SCXI-1125 & PXI-6289

[test_engineer]

http://www.ni.com/pdf/manuals/372425c.pdf 

On page 26 of above pdf, under "Gain/Input Range" at the bottom of the page, it states "In most applications NI-DAQ chooses and sets the gain for you determined by the input range."

Can someone help me to understand this sentence with an example?

Here is what I interpreted.

Assume I have a SCXI-1125 & PXI-6289 working together to capture a 0.5mV (max) voltage.

Because 1125 sees a 0.5mV voltage, it will select a +/-2.5mV range for me, which has a gain of 2000.

When 6289 sees the voltage, the voltage has become 0.5mV * 2000 = 1V.  So 6289 will select +/-1V normal range.

and all of this willbe performed behind the scene by NI drivers.

Is my understanding correct?

[Dennis_Knutson]

No. The actual gain is set by the min and max that you set. The driver will select the actual range that brst accommodates the expected signal. For example, if the DAQ card has ranges of +/- 1, +/- 5, and +/- 10 and you specify min of 0 and a max of 3, the actual range of the ADC will be +/- 5. There is no auto-scaling feature.

[test_engineer]

Thank you for your reply.  Does it mean SCXI-1125 doesn't do anything and it will always stay at Gain of 1?  in order to use 1125, I have to actively select the range the the gain setting for it?  especially when signal is low, say 0.5mV.  SCXI-1125 won't select gain of 2000 for me auotmatically, correct?

[Dennis_Knutson]

It will set the gain based on the min and max you provide with the DAQmx Create Channel function.

[test_engineer]

Dennis_Knutson, once again, I appreciate your reply.  Please forgive my stubbornness, but I want to be crystal clear on this.

If I have a signal which min is 0V and max is 0.5mV, and I pass in these two parameters with DAQmx Create Channel function, can you tell me the RANGE and the GAIN that DAQmx will select for me at SCXI-1125?

Will it be the first row (SCXI-1125 Gain = 1, Range = +/-5V) in following table or the last row (SCXI-1125 Gain = 2000, Range = +/-2.5mV)?

[Dennis_Knutson]

Neither. It would be the +/- 5mv.

[Eric-E]

Hello,

With a Min = 0 V and Max = 0.5 mV (not 5 mv)  we would see the 2.5 mV Overall Voltage range.

Also, you might want to consider the terminal block's gain. There is a high voltage attenuation on some terminal blocks. Make sure that the dip switch on the terminal block is set to feedthrough mode on each channel so that the voltage is not attenuatede as it goes through the terminal block.

Image is from page 4 through 5 of the following link:

SCXI -1327 High-Voltage Attenuator Terminal Block

https://www.ni.com/docs/en-US/bundle/321930c/resource/321930c.pdf

[Dennis_Knutson]

Oops, missed the decimal point. Sorry.

[test_engineer]

Thanks, Eric.

Yes, SCXI-1327 is set to 1:1.

A follow-up question.  After the 1125 (gain of 2000) & 1327 (1:1), the DAQ (PXI-6289) will see 0.5mV * 2000 = 1V, correct?

So I can follow following table (full scale 1V to -1V) to calculate the absolute accuracy for 6289, correct?

source of above table: page 6 of following pdf

http://www.ni.com/pdf/manuals/371292g.pdf 

Thanks, and appreciate your help.

ps. I understand the source of uncertainty from other sources like 1125 or 1327.  here I want to make sure the DAQ 6289 sees a 1V (amplified) signal instead of the original 0.5mV.

[Dennis_Knutson]

I don't see where you decide the 6289 would be using a +/- 1 volt range. You've set the SCSI to output a max of +/- 5. That is what the driver is going to use, not a measured value that could change sample to sample. You are assuming behavior that is current than what the documentation says.