Differential ADC Anomaly on nRF9160 DK

[bencra]

Hi,

I'm trying to read differential ADC input with inputs configured as:

NRF_SAADC_INPUT_AIN1 (P0.014) -> POSITIVE

NRF_SAADC_INPUT_AIN2 (P0.015) -> NEGATIVE (differential mode)

I'm also using 4x oversampling, gain of 1, +-0.6V reference, 12-bit ADC and 5us acquisition time.

Unfortunately, I seem to be getting very poor results.

By grounding both inputs, I'm receiving an output value of ~350, which equates to around 103mV, which is too poor for my use case.

Any advise would be appreciated.

Regards,

Ben

[Håkon Alseth]

Hi,

 

The UART1 interface occupies the P0.14 and P0.15 pins by default on the DK, as shown here: https://infocenter.nordicsemi.com/topic/ug_nrf91_dk/UG/nrf91_DK/if_connector.html

Could you try to use AIN0 and AIN3 (https://infocenter.nordicsemi.com/topic/ps_nrf9160/pin.html?cp=2_0_0_9_0) instead and see if these give the expected result?

 

Kind regards,

Håkon

[bencra]

Hi, that has reduced the error to ~25, which is not ideal but usable. I'm still seeing wild oscillations between samples. For example:

Sample 609: -335
Sample 610: 314
Sample 611: -333
Sample 612: 312
Sample 613: -336
Sample 614: 314
Sample 615: -337
Sample 616: 312
Sample 617: -338
Sample 618: 314
Sample 619: -337
Sample 620: 317
Sample 621: -340
Sample 622: 317
Sample 623: -336

No matter the sampling parameters, the samples alternate in this way. Is there a reason for this?

[Håkon Alseth]

Could you attach your configuration/sample so that I can try this at my end?

 

Kind regards,

Håkon