Go Down

Topic: Grounding a differential signal via a mosfet (Read 190 times) previous topic - next topic

Klagemauer

Hello again, I have a question regarding grounding one side of a differential ADC
to be able to use it as a single input converter.  The ADC specifies that in
case you want to use only a single input, the second input should be grounded.
I thought of using a jumper but I would like to refrain from having to open the
case every time I want to switch between signal input types.

Would it be a good idea to use a mosfet to connect the second signal input to ground in that case?
I am unsure regarding that since I would need a digital signal on the analogue side of my shield.
It would just be a steady signal provided by the MU so it shouldn't cause any interference right?
Also, are do you have any recommendations on which mosfet to use? (if my idea is not completely
stupid)

wvmarle

Why "open the case" to switch signals? I'm used to only see final projects to be placed in a case.

MOSFET: any small signal n-channel will do for this. BSS138 comes to mind (if you like to test your soldering skills).
Quality of answers is related to the quality of questions. Good questions will get good answers. Useless answers are a sign of a poor question.

Wawa

If you want to be able to change that with code then you could try to connect that A/D pin also to an Arduino I/O pin. Set the pin when needed to input or to output>low.
Leo..

Klagemauer

Why "open the case" to switch signals? I'm used to only see final projects to be placed in a case.

MOSFET: any small signal n-channel will do for this. BSS138 comes to mind (if you like to test your soldering skills).
If I use mosfets just to pull it to ground, it wont cause any problems with the signal when it's disconnected right?

If you want to be able to change that with code then you could try to connect that A/D pin also to an Arduino I/O pin. Set the pin when needed to input or to output>low.
Leo..
I'd rather not connect the micro-controller to the ADC. I am afraid it will cause too much noise.

wvmarle

If I use mosfets just to pull it to ground, it wont cause any problems with the signal when it's disconnected right?
Depends on the signals and the rest of the circuit.
Quality of answers is related to the quality of questions. Good questions will get good answers. Useless answers are a sign of a poor question.

Klagemauer

Depends on the signals and the rest of the circuit.
Could you give me an example?
I would like to use an n-type logic mosfet in parallel to a 100kohm resistor to keep it closed. so the signal trace has two nodes coming of it but passes directly to the adc.

wvmarle

Largely the output impedance of the analog signal is of importance, and with it how the particular sensor reacts to having its output shorted to ground (it may not be bothered, it may be killed).

You really have to come with concrete sensors and schematics for detailed suggestions.
Quality of answers is related to the quality of questions. Good questions will get good answers. Useless answers are a sign of a poor question.

Klagemauer

#7
Jul 12, 2018, 06:24 pm Last Edit: Jul 12, 2018, 06:25 pm by Klagemauer
Largely the output impedance of the analog signal is of importance, and with it how the particular sensor reacts to having its output shorted to ground (it may not be bothered, it may be killed).

You really have to come with concrete sensors and schematics for detailed suggestions.
Alright, Ill try and look into it. The whole point is to pull the ADC input to ground. I've just attached the general idea behind it.


Go Up