Increasing Input Impedance

[geo2342]

I am using a 6221 and I have discovered that powered down the impedance drops dramatically. The manual simply states that it is 10 Giga ohms.

Also I have seen a post here stating that the impedance is not 10 Giga ohms when used in the differential mode.

I need at LEAST 1 Mega ohm of impedance to prevent signal loss on a critical machine. I have learnt the hard way that this is not the case.

My questions are as follows:

1) Is this true that in differential mode the impedance is lower?

2) What is the best for me to ensure at LEAST 1 Mega ohm of impedance at all times (powered on , powered off)

[Sean_C.]

Hello,

Thank you for contacting National Instruments.

The input impedance should not change with respect to the input mode. Where did you see that posted?

As far as increasing the impedance while the device is powered down, you could simply tie all of your negative terminals to ground with 1 MOhm resistors. This would greatly affect your settling time.
Alternatively, you could tie your inputs to the device through a set of relays which close when the board is on and open when the board is off. You can use the 5V line on the board to power these relays.

I hope this helps,

Sean C.
Applications Engineer
National Instruments

[geo2342]

Sean,

Thanks for the help. I am working on the relay solution with our electrician and I think this will work great. As far as the input impedance in Differential mode this is the post title "input impedance in differential mode".

I may have been confused as it may be due to the bias resistors that this fellow was using. I was interpreting it that in differential mode the impedance was lower.


George