USB 6008 differential measurement of floating source & bias resistors

[beanbag]

Hello,

For a while now I have been measuring floating signal sources like Linear Velocity Transducers (LVT) in differential mode by plugging straight into the differential inputs.  The sensors have a source impedance of usually 1k or less.  Recently I read that if you want to measure a floating source in differential mode, you should use bias resistors, as recommended in some Field Guide here.  But my understanding is that the resistors are only to help prevent input bias currents from pushing the common mode signals out of range.  The recommended values were 10-100k.  However, the 6008 already has input impedance of 140k, so I figure that is "close enough".  I haven't noticed any problems with the signals I measured, at least in the sense that it swings back and forth between 0V and doesn't max out.  What other kind of errors would I be seeing in my signals?

So in this case were the bias resistors really necessary? 

[johnsold · Accepted solution]

Because of the input circuits on the USB-6008 you do not need to use the bias resistors which are ofter required on other devices with much higher input impedances.  The image below is from page 16 of the User Guide and Specifications document.

Lynn

[beanbag]

Thank you.  Now that you showed the diagram, I agree that the resistors aren't necessary.  Funny how the user manual still shows the resistors and directs you to use the field guide.

[johnsold]

The field guide contains much useful information. The input circuitry of the USB600x devices is quite atypical. Perhaps the person who wrote the manual was not aware of the differences or their implications.

Lynn