Measuring signals above and below VSS with the ADC

[Ohad Gal]

Hello,

I'm abit confused... while the reference is +/-0.6V and absolute ratings are not to go below VSS=0

so is it SAFE to measure -0.5V with the ADC?

it looks like we only use half the span of the ADC if we restrict to 0..+0.6V

Another question - if we use GAN1/4 then 1V reads as 0.25V by the ADC

so in such case, can we input -2V (read as -0.5V by the ADC when using gain=1/4)?

Is it the same answer for the nRF9160 and nRF9161?

the AI answer is not so clear (see below)

thanks

Ohad

[Kenneth]

Hi,

I totally agree with you that this was confusing, but I can also find that on the line below where you copied that it does say it's not possible "The AIN0-AIN7 inputs cannot exceed VDD_GPIO, or be lower than VSS":

Honestly speaking I am not entirely sure why there is an ± sign, but it might be because there is an differential mode, where you measure two the differential analog level between two inputs, so I guess it make sense in that mode.

Hope that helps,
Kenneth

[Ohad Gal]

We actually se when measuring single ended between A5 to GND that we get negative values. 
although small, they are real. 
maybe they are due to the AGC offset?

but you are saying that all GPIOs and all Analog inputs must not go below VSS. Is that correct?

[Kenneth]

Ohad Gal said:

maybe they are due to the AGC offset?

Very likely yes.

Ohad Gal said:

but you are saying that all GPIOs and all Analog inputs must not go below VSS. Is that correct?

Correct.

Kenneth