Thanks, Brandon.
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.
Jeffrey Zola
