Need to read a 30, 1.5A analog input signal using a NI-9205

[Agile]

Problem: I need to read two analog inputs with good resolution using one C-Series card. The two signal ranges are 0-30 V @ 1.5A and 0-5 V (not sure of the current). I need to be able to read the 5V signal with mV accuracy (i.e. 16 bit resolution).

I have looked at the NI-9221, which works great with the 30V signal, but doesn't give enough resolution for the 5V signal. The 9221 is 12-bit over the 60V range.

I have looked at the NI-9205, which works great with the 5V signal, but it can't handle the higher 60 V signal.

So a voltage divider circuit comes to mind, but has anyone found a good solution to this (other than getting both modules)?

[JÞB · Accepted solution]

Well, the current shouldn't matter.  The Analog input to the device should (Ideally) draw no current and any current on the source will pass through the UUT.

A 4:1 voltage divider should scale the signal well to the 9205 AI as long as you do not violate the absolute maximum voltage between any device pin and Ground.

[crossrulz]

We typically use a 10:1 divider with a buffer op-amp before the DAQ for any voltage above 10V.  I think the reasoning for the op-amp is that the switching of the DAQ could be seen when taking measurements.

[Agile]

Thank you Jeff·Þ·Bohrer and crossrulz. I could only mark one as a solution, but they are both great responses.

Crossrulz, would you explain further the need for the op-amp? I get the impression that the switching causes a glitch or a surge. Is this correct?

[crossrulz]

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@Agile wrote:

Crossrulz, would you explain further the need for the op-amp? I get the impression that the switching causes a glitch or a surge. Is this correct?

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I don't remember the full details.  It was right when I was getting into test engineering and I wasn't on the project.  And even thinking back, it doesn't make much sense to me.  But I want to say it had to do with an impedence mismach when the mux inside the DAQ switched and somebody saw a tiny glitch on a scope plot and freaked out.  But unless you have super tight limits because you are messing with space-grade products, it probably isn't necessary.

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@Agile wrote:

Thank you @JÞBand @crossrulz. I could only mark one as a solution, but they are both great responses.

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Kudos work as "Thank you" too.

[SnowMule]

My experience is with either high-impedance outputs (like optoelectronics) or low-impedance inputs ([cough]6008[/cough]).  A buffer inbetween the two lets you use the high-z input of the opamp and its healthy output signal to go into whatever measurement circuit you want. 

Also nice for scaling small signals to big signals, depending on the range you have available on the DAQ. 

If you're using RSE on a 6008, the only range you get is 5V.  By amplifying a 0-100mV signal 50x you can use the full range of the DAQ and get better resolution of the measured signal.

[Agile]

Thanks, Crossrulz. May the wind be ever at your back, and your system requirements always within budget. (eh, something like that)