Unpowered input impedance

4 posts / 0 new

Mon, 01/11/2016 - 09:36

ChrisLewins

Hello!

I've just been playing with our shiny new LabJack U6 and found that when the LabJack is unpowered, the input impedance of the analogue inputs appears to drop into the kohms range. Is this typical?

Thanks!

Mon, 01/11/2016 - 10:16

LabJack Support

The overvoltage protection on

The overvoltage protection on the analog inputs consists of a 2200 ohm series resistor and then schottky diodes to the Vm+/Vm- rails. When the U6 is powered Vm+/Vm- sit around +/-13 volts, so the inputs are very high impedance until the signal goes beyond these levels. When the U6 is not powered any driven signal will immediately be greater than Vm+/Vm- and so the inputs will act lower impedance as they will absorb some current.

Here I measured the milliamps going in to AIN0 as I used a power supply to drive a voltage into AIN0 on an unpowered U6. I also measured Vm+ at the same time.

V(AIN0)    mA(AIN0)    Vm+
1.0        0.2        0.05
2.5        0.7        0.7
5.0        1.7        1.0
7.5        2.8        1.1
10        4.0        1.3

This is fine as far as the U6 is concerned.

Mon, 01/11/2016 - 14:50

ChrisLewins

Thanks - that's consistent

Thanks - that's consistent with what I was measuring.

I wonder if this could be made clear in the documentation as it's important to know for us. We're monitoring process sensors - if we lose power, the sensor voltages read by the process controllers are wrong, so the process will fail!

Thanks again - Chris

Mon, 01/11/2016 - 16:21

LabJack Support

Yes, I would say this does

Yes, I would say this does have a place in the U6/T7 documentation and we will add it. This type of behavior is normal for a device with a nice amplifier input, rather than a resistive scaling input.

One quick thing to increase the unpowered input impedance would be adding a series resistor. Table 2 in the Settling Time App Note is a quick way to see how big of resistor you can use with autosettling for a particular Range and ResolutionIndex.

https://labjack.com/support/appnotes/SettlingTime

Another option would be taking the LJTick-InBuff and changing the R5/R6 resistors to a larger value such as 100k or 1M.

https://labjack.com/support/datasheets/accessories/ljtick-inbuff