Measuring Current with 1608HS DAQ

[sam56]

I have a small (< 100 uA) current from a photomultiplier tube that I would like to measure with my 1608HS DAQ. I have previously successfully measured this current with a 1616HS DAQ, but I purchased the 1608HS because I wanted a DAQ with simultaneous acquisition. When I try using a load resistor (see picture) to convert the current to a voltage, the reading I get floats around and eventually steadies at -5 Volts. AGND is attached to ground and I measured 0 voltage between ground and the PMT current source prior to entering any other circuit. My load resistor is 100 kohms.

 

The code I use for getting the voltage reading is ul.to_eng_units(board, ai_range, ul.a_in(board_number, input_channel, ai_range))

The circuit diagram is also attached. Basically the same circuit/code did work for the 1616HS DAQ, there just seem to be issues with my new 1608HS one. Any help would be appreciated.

[JRys]

Hello,

Please run the InstaCal program, double-click on the device, and let me know the channel's input mode (differential or single-ended). Could you also show how the channel is connected to your circuit?

Best regards,
John

[sam56]

2 hours ago, JRys said:

Hello,

Please run the InstaCal program, double-click on the device, and let me know the channel's input mode (differential or single-ended). Could you also show how the channel is connected to your circuit?

Best regards,
John

I've tried both single-ended and differential and a number of different combinations and none of them have given me good results.

 

For single-ended, I've tried:

• Connecting AGND to ground (the same ground the PMT is connected to) and ch0_H where V_out in my diagram is
• Connecting AGND in parallel to my current source (and before the resistor) and connecting ch0_H where V_out in my diagram is

For differential, I've tried:

• Connecting ch0_L (and AGND) to ground and ch0_H where V_out in my diagram is
• Connecting ch0_L in parallel to my current source and connecting ch0_H where V_out in my diagram is. In this scenario, I do actually get a stable result but it barely changes in response to different current inputs. I also get the same result when both ch0_H and ch0_L are connected to where V_out in my diagram is which makes me doubt whether the resistor is doing anything

I have also tried a couple of different circuits for measuring current (attached below). I've tried several different capacitance and resistance values, but the ones in the image are the ones that seemed to work the best. Circuit A was too noisy to get a good reading, but Circuit B was functional. The problem with Circuit B is that it still gave a much lower voltage value than expected for the level of current and changing the resistance value did not make much of a difference. Measurements were made with differential mode.

[JRys]

Hello,

I want to ensure the USB-1608HS is working, so could you test channel 0 using a 1.5v battery? Set the channel to single-ended mode and connect the battery across CH0_H and AGND. Do a second test with the channel in differential mode and the battery connected to CH0_H and CH0_L. If the measurement is inaccurate or drifts, you may need to connect a 100k reference resistor from CH0_L to AGND.  If you have other connections to the device, please remove them.

Assuming the battery test went well, set channel 0 to differential mode and connect your amplifier output to CH0_H and the amplifier ground to CH0_L. There could be a ground loop between the amplifier power supply and the USB-1608HS, and I hope the differential input removes it.

Best regards,
John

[sam56]

2 hours ago, JRys said:

Hello,

I want to ensure the USB-1608HS is working, so could you test channel 0 using a 1.5v battery? Set the channel to single-ended mode and connect the battery across CH0_H and AGND. Do a second test with the channel in differential mode and the battery connected to CH0_H and CH0_L. If the measurement is inaccurate or drifts, you may need to connect a 100k reference resistor from CH0_L to AGND.  If you have other connections to the device, please remove them.

Assuming the battery test went well, set channel 0 to differential mode and connect your amplifier output to CH0_H and the amplifier ground to CH0_L. There could be a ground loop between the amplifier power supply and the USB-1608HS, and I hope the differential input removes it.

Best regards,
John

The battery test worked. For the differential, I did need to connect the 100k reference resistor between CH0_L and AGND. 

The issue I was having with Circuit B remains. It's output still does not change very much in response to different current inputs.

It's also still unclear to me why simply using a load resistor for converting the current to a voltage does not work with this DAQ.

[JRys]

Hello,

This morning, I tested a USB-1608HS and USB-1616HS using a precision current source and a 100k ohm resistor connected across a differential input. Both units produced a noisy measurement. Averaging the data produced a value close to the correct value. For example, 50 uA produced a 4.94-volt value. Both devices had approximately 20 mV p-p noise. If I switched from current to voltage, removed the resistor, and applied 5 volts, I measured 5 with less than 2 mV of noise with the USB-1608HS and less than 3 mV with the USB-1616HS. Connecting a current source to the device inputs causes a disturbance I cannot explain. Using an op-amp to convert the current to a low-impedance voltage source is better. 

If you have a handheld voltmeter, connect it to your circuit A or B output. Measure the output with it disconnected from the device. The voltmeter will produce an averaged value, so it's no indication of noise, but it should show that your amplifier is converting the current to voltage. 

Best regards,

John