What is the analog input impedance of the NI PCI-6229?

[RWB]

I am trying to determine the affect of a 12K resistor that is in series with an analog input of a NI PCI-6229 data acquisition board.  The 12K resistor appears to be part of an RC filter.  I have a 0 - 10 VDC source feeding this circuit.  What is the analog input impedance of the NI PCI-6229 data acquisition board?  If it makes any difference, the analog input is connected in differential mode with a 180K resistor to AI Gnd.

Thanks,

RWB

[aNIta B · Accepted solution]

Hi RWB,

        The input impedance is listed in the specifications as 10 GOhm.  So, the affect of your 12k resistor should be relatively small.  Take care!

[EBL]

RWB,

The input impedance of the 6229 is listed as 10 Gohms in parallel with 100 pF. While the 10 Gohms will probably not affect your measurement, the 100 pF may. Your 12 k resistor and the 100 pF will form a lowpass filter with a -3 dB cutoff of 133 kHz, which is significantly lower than the board's nominal 700 kHz small-signal bandwidth. So your intuition that you have an RC filter is correct. (The actual cutoff frequency may be somewhat lower than 133 kHz, due to the protection circuits.)

The other effect of the 12 k resistor that you may notice is unusual settling behavior due to charge injection from the multiplexers. Your high impedance allows the charge generated during channel-switching to be accumulated on the internal capacitance. Hence you may experience transients with 133 kHz (1.2 us) time constants as your source dissipates the charge from the capacitance. If you're not scanning among channels, this shouldn't be a problem for you.

Hope this helps,

EBL