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1 What is the difference between both functions (REL & ZCOR) in Keithley 8465 ? Manual does not make it clear
6487-900-01 Rev. C / March 2011
Step 3. Perform zero correction
To achieve optimum accuracy for low current measurements, it is recommended that you
zero correct the -. To do so, make sure the 2nA range is selected, then press
the ZCOR key (Model 6485) or REL key (Model 6487) to perform zero correction
(“ZZ” indicated for Model 6485; MON indicator on for Model 6487).
but still I have REL button in 6485 panel
2. If I run 6485 manually ( using buttons in panel) how ZCOR (or REL) does work?
6487-900-01 Rev. C / March 2011 . 3-9
Programming example — current measurements
The following command sequence will perform one zero-corrected current measurement:
*RST ' Return 6485/6487 to GPIB defaults.
SYST:ZCH ON ' Enable zero check.
RANG 2e-9 ' Select the 2nA range.
INIT ' Trigger reading to be used as zero' correction.
SYST:ZCOR:ACQ ' Use last reading taken as zero' correct value.
SYST:ZCOR ON ' Perform zero correction.
RANG:AUTO ON ' Enable auto range.
SYST:ZCH OFF ' Disable zero check.
READ? ' Trigger and return one reading.
above soft example shows that is not so simple to perform it manually. I mean should I press buttons to perform
SYST:ZCOR:ACQ ' Use last reading taken as zero' correct value. ?
- Keithley Applications
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Suppose a photodetector is the device from which you want to measure the current vs. time (in response to light).
The PD might have some level of "dark current" that is not a response to incident light. The REL feature could be used to store this offset value and subtract it from readings. This way, if you are doing area under the current vs. time curve, the cumulative dark current does not contribute.
The zero correct (ZCOR) is for minimizing a different source of error.
A perfect amp meter would be invisible to the measuring circuit/device. But real amp meters are not invisible and they will develop some delta V for the current that flows through them. This delta V is called the voltage burden.
In the case of the PD, the delta V could affect how much current flows.
Zero Correct is an internal adjustment to try to minimize voltage burden.
For the uA and lower ranges on 6485, voltage burden will be 200uV or less if you use the ZCOR feature.
A SMU forcing 0V and measuring current is another way to have a very low voltage burden amp meter.
In contrast, a good quality multimeter current measure range might have voltage burden of several mV because the DMM uses resistive shunt.
Here is diagram of the 6485:
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