Noisy low voltage signal with 9215 BNC module

[Shashi.]

I am using NI 9215 BNC module with cDAQ-9172, to acquire low voltage signals (8mV-50mV) from temp. sensors. The signal has a noise envelop of about +/-2.5mV. This noise level appears even when there is only a resistor connected across AI+ & AI-, or even AI+ & AI- are just short.

Has anyone faced a similar problem with this module? Suggestions are welcome.

Shashi.

[johnsold]

Shashi,

According to the specifications for the NI 9215, the input noise is 1.2 LSB rms or 7 LSB peak-to-peak. With a 20 V input range (+/-10 V) and 16 bit resolution 1 LSB = 20/65536 = 0.31 mV. Thus specified noise is then 7*0.31 mv = 2.1 mV pp.

What you are reporting is slightly higher than this. The difference may be due to a different chassis configuration or other factors which add some noise to the system.

Lynn

[sth]

Is this a diffential signal or a single ended measurement?  Is there a path to ground for the high impedance input of the digitizers? 

Digitizers pump out a bit of noise into the inputs and can heat low temperature sensors if you are measuring such you should have an amplifier stage between your sensor and the digitizer input.

What is the spectrum of the noise?  Are you picking up 60 Hz?  Do you have a ground loop in the system?