voltage measurement

[rajade]

I find a peculiar problem while acquiring DC voltage from sensors in differential input mode.

Eventhough the circuit is open, the MAX is showing some arbitrary value (constant all the time)

When the circuit is closed, the value shown is lesser than the actual.

I am unable to understand this phenomenan, Can it be due to grounding problems-because  in differential mode when the negative of the signal is grounded it showed the value little lesser than exact value.

Consider this, when the input to the card is 6.3 v, it still reads 6.01v in max whereas near the card terminals it reads 6.3v

This difference is constant almost at all input voltage levels.

How to sort out this

rajesh

[Henrik_Volkers]

Hi,

first guess: no return path for the bias current. have a look into the manual (or here in the KB) about this effect.

If your source allow a 100k to 1M Ohm resistant to ground , try it 🙂

[Mark_E]

Hi Rajesh,

How does the signal look in RSE mode?

If it’s fine in RSE, but not differential, my first tip would to be to make sure your grounding is good. You can check out the following DevZone article on wiring and noise on signals, especially Table 1 (about 1/3 of the way down, in the third section) for recommendations on how to connect signals, depending on their grounding. The recommendation for bias resistors (for differential or NRSE with a floating source) is any value between 10k and 100k.