! Crooked chips?

There may be an issue with the chip you are using since there seems to be a considerable difference between the readings (1500 and 230).

Have you checked if something may be shorted with the circuitry? When I tried to replicate the schematic shown, I read a different voltage at REF+ and REF- pins compared to 1.144V and 1.324V you mentioned.

The measurements are correct. Voltage relative to the ground for Iref = 0.6mA:

1. VDD = 0.580 (BAV90)

2. I-=I+=U-=U+= 0.580 + 470 Ohm * 0.6 mA = 0.580 + 0.282 = 0.862

3. Ref- = 0.862 + 470 Ohm * 0.6 mA = 0.862 + 0.282 = 1.144

4. Ref+ =  1.144 + 300 Ohm * 0.6 mA = 1.144 + 0.180 = 1.324