Input resistance of the DAQ channels

[student_1]

I am trying to sample an anlog output using NI PCI-6220, through a BNC-2110.

The signal can be chosen to be either bipolar (-10 V to +10 V) or unipolar (0 to 10 V), and is coming from a LakeShore 336 temperature controller. The manual specifies the following:

" The unpowered analog outputs are not designed to provide heater power, and although they are short-protected, should not be used to drive a resistance lower than 1 k Ohm"

I was planning to directly connect the instrument output to the BNC-2110, but now I am wondering about this 1 k Ohm thing. How do I know what the input resistance of the instrument is? 

[AnalogKid2DigitalMan]

The BNC-2110 is just a connector block, you need to look up the manual for what DAQ card you have it connected to.

The lowest input resistance DAQ that I know of that NI produces is the USB 6008/6009 series, and that is at 144Kohm.

The majority of the DAQ cards NI makes are in the 1-10Gigohm range.

-AK2DM

[student_1]

The manual says:

>10 GΩ in parallel with 100 pF

Obviously this is way too much higher than the 1 k Ohm required. How big of a deal can this be in terms of accuracy of measurement?

[crossrulz]

That 1kOhm requirement sounds more like a load current requirement than an accuracy requirement.  Connecting directly to the DAQ should not do anything to hurt your measurements.

[student_1]

When you say "load current requirement", this is a resistance, so then I would say I=V/R=10/1k=10 mA, so I guess the manual wants no more than 10 mA to be sunk from the instrument, or, in another way, no less than 1 k Ohm resistance. In our case, with 1 G Ohm, there should be no worries. Correct?

On a similar note, this make me think: I=10/1 G = 10 n A. The manual of the 6220 DAQ specifies " Input bias current ±100 pA". So this is going to be higher than allowed...?

[crossrulz · Accepted solution]

Correct, no worries.

I'm pretty sure that the bias current is the minimum ammount of currnent that is needed to get your signal through the MUX and ADC in the DAQ.  Again, you shouldn't have any worries here.