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Submitted by chriswilliams186 on Wed, 03/08/2017 - 08:23

4045

We (are/were) planning to use the inputs for a 'follower' application using a precision 10k potentiometer but we have excessively big offsets on all five inputs

a) when floating - 2.568 volts indicated for all five channels in Galil Tools - Watch -( identical within 'noise effects)
b) with 18 K across terminals indicates about 1.2 volts
c) with 5 k ohm about 0.3 volts
d) in fact value is near linear with resistance across the terminal

Is this acceptable - how it is' or have we got a problem with these channels

Can offset be adjusted by opening the box (offset pot??)

What is likely drift in offset

We Note (belatedly) that the data sheet gives the input impedance as only about 30 k ohms - this seems far too low for a voltage input - would have expected more like a meg ohm

I notice that Galil tools has an offset column - how is that being implemented - is it just for display or is it sent to the DMC somehow

Comments - advice please

Comments 1

AndrewS on 03/08/2017 - 15:25

Hi chriswilliams186,
The answer to your questions is split into hardware and software:

Hardware: yes, you will see that the analog inputs float. To properly use the analog inputs for a potentiometer, you'll need to use a reference voltage, such as the +/-12V reference that is provided on the same D-sub connector. For a 10k pot, wire it in series with 2 1k resistors, one on each side of the pot. Use the +/-12V, and you'll see that with the pot turned to one side, you'll read 10V, and then turn the pot to the other side, you'll see -10V.

Software: The watch window in GalilTools uses the data record. The data record returns the ADC reading represented as a 16bit value. GalilTools allows you to scale and offset that value as needed. This scaling and offset is not sent to the controller.

I am curious to know more about your application and would like to help further. Can you send an email to support@galil.com and ask it to be forwarded to Andrew.

Regards,
Andrew