connect smu output lo to ground

[nickstorton]

Is there any reason why I cannot connect the output lo output from a PXIe-4140 SMU to a ground plane on my test board which is ultimately connected to chassis ground?

Thanks,

Nick

[hicksc]

Hi Nick

It should be fine, but I'm not sure that the measurement would be exactly what you were expecting due to the loop between Output Lo and Output Hi also being a loop to ground.

Why do you want to do this?

Kind regards

[nickstorton]

Hi Chris,

Thanks for the reply. I'm going to be testing a new ASIC and will be using the SMU to power the chip and monitor the current consumtion etc. in different modes of operation. I will also be using an M-Series DAQ card to measure some Analogue functionality of the chip at the same time. I'm just trying to be sure that if I connect the AI GND from the M_Series to the SMU Lo output so that all references are equal I won't get any standing currents flowing from the SMU through the DAQ card to chassis ground, as this will affect my measurements.

Thanks,

Nick

[hicksc]

Hi Nick

I don't think we could be certain of that, can't you just have common grounds between the two but keep Output Lo and Output Hi floating relative to ground?

Kind regards

[nickstorton]

Hi Chris,

Thanks for the reply. The issue is that I will be using the SMU to power the chip and the chip is designed to be powered from a battery so truly floating. So I can connect the chip to the SMU in a floating connection no problem. Now I need to measure one of the analogue outputs from the chip and I'm using an M-Series DAQ card. The chip output is referenced to the Lo output from the SMU so I have to measure the output w.r.t. the SMU Lo connection but the AI_GND which is the reference for the DAQ AI is connected to chassis ground on board so this will now ground the SMU Lo connection and the system is no longer floating. I guess what I need to know is this:- could this connection result in a standing current flowing in this ground path or not? If there is a standing current then I might need to look at adding isolation, but I don't want to do this unless it is absolutely necessary.

Thanks again,

Nick

[hicksc]

Hi Nick

So I've taken a look at your message with two colleagues and our consensus is that there should not be any standing current flowing to ground.

Which DAQ card are you using?

Could you use AI SENSE instead of AI GND?

Kind regards

[nickstorton]

Hi Chris,

That might be a possibility but I still have the problem that the chip is programmed using the DIO of the M_Series and the user manual states that AI_GND, AO_GND and D_GND are all connected together and to chassis so I will still have the path. I'm beginning to think that the only way around this is to give it a go and see if isolation is required.

Thanks again,

Nick

[Jeffrey_Zola]

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@hicksc wrote:

Hi Nick

I don't think we could be certain of that, can't you just have common grounds between the two but keep Output Lo and Output Hi floating relative to ground?

Kind regards

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Are the LO sides of the four channels connected to one another on the 4140 like the 4141?  Are you using multiple channels in your setup?

I use a 4141 as a zero-burden ammeter in a multi-channel application and need to be sure to only have one channel "live" at a time because the LO terminals are all interconnected. I've been burned a few times when I've forgotten this "minor" detail. Wasted a few hours and pulled out a few hairs...... 

Jeff

[nickstorton]

Hi Jeff,

Yes I will be using multiple channels as the DUT requires reference supplies (voltage & current) as well as power supplies, and yes the 4140 has the same architecture as the 4141 so all LO channels are connected together. I would have preferred a truly independent and floating SMU, as I'm sure you would as well. I may well be compounding the issue but it looks like I might be using a 4141 as well as the 4140 in this system because it has better DC performance than a DSA.

BR,

Nick