usb 6008 current measurement

[-SK-]

Hello, I hope someone can offer some insight here.

I am using the USB-6008 to measure a bunch of analog data, one part of which is a current. I measure it via a sensor that I concocted, shown in the attached (poorly drawn in paint) circuit diagram. I don't understand this one:

When the sensor output is measured with a voltmeter, it works normally, giving ~0.7V for ~20mA and ~10.7V for ~500mA. When I connect the output of the sensor circuit to the DAQ, it jumps up to 11.8V and sits there no matter what current is being measured. Another odd thing is that when I disconnected the circuit from the DAQ, the circuit output remains at 11.8V even if I cycle the power. To get it back to a working state, I have been de- and re-soldering the transistor.

I have tried different analog inputs on the 6008 but they have the same effect.

When I built this circuit on a breadboard, it worked perfectly even when connected to the 6008. I just assembled the same circuit, with the same parts, on a new pcb, and now I see this behavior. Any ideas?? Thanks.

[DavidMN]

Hi -SK-,

This sounds like a grounding issue.  What is are the ground reference differences between the breadboard and PCB setup?

[-SK-]

On the breadboard, all ground comes off a common rail that is connected to a power supply ground. One of the USB 6008 ground terminals is connected to this rail as well. In the current sensor circuit specifically, the op amp negative supply and the emitter resistor are connected to this ground rail.

On the pcb, a ground plane provides this same function. One of the pins in the wiring harness connected to the USB 6008 is connected to this plane, as are the emitter resistor and op amp negative supply pins.

[Doug_L._Bear]

The 6008 can only handle currents of 1 mA.  Testing such large currents could be doing this.

http://digital.ni.com/public.nsf/allkb/7D8EDCD0C3887EEA86257075004CE733?OpenDocument

[-SK-]

I have tried disconnecting all other analog inputs, and further limiting the current for this input, but this does not correct the problem. Also, I should think that I would see this problem on my breadboarded circuit too, since the circuits are the same.

[AnalogKid2DigitalMan]

Have you verified the traces and ground plane are all routed correctly on a bare PCB by using an ohmmeter?

Any inadvertent through hole vias tied to the ground plane that should not be?

-AK2DM

[-SK-]

AK2DM, yes I have checked the traces, vias, and ground plane. Everything is connected where it should be.

[timoteo]

hello  Doug,

I must discord of your intervention because i use the USB 6008 to measure a current from a pressure traducer that is from 4-20mA and work for me...

SK maibe the problem is the oposite, sometimes in the breadboard the pins are not well connect and that can make your circuit to work well and you need a flowting ground.

Tim

Message Edited by timoteo on 05-10-2009 06:09 AM

[Mohammed.Ashraf]

timoteo

how come you are using the 6008 to measure current , although it's measurement type is Voltage not current ?

how do you convert the 4-20mA to 0-10 V ??? 

[AnalogKid2DigitalMan]

Mohammed:

Put a precision 250 ohm resistor across the analog input terminals. The current will generate a voltage by Ohms law (E=I*R).

4mA * 250 ohms = 1V, 20mA * 250 ohms = 5V.

-AK2DM