NI 9502 ADC conversion

[menzi74]

I have the following short questions regarding the NI 9205 module, which I use in connection witht the RIO:

• Floating input, how to connect COM?

      In the manual of the NI-9215, a bias resitor of about 1M (connected to AI-) was suggested to provide a defined common mode voltage if the source was floating. This resitor was             even in-built in the version with the BNC housing. This makes sense, but it was not mentioned inside the NI-9205 manual; can I use the same method to connect floating inputs?

• Sampling rate.
  

    The sampling rate was mentioned to lay inbetween 200kS/s and s250kS/s. In the FPGA, module setup, I can adjust a time between conversions between 4us and 8us. I am not       quite sure about the precision issue. If you sample multiple channels at <8us, probably the precision will decrease, due to the settling time. However, If I sample only a single channel, I could utilize the full 4us/250KS/s without loss of precision, right?

[Eirikur]

Hi menzi74

The Measuring Floating (Nonreferenced) Sources section of the Field Wiring and Noise Considerations for Analog Signals Guide gives an excellent explanation of when and how to use bias resistors. Basically yes, a bias resistor is recommended when measuring differentially to keep the common mode voltage level within the input range of the device. Bias resistors should be used when measuring floating signal sources in DIFF and NRSE configurations no matter the device. Failure to do so will result in erratic or saturated (positive full-scale or negative full-scale) readings.

Regarding your other question you are right.

[menzi74]

Dear Erik,

thank you for your help. The device-independent range of posssible bias resitor values seems huge, allowing great flexibility. Since the source resitance that I am dealing is relatively high (1k-5k), I will use two rather large bias resistor of each 1M. This should keep the common mode voltage low (for bias currents of only 100pA).

Best regards,
Clemens

[Eirikur]

You can take a look at page 4-17 of the M-Series User Manual for a discussion of the resistor value that should  be used when connecting AI- to AI GND.

[R6]

In case anybody digs this up.  This feed should be refering to a 9205 and not a 9502....

         -PBD