rf52840 saadc vreference

RE: rf52840 saadc vreference

Jared — Wed, 13 Jan 2021 13:53:15 GMT

Hi,

I understand. The main issue here is that you would want the internal reference to be at 3.3 V. This would mean that VDD would have to be 13.2 V if you chose VDD/4 as the internal reference. This is obviously way over the maximum ratings for VDD at 3.6 V. In short we don't support this. The internal reference can either be VDD/4 or 0.6 V. While VDD can be up 3.6V

best regards

Jared

RE: rf52840 saadc vreference

ziv123 — Wed, 13 Jan 2021 10:01:00 GMT

hi Joakim

are you also going to attend this issue or did you passed this subject to Jared ?

RE: rf52840 saadc vreference

ziv123 — Wed, 13 Jan 2021 09:59:32 GMT

sorry to say but this discussion does not advance me in any way, i already know that in order to get 0-3.3v range i have to use gain 1/4 if my V ref is VDD/4, the point was that in that way i loos some accuracy of input, the input was reduced to VDD/4 (~3.3/4 ~0.825v) and bringing it back to 3.3v is done by some calculation, like a camera chip that compensate on less accurate lens or sensor, so missed pixels are calculated in and were not captured by the input itself (hope i made my self more clear with this example)

so my wonder was that with the nrf52 there is no way to get the full original range of input and my question was am i missing something ?

RE: rf52840 saadc vreference

Jared — Wed, 13 Jan 2021 09:43:57 GMT

Hi,

With a gain of 1 and using VDD/4 as internal reference you'll get an input range +- VDD/4:

Input range = (+-VDD/4)/Gain ------------> Input range = +- VDD/4*1 = +- VDD/4

A lower gain will enable you to have a higher input without going over the internal reference. If you want an input range up to VDD = 3.3 then you have to lower the gain to 1/4 if the SAADC is using VDD/4 as it's internal reference:

Input range = (+-VDD/4)/Gain ------------> Input range = +- (VDD*4)/(4*1) = +- VDD

regards

Jared

RE: rf52840 saadc vreference

ziv123 — Tue, 12 Jan 2021 15:03:14 GMT

can you explain what's the values you expect to read when v ref is VDD/4 and gain is 1 ? because to my understanding it will not be in the range equivalent of 0-3.3 v

RE: rf52840 saadc vreference

Jared — Tue, 12 Jan 2021 13:13:55 GMT

Hi,

You're correct in that the internal reference should be either VDD/4 or the 0.6V but the gain is not dependent on the internal reference. You can chose any gain from 1/6 to 4, see the config register.

regards

Jared

RE: rf52840 saadc vreference

Joakim Jakobsen — Sun, 03 Jan 2021 23:15:29 GMT

Hi.

Sorry about the delay here.

Due to christmas vacation there has been low staffing.

I will take a look at your ticket tomorrow.

Br,
Joakim