USB6008 voltage error

[dangrenade]

Hi

I'm currently on my way through my uni coursework and i'm having some issues with the USB6008.

I have a breadboard with an AD595 that I'm using with a type K thermocouple.

I'm wired up to AI0 on the USB6008.

With the USB cable disconnected from my PC, and measuring the AD595 output with a DMM i'm getting sensible readings.

As soon as i plug in the USB cable a voltage of ~320mV is present on the output. If I measure temperature above 33 Deg C the USB6008 the voltage is fine, however I can't measure below 32 DegC due to this minimum voltage always being present.

I'm using differential mode to measure the voltage.

It's definately something to do with the interaction between the voltage supplied via the USB cable, and the characteristics of the USB6008.

PS the AD595 is being powered by a 9V battery, this device is working correctly as the correct output voltage is present as soon as the USB cable is disconnected.

Has anyone experienced similar problems, and if so how did you get around them?

I appreciate any help.

Kind Regards

Danny

[johnsold]

Danny,

I have not done a complete analysis, but I think I know where the problem is. I am assuming that you have the AD595 V- and battery negative connected to the USB-6008 GND.  If not, that is certainly part of your problem.

Two things combine to create a situation where an error such as you are seeing might occur. First is the input circuit of the USB-6008 analog inputs.  It is shown in Figure 7 on page 16 of the User Guide and Specifications. The input impedance is 144 kohms and it has an equivalent voltage source of about 1.4 V behind that impedance.

Second is a footnote (4) on page 2 of the AD595 data sheet: "Current Sink Capability in single supply configuration is limited to current drawn to ground through a 50 kohm resistor at  output voltages below 2.5 V."

I think that the combination of these two conditions may produce the effect you are seeing. The solution would be to buffer the output of the AD595 with a unity gain op amp which can drive the 6008 input circuit.

Lynn