# Multifunction DAQ

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## 6008 floating voltage solution

Hi there are many questions regarding the analogue input volatages of the usb 6008 floating to 1.4 volts which makes sense due to the electronics.

What ways have people come up with to get round this?

My circuit is shown in the attached Jpeg. I effectivly have a voltage divider going from 0-200 volts down to 0-5V using resistor values of 470K and 12K. One solution would be to drop the resistor values down so it pulls down the daq resistor network as well. I would like to avoid droping the values down too much as I want to minimise the current being drawn by the circuit. I am currently seeing about 100mV on my input.

It would be good to have a few solutions in the following areas:

A) Hardware electronics

B) NI Hardware ie which DAQ would be better

C)Software solution (obviosuly there are limits but there might be a combined hardware/software solution)

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Accepted by topic author shavecp

## Re: 6008 floating voltage solution

A low output impedance buffer amplifier is probably the most straighforward solution and costs quite a bit less than buying another DAQ device.  I have not checked all the other devices, but I think only the USB-600x devices have this strange input circuit.

Since you know the circuit of your divider, it should be possible to calculate the effect and compensate in software.  If you attempt this, run some calibration checks.  On page 18 (last paragraph) of the USB-6008-9 User Guide it mentions the input circuit pulling to 1.4 V and adds: "

This behavior is normal and does not affect the measurement when a signal is connected."  This may suggest that the devices do some internal compensation under the assumption that the source impedance is zero.  If they do that, it will complicate any effort to compensate for the external source impedance.

Lynn

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## Re: 6008 floating voltage solution

Hi Lynn

I have just made a voltage follower\buffer on to the input of the DAQ. This has solved the problem. The down side is that I will have a dual rail supply for my application making it a little more complicated.

Cheers

Ceri

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## Re: 6008 floating voltage solution

Ceri,

I am glad you were able to get it working.

Often by the time you do things like figuring out why it does not work as expected and fix it by adding buffers, the cost advantage of the USB-600x devices is lost.

Lynn

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