When using the SMU2450 as a ammeter, do I need to the chassis connection to the source meter?

[Gotleyb]

Hello,

I'm using an external source meter to supply voltage to my device. I'm measureing the current (Very low currents, below 10nA and sometimes even lower) with the SMU 2450. The SMU is at 0V and it only used to measure the carrent that goes through it. It is connected in series between the Plus terminal of the voltage source and the cathode of the device. My quastion is, do I need to connect the chassis connection of the 2450 (Green/Ground terminal) to the GND of the source meter and if so why?

I see differences in measurements between the two configurations but not sure which is the correct one.
I'll also mention that sometimes when the chassis connection is not connected to the GND I see OVERFLOW in the SMU245 eventhough the currents are very small and below the limit/range that I defined.

Thanks

[Andrea C]

You are describing the 2450 is behaving like amp meter.
It must be inserted into your circuit.
You have it riding on your power supply voltage.

In this case, you need the 2450 to be a floating instrument with HI and LO terminals.

The chassis ground should not be connected to the LO.
If you do this, it would no longer be floating and current runs to GND.

Try to give us a sketch of your connections. Did you break a connection and insert the “amp meter”? Or are you connecting across like a voltmeter?

Question: since the SMU can source V and measure I, why the power supply and floating the SMU? Why not just use the SMU to do both?

[Gotleyb]

Thank you for your replay. My device has several contacts, some of them need the same voltage wherase I need to prob the current at only one contact, has you said I broke a connection and added it has amp meter, attached is a simple connectivity scheme.

Shouldn't I connect the GND (the chassis in the SMU2450) of both instruments (Red line in the attached scheme) so that they'll be at the same reference voltage?
When the GNDs are not connected to each other the SMU sometimes blinks the OVERFLOW eventough I set the current limmit to 100nA which is way above the actual current (several nA and below). When I increase the limit to 10-100uA it suttles but the resolution is not enoght for a meaningful current

[Gotleyb]

The scheme attached

[Andrea C]

For the chassis ground of the external power supply and the 2450, by plugging them into the same AC power circuit, they will have shared chassis ground through the power cord.

The 2450 Reference Manual has a few diagrams on floating the SMU. Here is one of them:

floating 2450 on external voltage source

2450_floating.PNG (40.74 KiB) Viewed 48 times

In your sketch, you are showing that the current that leaves the + terminal of power supply will be divided. Some will go to C1 and some will take parallel path through the floating 2450 and to C2. Is that what you intend?
Also from your sketch, the red line for chassis connection is likely already achieved via the power line cords. You could use a simple hand held battery powered DMM and measure the ohms between the two chassis.

[Gotleyb]

Hi Andrea,

The connectivity you described is correct.
There is something I'm still missing. If the chassis of the two instruments are already connected trough the AC power chord, why do I get different behavior when connecting the two chassis (the red line in scheme), and in what cases these the chassis is important?
Maybe I'm having a ground closed loop?

When the chassis are connected to each other the measurement seems more stable, but in the other hand it could be a false reading?

Thank you very much for your help,
Dor

[Andrea C]

Curious for how much voltage is the 2450 sitting on? If more than 250V, you need to stop as that will violate the common mode voltage spec.

The isolation of our LO terminal to the chassis ground is > 1GΩ. So if LO is sitting at 10V, you could have as much as 10nA of common mode current (leakage to chassis).

As a quick test: remove your DUT from the situation.
Get a resistor (1MΩ or so) and use that to simulate the C2 to C3 pathway of your DUT.
Is the floating 2450 able to report the expected current for the value of the voltage source and the resistor?
Is the value affected by the presence or not of the extra/redundant chassis ground wire?

When testing with your actual DUT, does the 2450 display any limit indicators? Are you encountering the current limit?
I think you indicated needing to set the current range higher than you originally expected to need.
Both your C1 to C3 pathway and the C2 to C3 pathway need to have enough R to drop the value of the applied voltage source.
I suspect one of the pathways does not have enough R for the resulting I*R drop to equal the applied V from your source and the 2450 is likely going into current limiting and is developing a non-zero voltage potential.