Issue with PXI 4132 SMU

[Brandon_G.]

I misspoke earlier and wanted to clarify...

If you don't have a scope available can you measure the AC voltage and DC voltage across the DUT with a DMM when this behavior occurs?

[phx_tech]

Hi Brandon,

  I am sorry about the late reply. I have attached all the details that you had asked for in the word document. The problem occurs only on this DUT . I have tested other Bi-directional devices but they read accurately on the 4132. This DUT which has a problem has a negative slope in the I-V characterstic in one direction. The other DUT's that i tested do not have the negative slope. 

[phx_tech]

Hi Brandon,

   Here is another update. I used the keithley 2611A to perform the same test across the DUT. I forced a 1mA across the DUT and read the current and voltage across it. Using the keithley I read 1mA across the device in both the directions.

[Brandon_G.]

Hey phx_tech,

The signal shown in Figure 1 looks pretty noisy but I wonder if you zoom in, if it's actually oscillating?  Can you zoom in both the time scale and the voltage scale?  If it is oscillating, there maybe some external circuitry we can add to stabilize the output.  Stability in traditional SMUs has always been a challenge because the DUT itself is in the control loop.  An SMU maybe optimized for stability under one type of load but then be less stable with another type-capacitive DUTs are notorious for causing instability.  This is why one SMU might work fine for a particular DUT and another causes instability.  If we can verify that the output is actually oscillating, then we can look at possible solutions for this issue.

Thanks!

[phx_tech]

Hi Brandon,

   Please find attached the information you requested. Do let me know if you have any suggestions to overcome this problem.

thank you.

[Brandon_G. · Accepted solution]

Hey Phx_tech,

Thank you for verifying the scope shot- it definitely clear now that there's a 1 V pk-pk oscillation when applying a negative current. One possible workaround that we might be able to try is to place a 100 ohm or 1 k resistor in series to hopefully slow down the loop and prevent the output from oscillating.  The sense leads can be use to sense around the resistor such that the SMU measures the voltage right across the DUT.  When doing this, be sure to enable remote sense in the SFP.  Below is a wiring diagram on how the connections should look.

If you intend to force higher currents through the SMU, you may have to scale back the resistance such that the high lead drop doesn't exceed 1 V.  I picked a resistance value of 100 ohms to 1 k so that the lead drop will be from 0.1 - 1 V at 1 mA, but if you intend to output higher currents, you may want to scale back your series resistance appropriately such that your high sense lead drop doesn't exceed 1V.  You can also add split up your series resistance on the Hi side and Lo side, such that neither lead exceeds 1 V of drop. 

Let me know what you find when you get a chance.

Thanks!

Brandon G

[phx_tech]

Hi Brandon,

   Thank you for all the help. When I connect a 500 ohm resistor in series with force hi lead the problem is eliminated. Appreciate your help.