Reading the output voltage/current of a test equipment affects the measurement

[wedjati]

Hi all,

I am trying to perform some electrochemical measurements, and while the measurement details are a bit technical I think I'm having some fundamental problem with my measurement setup, which is why I'm posting this on the beginners board.

I am trying to read the voltage and current values of a somewhat old piece of equipment (Solartron Instruments 1287 electrochemical interface). The workstation came with a software that communicates with the instrument through GPIB, however the buffer of the machine is so slow it is causing problems with some measurements. To overcome this, I thought I could use the voltage and current output terminals and read their values using DMMs and achieve a much faster sampling rate. The machine has two BNC outputs, one for voltage and one for current (+/-200 mV for full range). The machine has separate connectors for connecting the sample, I am trying to use separate ones that I assume are for direct monitoring of the voltage and current. Reading the voltage seems to be fine, but reading the current sometimes affects the measurements. I'm using a Keithley DMM (7510) and an SMU (2450) to read the outputs. Overkill I'm sure, but our lab had them lying around...    I am trying to do two types of measurements, and both have problems:

1) Cyclic voltammetry (use a linear ramp sweep, e.g 100 mV/s between 0V->1V->0V for x number of cycles).
Connecting the current output leads to the DMM leads to a shift in the current output voltage.

2) Charge-discharge measurement (apply a constant current until the voltage reaches a specified limit, then reverse the current and repeat).
Connecting the current output leads to the DMM also leads to a shift in the current output voltage, but also seems to affect the actual current applied to the sample.

The strange part is that I can't always reproduce this problem. And even when it is happening, if I switch the DMM to a basic battery-operated multimeter, the problem goes away. Switching between the DMM and SMU or changing cables does not help. The cable is a BNC to dual banana jack type. I am more of a software kind of guy and I'm completely at lost of what the problem is. Any help would be greatly appreciated!

Some thoughts I had
- Something to do with the input impedance? Although why aren't the results consistent?
- Dirty contacts? I tried cleaning them, but didn't see any change.

tl;dr Why would reading the output current (as a voltage relative to the current range) of a testing interface sometimes change the actual current applied to the device the interface is connected to?

[alm]

Stupid question, but you are connecting the current monitoring output to the voltage terminals of the DMM and set it to voltage mode, right? In the case of the SMU, you would configure it as voltmeter (e.g. source 0 nA current in the lowest current range, measure voltage). The SMU may provide a higher impedance, if that matters.

Looking at the block diagram in the user's guide, the volts and current outputs are buffered, so they should not affect the measurements.

What do you mean with connecting the current output leads leads to a shift in current output voltage? How do you measure the current output voltage without the DMM connected?

A bunch of things you could try and see what sticks :
- Are the volts and current monitoring outputs connected to anything except the voltmeter inputs? Any connection between the volts and current inputs? For example, is the shield of the BNC cable connected to earth anywhere (the instrument claims to be floating, so the shield may not be at earth potential)? Any kind of multiplexer between the outputs and the meter?
- Does switching the polarity of the voltmeter connections, i.e. connect BNC center to the negative terminal, and shield to the positive terminal, make a difference? The negative terminal is usually tied to guard on the DMM, and hence will have a larger capacitance to earth.
- Does reducing the control loop bandwidth make a difference? I could imagine that the capacitance of the DMM inputs to earth destabilizes the control loop somehow. I understand that this may interfere with the actual experiment, but it is just for troubleshooting purposes.
- Does enabling the low pass filter on the monitoring outputs make a difference?
- Does adding a say 100 kOhm resistor between the DMM and the electrochemical instrument for both the positive and negative terminals make a difference? Make sure the DMM is set to its high-impedance (> 10 GOhm mode), not 10 MOhm. This could help against any noise that the DMM might inject into the instrument.
- Does lowish-value resistor (.e.g. 10 kOhm) between the inputs near the voltmeter make a difference?

Do you have access to one of the frequency response analyzers these outputs were designed to work with? Then you could measure some properties like input impedance and if the shields are connected together or to earth?

[wedjati]

Stupid question, but you are connecting the current monitoring output to the voltage terminals of the DMM and set it to voltage mode, right? In the case of the SMU, you would configure it as voltmeter (e.g. source 0 nA current in the lowest current range, measure voltage)

Yes, that's exactly what I'm doing  Using the voltage and the known current range I can convert the voltage to the current.

What do you mean with connecting the current output leads leads to a shift in current output voltage? How do you measure the current output voltage without the DMM connected?

Sorry, I should have mentioned it, for testing this I used the attached software (CorrWare) to run the measurements and monitored the output both on the software and on the DMM/SMU at the same time. As for the shift in the current output voltage, I mean that the value shifts (maybe around 10 mV) when I connect the leads to the DMM. Unfortunately I don't have the actual data at hand, but this graph below shows/simulates approximately what happens.



I will try your suggestions, thanks a lot!

Do you have access to one of the frequency response analyzers these outputs were designed to work with? Then you could measure some properties like input impedance and if the shields are connected together or to earth?

I do, if I luck out with your other suggestions I will give this a try 

[alm]

Oh, and when using the SMU, make sure the low terminal is not connected to earth (I do not remember if this is the default on the Keithley 2450). Measuring resistance with a DMM from the force/sense lo to the earth terminal should give a very high resistance, if not open circuit.

[wedjati]

Measuring resistance with a DMM from the force/sense lo to the earth terminal should give a very high resistance, if not open circuit.

I will test that too, although according to the Keithley 2450's reference manual

There is no internal connection between protective earth (safety ground) and the LO terminals of the Model 2450