11-30-2012 04:49 PM
I would like to do something similar to what is discussed in the following thread using multiple channels of a PXI-4141 4-channel SMU:
I would like to understand the statement in the final post of that thread regarding burden voltage/shunt resistance. In my application, it is critical that I compensate for the presence of a shunt resistor when I take a current measurement. How do I determine what the effective shunt resistance or the effective burden voltage might be when using the current-measurement capabilities of the SMU?
11-30-2012 06:27 PM
Any burden voltage generated across the internal shunt resistor of the SMU will be cancelled out by the internal supply of the SMU, leaving your overall voltage drop essentially 0V. If you program your SMU voltage to 0V, the SMU will try to maintain this value across its terminals. The only voltage drop you may want to consider is the offset accuracy of the SMU which is spec'd at +/-150 uV. This is not a function of the voltage drop across the SMU shunt but rather just the measurement/output accuracy of the device itself.
As far as whether to disable the output vs programming 0 V, when the 4141 is in the disabled state, the SMU will be set to 0V (just like the 4130), but will be set to 2% of current range instead of 20 mA. See this KB for more information on that. Therefore, on the 4141, when deciding to use the 'output disabled' vs 'output enabled' at 0V, the 'output enabled' case will allow customizable control of the current level whereas 'output disabled' will always be 2% of range. In either case, I suggest that you pick the lowest current range possible for your measurement so that you have the best resolution and accuracy possible.
Can you explain how your application involves "multiple channels"? Are these channels connected together in some way, or do you have multiple channels that are independent of each other?
11-30-2012 07:30 PM
I am currently using a setup two paired PXI-4071 DMMs and PXI switch modules (in matrix mode) for external current measurements & an additional pair for MUX'ed voltage measurements to verify that internal meters on my equipment are performing as desired during acceptance & performance testing. The V-I results that are logged during execution of my LabVIEW program are compared with both setpoints and the results logged by the internal voltmeters and ammeters.
The current measurements need to be performed using a matrix switch because the test equipment requires a continuous current path through each DUT, or it will call that DUT "dead" and discontinue testing it. It will also call a DUT "dead" if a shift in resistance above a certain threshold is detected. I have taken care, therefore to design an external resistor that "looks like" the internal shunt resistor of the DMM. The matrix switches this resistor in and out of the continuous current path depending on whether the DMM is taking a measurement or not. Because the equipment was not designed for this type of external monitoring, triggering is not a feasible option for synchronizing external and internal measurements, so the external "shunt" resistor becomes critical to ensure that the populations of the current measurements can be considered comparable.
The number of sources I can monitor during any given data acquisition "run" is limited by the size of the matrix accommodated by the switch modules. I am presently using PXI-2530's with 2632 terminal blocks that are configured for an 8X16 matrix. I use one row for each DUTs current return path from the DMM, and two columns for each DUTs current supply path -one with the external "shunt" resistor and one without. When current is not being measured, the crosspoint corresponding to the DUTs return row and its shunt supply column is closed. To measure current, crosspoints to connect a dedicated "read" row to the non-shunted current supply path and the DMM+ terminal (which takes up one column) and the DUT's return row is connected to the DMM- terminal (which takes up another column). In this way, the 8X16 matrix can measure current from 7 sources, for a total of 14.
I am looking at ways of increasing the number of sources I can monitor during a "run," but want to reduce the cost if possible (who doesn't?). When I originally posted the question, I was looking for ways to use three matrix modules to switch current and still measure voltage. But I only have three PXI DMMs. I recalled having seen the thread I linked to, so sought clarification. What I learned is very interesting, as I potentially would not have to use two columns for every row, which opens up my options to have a square matrix and FURTHER increase the number of sources I can test should we opt to purchase a Switch Block with the 16X22 cards.
I would replace each DMM with a channel of the 4-channel SMU. That's a very long-winded explanation how "multiple channels" come into play.
11-30-2012 09:55 PM
Note that the four 4141 channels share a common LO terminal... I don't know if this is an issue in your setup, but now you know :).
As Brandon mentioned, the voltage across the 4141 will be close to 0; at peak current - 100mA - the 4141 develops less than 5mV burden voltage when set to 0V... the drop across your switch matrix is likely higher, depending on your test current.
You mention that your DUT must maintain a particular resistance, and that you've created a dummy resistance to mimic the DMM's shunt resistor... I'm having a hard time visualizing how you've routed these connections through your matrix... can you give us a summary diagram of maybe two of your DUTs and how they interact with the matrix? If all the DUTs share a common ground or power tap, then we might be able to get rid of the matrix and use a mux or two instead (by connecting multiple relays at once). Without a diagram, I trust your judgment using a matrix, but I'm always looking to optimize :).
12-01-2012 06:38 PM
Hi, John. Not sure i understand what you mean by the statement that the four channels share a common LO terminal. The pinout includes separate "LO" and "LO S" connections for the low-side of each channel's force and sense. Are you saying that these are connected internally to the module somehow?
My current levels are going to be in the vicinity of one microamp. Are you able to guestimate the burden voltage that might result? I do plan to compare using the 4071 and the 4141 modules to measure currents early next week.
As I am using reed switches, there is some thermal EMF generated. When I worked out the polarities of the various switches that are actuated in 2 wire multiplexer mode, I identified that the thermal EMF canceled out for all but one switch. I should repeat this for a matrix configuration.
As I am on my home computer right now, I do not have access to some shematics of my setup. But I can post a few when I get back to work next week. I will aim to capture images of the Soft Front Panel with my external shunt resistor connected and with the ammeter connected.
When I developed the scheme I am using, I did a lot of brainstorming to identify the simplest way to meet the requirements. I explored the use of a SPDT MUX- but I determined that I would need one SPDT MUX to maintain a continuous current path for each DUT and a SPST MUX to multiplex the ammeter to the SPDT MUX channels. In my application, I am measuring the current supplied to independent test sockets and the voltage across them. And the test setup needs to operate unbeknownst to the system to which it is connected. I would love to see alternatative approaches!
12-03-2012 11:16 AM
Assuming nobody lost any sleep since Saturday because of all of the suspense..... Here are the images!!!
Wiring goes like this:
r0: No connection
rX: Channel X current return to DUT circuit.
c0: DVM- terminal
c1: DVM+ terminal
c(2X): Channel X current supply from DUT circuit
c(2X+1): Channel X current supply from DUT circuit thru external shunt resistor
I've shown the "bypass" switching configuration I use and the switching configuration for measuring current on the first two channels. The others follow the same pattern moving across/down the matrix.