PXIe-4162/4163 Outputting Unexpected Voltage When in Remote Sense

Issue Details

I am using a PXIe-4162/4163 in remote sense to output a voltage. When I read the voltage between HI and LO using a DMM it is very different then what the SMU is returning. What is happening?
 

Solution

There are a couple different scenarios that could be happening:
 

1. Both sense leads are left open
   1. Output programmed to >0V in CV mode or >0A in CC mode:  Voltage measurement from SMU will likely be near 0V.  Output will most likely drive to maximum positive voltage (~ +25V).
   2. Output programmed to <0V in CV mode or <0A in CC mode:  Voltage measurement from SMU will likely be near 0V.  Output will most likely drive to maximum negative voltage (~ -25V).
   3. Output programmed near 0V in CV mode or near 0A in CC mode:  Voltage measurement from SMU will likely be near 0V.  Output will be indeterministic and could drive anywhere between maximum and minimum voltage.
2. One sense lead is left floating
   1. The voltage can go anywhere between ±25 volts depending on impedance between the sense and it's corresponding force line.
3. A HI/LO line is left floating
   1. It will rail to ±25 volts depending on configuration

Provided are some suggested fixes:

• The recommended solution is to figure out why Force/Sense lines are not connecting properly to the DUT. Once you have found the problem, fix it and see if the SMU is now working properly. 
• If you do not need remote sense, configure the SMU into local sense. 
• If you are unable to resolve the connectivity issues of the sense leads and you require remote sense measurements, you can connect resistors between the HI and HI Sense, and LO and LO Sense lines (recommended values are between 100kΩ and 1MΩ). This will provide a weak local sense fail-safe path to drive the sense measurement if the Sense leads become open, and prevent an open control loop condition.
  • If you are running into scenario 3, adding a resistor may not help if the DUT impedance is too low.
  • As a note, this resistor is used as a fail-safe resistor to protect your DUT from potential high voltages. If connectivity issues are not resolved you risk getting back incorrect readings about your DUT. For example, if you output voltage and the Force leads are open, the SMU will return voltage and current readings that don’t correspond to your DUT.

Additional Information

When in remote sense the SMU takes the measurement from the HI Sense and LO Sense lines, rather than the HI and LO lines. So if your HI Sense and LO Sense lines are not properly connected to your DUT, such as an open/short, it will not read the proper voltage. As stated in the DC Power Help , "Configuring a channel for remote sense operation without connecting the sense leads to the DUT can result in measurements that do not meet the published specifications. If a channel is configured for remote sense and the remote sense leads are left open, the channel may source a voltage higher than the voltage level or voltage limit."

Many of National Instruments SMUs have internal resistors between HI and HI Sense, and LO and LO Sense. This is to help prevent the SMU from damaging a DUT if the sense leads do become disconnected. The PXIe-4162/4163 SMU differ from other SMUs in architecture. The 416x have a dedicated analog to digital converter per force and sense pin. This allows the SMU to have high impedance between pins thus Force and Sense do not have a sense resistor as both are being measured continuously. This reduces the error caused by the auto sense resistor that other SMUs have between sense and force but now the user must ensure Force and Sense are connected.