Measuring 16V using external reference point (of -10 or -5 V)

[DennisVoort]

I'm currently in posession of a PCI DAQ 6281 card, hooked up to a control system. I'm trying to measure an voltage, ranging from 3V-16V, depending on the power consumption of the thing to be controlled (sorry, can't really get into it, confidentiality and such).

A voltage divider is not possible, as another strand to the control system will disrupt it's function (apparently, I tried).

My next step was to change the internal reference point to a -5V or -10V source, so it would register as 6V, instead of 16. 

the card should be able to measure a 20V difference, so it's mostly a driver issue, but I was wondering if it was possible to change the reference point on this card?

Thanks for the help,

Dennis vd Voort

Department Biomedical Engineering 

Eindhoven University of Technology. 

[Norbert_B]

Dennis,

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DennisVoort wrote:[...]

the card should be able to measure a 20V difference, [...] 

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This is not true. The device is capable of measuring differences between + 10 V and - 10 V between two different wires. This makes an amplitude of +-10 V for the signal. It might alternate up to 20 V in total, yes, but the difference between wire + and wire - never exceeds |U| = 10 V.

Furthermore, the device 6281 is not isolated; this means that the ADC is grounded to the ground potential of the measurement system. If any signal (regardless of connection type or input pin) exceeds the about 12V in regard to the ground potential of the measurement system, the ADC will be saturated.

To solve your issue, you have to "scale down" your voltage (Signal Conditioning). 

hope this helps,

Norbert 

[DennisVoort]

As I said, I tried that, but signal conditioning will alter the function of (not to mention, slow down) the controller.

Is there no way to change it to a Unipolar function? If I pay 1500 euros for a card, I'd expect to be able to change its internal reference.

Message Edited by DennisVoort on 12-01-2009 07:10 AM

[Norbert_B]

No, the device is bipolar only. And you cannot set another reference for analog input. You can do this for analog output, but this does not help you in your situation..... (see here)

Here the important excerp of the linked manual:

Maximum working voltage for analog inputs
(signal + common mode) ................ ±11 V of AI GND

AI GND is the ground potential of the measurement system.

What i do not understand from your reply is: [...]but signal conditioning will alter the function of (not to mention, slow down) the controller. 

Why should signal conditioning alter the function of the controller? If termination is the issue, you have to include isolation using the proper termination on the primary (conroller) side.

Just using a simple potential divider made off simple ohmic resistor will do no good, that's for sure!

Norbert 

[DennisVoort]

The controller is based on a wheatstone bridge principle, and because of power requirement issues I had to create an instability, which means that even large resistors still effect the system to a degree I'm not comfortable with (health issues and such), not to mention adding another strand. 

But I'm going to try buffering before the readout and see what happens. 

[ChrisR.]

It's hard to make suggestions without knowing the exact nature of what you're trying to measure.  Of course you can make measurements with respect to whatever voltage you like, but as Norbert pointed out, the inputs to the board have to be within ±11V of the computer's ground.  Now if your source is isolated from this ground, then you can "ground" it to -10V (use one of the board's DACs to generate this), and your 16V signal will be at 6V and you can measure it.  But it sounds like the system is probably already grounded and you want to measure signals more than 11V away from ground, in which case there's nothing you can do without building some sort of external attenuator or level shifter.

Chris 

[DennisVoort]

Alright, the buffer worked to seperate the voltage divider from the controller. Thanks for all the help!

Kind regards,

Dennis van der Voort 

[aeastet]

did you try giving it a scale and off set it buy 6 volts? Just a shot in the dark. That way you are doing nothing to the signal but giving it an offset.