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Analog input unexpected behavior

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I'm working on taking +/- 50 voltage and decreasing it by 10X (I.E 50 V is 5VDC on Analog In). I'm using a voltage divider to perform this with 9 Megaohm resistor in series with a 1 MegaOhm resistor. The general schematic is shown in the picture. This setup works fine for 0-50VDC but once I get to about -13VDC Vin, the analog voltage reading jumps from -1.3 VDC to 0.6VDC. I am using PCIe 6353 for this and labview 2017. Is this a high impedance issue? Perhaps an over voltage protection device causing this on the negative end? Thanks in advance.

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Can you add your ground reference to the schematic?  Do you have anything wired to any of the DAQ card's ground pins?  Is your signal source referenced to an earth ground?  Right now the signal doesn't appear to be referenced to anything.  If that's the actual case, then you could well be floating out of the common mode range of the DAQ card. 

 

Also, I assume the DAQ card is configured for differential input since you used AI+/- terminology, but I want to make sure.  Can you confirm?

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I do have it set to differential. The DAQ card's ground pins are floating but for differential measurement does that matter? I've tried connecting the earth ground of my power supply to the analog gnd rail but the issue still persists. Thanks.

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Solution
Accepted by topic author LabviewRat
04-18-2019 11:35 AM

What matters is the voltage of V+ and V- with respect to the DAQ board's ground.  For example, if V- is at -10 and V+ is at 40 V with respect to DAQ ground, you have a differential voltage of 50 V.  After your voltage divider, the DAQ board will read -10 on AI- and -5 on AI+, so 5 V differentially, and everything is hunky-dory.  If you then keep V- at -10 but lower V+ to -60 V to get a differential voltage of -50 V, your DAQ board will see -10 on AI- and -15 V on AI+.  This breaks the Maximum working voltage spec, which states that each AI pin must be within +/- 11 V of DAQ ground, and you could see weird readings at this point.  Even if your differential input voltage is within the range you select (+/-5 or +/-10 or whatever), each pin must always remain within +/- 11 V of DAQ ground.

 

By using the terminology V+ and V- for your source instead of V+ and Gnd, you're implying that V- is floating (i.e., not grounded).  (I don't know if this is actually true, which is why I'm asking.)  If both your source AND your measurement device are not grounding the signal, then either end of the signal could easily float out of range of the DAQ board's max working voltage.

 

If you're sure that V- is tied to your power supply ground, then I'd just double check the voltages between AI+ and AI ground and then AI- and AI ground when you're sourcing near the problem voltage (-13 V) just to confirm.  If V- is floating, try connecting it directly to your power supply ground or DAQ ground.  Connecting the power supply earth ground to DAQ ground isn't going to help if the power supply's output is isolated from its ground (a common situation if you're using an SMU, for example).

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