We noticed strange behavior when measuring voltages with the SAADC with our custom board.
When VDD_GPIO is at 1.8V (which is our target), the ADC measurement error is about 8%, which is quite high. If we increase VDD_GPIO to about 2.1V or above the ADC measurement error drops below 1%.We did also the same measurements with the DK 0.8.5 and the error there was below 1% with VDD_GPIO even at 1.8V.
We used this adc sample with a few changes:
- GAIN = 1/3
- Calibration bevor each measurement
-
Oversampling = 7 (x128 for smother readings)
The DK 0.8.5 and our custom board have different HV revisions of the nrf960
| DK 0.8.5: | INFO.VARIANT = 0x41414143 | (AAAC) |
| Custom board: | INFO.VARIANT = 0x41414230 | (AAB0) |
Measurements with the custom board:
| VDDIO = 1.8V | VDDIO=2.1V | |||
| Input [V] | ADC value [V] | Error | ADC value [V] | Error |
| 0.0 | 0.000 | 0.000 | ||
| 0.3 | 0.274 | 8.7% | 0.300 | 0.0% |
| 0.6 | 0.548 | 8.7% | 0.600 | 0.0% |
| 0.9 | 0.823 | 8.6% | 0.900 | 0.0% |
| 1.2 | 1.099 | 8.4% | 1.201 | -0.1% |
| 1.5 | 1.374 | 8.4% | 1.504 | -0.3% |
| 1.8 | 1.648 | 8.4% | 1.798 | 0.1% |
Measurements with the DK 0.8.5:
| VDDIO = 1.8V | ||
| Input [V] | ADC value [V] | Error |
| 0.0 | 0.000 | |
| 0.3 | 0.302 | -0.7% |
| 0.6 | 0.598 | 0.3% |
| 0.9 | 0.899 | 0.1% |
| 1.2 | 1.201 | -0.1% |
| 1.5 | 1.499 | 0.1% |
| 1.8 | 1.794 | 0.3% |
Is there an explanation for this strange behavior?
Best Regards,
Ueli
