When using the SourceMeter to measure only (no sourcing), why are the polarities of my current measurements negative?
The instrument always assumes that it is measuring a current from its internal source. In this case, a POSITIVE current will flow OUT of its terminals and a NEGATIVE current will flow IN to them. As a result of this convention, a Source Measure Unit (SMU) operating in the "measure current only" mode will measure polarities opposite from what is expected with a typical ammeter. A positive current flowing from your circuit into the SMU will be measured as a negative current and vice versa.
If you are controlling the SourceMeter with a PC, you have a couple of options to change the sign of the readings. You can read the data into the PC as is and use software to post-process the measured data and simply change the sign, or you can define a math function for the SourceMeter that will change the sign for you automatically. This latter approach has the advantage that the SourceMeter display will show the proper polarity.
The following commands can be used to set up a user-defined math function to change the sign of the measured current (refer to the SourceMeter User's Manual for additional information on user-defined math functions):
:CALCulate:MATH:UNITs 'A '
Note that the defined "units" can consist of up to three characters. If less than three characters are used, then trailing spaces must be included. In the example above, the A is followed by 2 spaces. Also note that you must use the "CALCulate:DATA?" query to retrieve data processed through the math function. A "READ?" or "FETCH?" query will return raw data without the sign correction.