nrf5340 adc with internal reference

[MaximSh]

Hi,

I have 1.8V VDD with 3 analog inputs, channels 1,2,6.

my question is can i use them with internal reference and read from ADC by sequence. for channel 6

1. I use gain 1/2 for channels 1 and 2where the maximum input voltage is 1.2  and gain 1 for channel 6 where maximum voltage is 0.45. Is it ok?

2. The source input for channel 6 is go throw voltage divider where R1 is 1.5 MOhm and R2 is 800kOhm. should i increase Acquisition time and what is the maximum source resistance?

Thanks.

[Kenneth]

1. This should work yes.

2. You may either considering adding a small capacitor (e.g. 10nF) in series with R2 (this capacitor will charge the internal sampling capacitor when performing an ADC measurement) , or alternatively you can increase acqusition time, you can find a table that show acquisition time as function of source resistance:
https://infocenter.nordicsemi.com/topic/ps_nrf5340/saadc.html#concept_qh3_spp_qr 

Best regards,
Kenneth

[MaximSh]

1. After the increase of the Acquisition time it is not working. looks like 40us is not enough for 1.5MOhm Resistor.
2. when i set bits in  adc_sequence channel, the samples to the output buffer written by setup sequence of channels or by channels order? for example setup for channel setup sequence  channel_1,channel_0,channel_6 -> buffer={channel_1,channel_0,channel_6} or buffer={channel_0,channel_1,channel_6} 

[Kenneth]

1. Try to add the capacitor I mentioned on the analog inputs you are using.

2. You should configure CH[] registers in the order you want to sample the analog input, for instance you can:

CH[0] = analog input 1
CH[1] = analog input 0
CH[2] = analog input 6

Please see that there is no direct relation between CH[n] and analog input m.

Best regards,
Kenneth