USB-1808x analog input stuck at +10V or -10V

[Francois Ouellette]

Here we go again, I had a similar problem last year, thought I'd solved it, but now it has suddenly come back.

I connect the analog input (Ch 0, but it doesn't matter which one) USB-1808X to a photodetector. All of a sudden the voltage rises to +10V. OK, I disconnect, short it, voltage goes to 0 as it should. Open the circuit, the voltage starts to go negative, until it hits -10V. 

Test it with Instacal: same results. And all channels go crazy like this. Either all 10V or all -10V. 

On the other hand, I tried another V source, worked perfectly. Also if I connect the analog output to the input, generate a sine wave, works like a charm. 

I've used that photodetector like this for more than one year with the USB-1808X without problem. Changed the battery in case. If I test it with a voltmeter, I get a normal behavior. 

What's going on? 

Thanks for your help! 

(Context: I use this in a feedback loop to stabilize a laser beam pointing, with a piezo on a mirror. Generate a sine wave in channel 0, measure the beam through a slit, use it as error signal in a PID using Labview). 

[Fausto]

Hello @Francois Ouellette.

If the signal to the analog input channel is removed or the connection appears open, then the voltage reading will float.  For each enabled analog input channel, you must provide a signal or else short the channel to ground.  From your post, it appears the USB-1808X functions correctly when the analog input channel is either grounded or connected with a valid voltage signal.

Check the cable connection from the photodetector to the USB-1808X as well as a different cable.

Regards,

Fausto

[Fausto]

The USB-1808X device has an analog input impedance >1 GΩ min.  Typical signal connections to our daq devices have an impedance <1000 Ω.

For some customer applications, the input voltage to a data acquisition device comes from a high impedance source.  In this situation, it is necessary to introduce an operational amplifier (op amp) between the voltage source and the daq device's voltage input channel.  Was an op amp used as a solution last year?

Regards,

Fausto