Detecting negative voltages with SAADC

RE: Detecting negative voltages with SAADC

Noah — Fri, 26 Jun 2020 05:35:41 GMT

This is the solution to my problem, thanks all for the help :)

RE: Detecting negative voltages with SAADC

hmolesworth — Thu, 25 Jun 2020 17:36:29 GMT

Typically you can use a flying capacitor to level shift from a range outside the ADC to a range within the range of the ADC, by using analogue switches. A c/o (changeover) switches the lower end of the capacitor from the negative signal (perhaps its -ve supply) to (typically) nRF52832 Gnd and a second c/o switches from the positive signal to the SAADC analogue input pin. This shifts the sensor sampled voltage on the flying capacitor from +-250mV to +500mV above nRF52 Gnd.

Without a schematic I can't give more details, but this works very well when shifting small signals sitting on top of +ve or -ve pedestal voltages, which can in some cases be quite large (the pedestal in this case is -250mV, quite small). A quad sp or dual c/o analogue switch works well, with supply voltages taken from the sensor rails.

RE: Detecting negative voltages with SAADC

haakonsh — Thu, 25 Jun 2020 10:30:30 GMT

Do you have a datasheet of the sensor?

[quote user="nhill03"]What would I do once I know the output impedance of the sensor?[/quote]

Choose a suitable DC bias circuit.

[quote user="nhill03"]And what do you mean by max/min frequencies, it is an iridium oxide pH sensor which produces a varying voltage output based on pH of solution. [/quote]

Then I guess you have no more than a few Hz as the max frequencies.

RE: Detecting negative voltages with SAADC

Noah — Thu, 25 Jun 2020 09:38:02 GMT

What would I do once I know the output impedance of the sensor? And what do you mean by max/min frequencies, it is an iridium oxide pH sensor which produces a varying voltage output based on pH of solution. I would like to take 20-30 samples to average per reading, and perform one reading every 15 minutes or so

RE: Detecting negative voltages with SAADC

haakonsh — Thu, 25 Jun 2020 08:14:03 GMT

You need to know the output impedance of your sensor and the maximum and minimum frequencies you wish to resolve.

RE: Detecting negative voltages with SAADC

Turbo J — Thu, 25 Jun 2020 07:47:54 GMT

AIN pins cannot go below 0V (GND) - thus you will need to shift your signal to be in 0V...VCC range.

We don't know enough yet in order suggest a specific circuit (e.g. using opamps or instrumentation amps).