pci 6052e scxi 1520 input limit relationship?

[cb34]

Hi!

In the sample programs for DAQmx Analog Input, you can specify the input range for a given physical channel. For example, if you have only a PCI 6052e, then you can browse all channels.

But what if you have an SCXI 1520 connected to it and you choose its channels for which the input ranges will take effect. Will the DAQ card take on its default input range (say +/- 10 V)? Is there a way to specify input ranges for both scxi 1520 and pci 6052e?

Please help me on this. Thanks!

[JaredA]

Hello,

The DAQmx driver is an intelligent piece of software that will set the gains on the SCXI and DAQ device automatically by simply setting the input range on your SCXI DAQmx Task. When your signal is first acquired on your SCXI module, the maximum amount of gain will first be applied by the SCXI module. This helps to reduce the total amount of noise present in the measurement system. After the gain is applied, the signal is sent to the DAQ card at which point the post-SCXI amplified signal is analyzed to see if an additional gain setting can be applied by the DAQ card and still have it fall within the input range.

So you can see, the driver maximizes the gains of the signal simply by giving it the input range that you are expecting and the actual signa
l acquired.

In particular, why are you trying to change the ranges manually? I'd be interested to learn why and possibly research farther to see if there's anything that could be done depending on your application. Let me know.

Jared A

[RDonaldson]

I am converting an application which uses a PCI-6030E and SCXI 1141 modules from Traditional to DAQmx .  With the traditional setup I can measure ±6.2v from the SCXI 1141 modules. I think the DAQmx "intelligent" gain control is preventing me from setting the PCI-6030E card to ±10 volts which would allow the SCXI 1141 modules to have the same range of ±6.2v rather than ±5v. This reduction of range comes into play when I use DAQmxCreateAIVoltageChan() with a "SC1ModX" identifier in the device list.  DAQmxGetAIGain() tells me the SCXI module is at a gain of 1 and DAQmxGetAIRnagHigh tells me that range is 10 but the range rails at ±5v. Using DAQmxReadRaw tells me the rail is reaching the signed integer numerical range limit of 32768. I have tried using DAQmxSetAIRngHIgh(), DAQmxSetAIMax(), and DAQmxSetAIMin to force the PCI-6030E range to 10/±10 with no raised errors but no change. Any ideas on how I can get my full SCXI 1141 range back using DAQmx?

[TheWoost]

Hi RDonaldson,

The ability for the Traditional DAQ drivers to allow a returned reading in excess of the rated input range of the SCXI module should have not been allowed.  The NI-DAQmx drivers will not return voltages outside of the rated range simply because the data cannot be certain.  It is preferred for the DAQmx driver to return a railed reading rather than inaccurate data.  Perhaps you could consider modifying your setup with a voltage divider at the input to put the acquired signal within the rated range of the SCXI module.  The software could then be modified accordingly to perform scaling or calculations off of the new analog input signal.

Regards,
Andrew W
National Instruments