09-23-2009 07:57 PM
09-24-2009 09:15 AM - edited 09-24-2009 09:17 AM
There is resistive biasing on the inputs that can raise the measured voltage of a signal.
This is especially noticeable with higher output resistance signals.
This is explained in the manual, but the summary is that the design uses resistors to bias the open-circuit voltage on the 6009 up to around the middle of the conversion range so that differential signals can be measured.
Try using differential inputs, with the + input connected to your AD595 output and the - input connected to the AD595 ground.
09-24-2009 04:30 PM
"Try using differential inputs, with the + input connected to your AD595 output and the - input connected to the AD595 ground. "
I have attempted this, but it did not result in any change. I do not understand why there is a lower cutoff limit of 330mV. I have used a lighter to heat the thermocouple probe and it responded just fine. Is it a grounding issue? I have searched these forums and saw mentioning of this. Any other suggestions would be most helpful, thank you.
09-24-2009 08:44 PM
If you could show a schematic of how you've got this hooked up, it would help.
Actually, a schematic of the whole thing would help.
If you have this on a breadboard, you could hook the power supply inputs to your circuit to the 5V and ground terminals on the 6009 (I believe that the supply should be able to source a couple of mA easily).
Grounding shouldn't be an issue if you have no external power supplies hooked up, etc.; however, when you have multiple systems, it can be an issue. Which is one of the nice things about a differential connection -- it can help if you have small amounts of ground difference.
Have you done a "sanity check" and just tied the inverting and non-inverting inputs of your 6009 together and then read the input (remember that you have to re-configure the 6009 to do differential input)?
03-01-2010 11:19 PM
03-02-2010 09:22 AM
03-02-2010 09:27 AM
As I suggested earlier:
Have you done a "sanity check" and just tied the inverting and non-inverting inputs of your 6009 together and then read the input (remember that you have to re-configure the 6009 to do differential input)?
03-23-2010 11:31 PM - edited 03-23-2010 11:39 PM
Sorry for the late reply, but this issue has now surfaced again with me doing the same testing.
But yes I have tied the two inputs together and get about a 1 mV reading, so that does not seem to be an issue. I'm almost running out of ideas and I swear my lab partner and I was using this same setup just the other day and got the correct results. Trying to figure out what could be wrong.
03-24-2010 06:57 AM - edited 03-24-2010 07:01 AM
Could you post your schematic and maybe a picture of your setup?
I'm out of ideas, frankly.
Have you put an oscilloscope on your USB-6009 inputs? Maybe you have oscillation. Have you bypassed the power supply pin 11 to ground with a capacitor? Though the example circuits in the data sheet don't show capacitors, I'd certainly add one.
I'd recommend something like the AD595 data sheet's basic connection circuit (Figure 1) (shown here), with the addition of a 0.1μF capacitor between pin 11 (the power supply input) and pin 7 and 4 (ground).
Your USB-6009 should be connected differentially:
If you duplicate the schematic given in the AD595 data sheet as Figure 13 (shown below) (connect pin 7 to ground) you will have a simple thermometer (this circuit replaces the thermocouple with a short circuit). Does this work? If you heat the chip, do you see the output increase?
03-24-2010 05:44 PM