How do I dynamically set channel gain(s) on an SCXI-1100?

[Biobartman]

I am using a PCI 6036E with an SCXI-1100 to multiplex trough 25-32 channels with low current signals from a photodiode array. I tried using the DAQ Assistant Express to measure current, but it won't let me do the following (even when I convert it to a standard subVI):
1) Select shunt resistor values greater than 1kOhm. (I am using 100MOhm resistors to get the highest gain possible from the low current photodiodes. I want to be able to programatically adjust the gain depending on the different low intensity light sources).
2) Change the gain selection so I can programatically create an auto range select.

John Feiereisen posted a question "Can I set dynamic gains on an SCXI-1100?" on 11/24/1999 wherein he stat
ed:
"In the past I used a single SCXI-1100 as a 'dumb' mux/amp by passing the OUTPUT and AOREF lines to the front panel connector (jumpers W7 & W8), controlling the SCXI with an MIO-16, and passing the mux'ed output to an A2000 with a BNC cable. I pulled CINs out of some LabVIEW-supplied VIs (v2.2?) to select channel and gain. This setup worked great and allowed me to march through a bunch of transducers and select an appropriate gain for each one. (And allowed much faster acquisition rates than if I did the A/D with the MIO-16.)"

It says that he was able to do what I want to do, but I don't understand how he did it.

Any suggestions or sample code? What is a MIO and a CIN anyways?

Thanks

[JoshuaP]

Hey Biobartman,

I tested creating a current global channel in MAX using NI-DAQ 7.1 and it did not have a problem using a 100 MOhm external shunt resistor. However, you do need to make sure that the current range is equal to or smaller than +/-100 nA. MAX calculates the voltage range by multiplying the shunt by the current range. The voltage range must be equal or smaller that +/-10 V.

The gain for the DAQ device can not be programmatically changed. You must change the minimum and maximum range in order the change the gain settings used on the DAQ device in DAQmx. However, you can change the SCXI gain settings programmatically when the task is not running using the Channel Property Node. The property can be found under Analog Input: General Properties:
Advanced: Gain: Gain Value.

I hope this helps out.

Joshua P.
Application Engineering
National Instruments