Keithley 6482, Справочное руководство

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6-4 Return to Section Topics 6482-901-01 Rev. A / August 2012
Section 6: Data Store Model 6482 Dual-Channel Picoammeter / Voltage Source Reference Manual
Standard deviation
This mode displays the standard deviation of buffered readings. The following equation is used to
calculate standard deviation:
Where: y = The average.
X
i
= A stored reading.
n = The number of stored readings.
Timestamp format
Buffer readings can be recalled using the absolute timestamp format or the delta format. For the
absolute format, readings are referenced to zero (0) seconds. For the delta format, the timestamp
indicates the time between the displayed reading and the previous reading.
Perform the following steps to set the timestamp format:
1. While in the normal display mode, press the CONFIG key and then the STORE key to display
the timestamp choices.
2. Place the cursor on ABSOLUTE or DELTA and press the ENTER key.
Timestamp accuracy
Because of internal timing methods, the timestamp value is only approximate. The method in
which the timestamp is implemented limits its use in time-critical applications. If accurate test
timing is crucial, it is recommended that an external timer be used in conjunction with the
Model 6482.
The timestamp is based on an oscillator with a frequency of approximately 8 kHz. This oscillator is
used as the system clock and is divided by eight to generate system “ticks” every millisecond.
Therefore, the timestamp should provide 1 ms resolution for test timing. Because the actual
oscillator frequency is 8.192 kHz (instead of 8.000 kHz), a system tick occurs every 8.192 kHz/8,
or 1,024 times a second, which results in a system tick every 0.9765625 ms. As a result, the
reported timestamp value deviates by 24 ms every second. Thus, to obtain more accurate
timestamp values, multiply the timestamp displayed on the front panel or returned through a
remote interface by a factor of 0.9765625.
Buffer considerations
From the front panel, 3000 source and measure readings can be stored and accessed using the
method described earlier in this section. Over the bus, however, there are two separate 3000-
reading buffers, for a total of 6000 readings. The :TRACe buffer is a 3000-reading buffer used by
front-panel data store, bus :TRACe commands, and to store sweep data. The READ? buffer is a
separate 3000-reading buffer that can only be accessed over the bus using the :READ? command
(or alternatively by using an :INITiate followed by :FETCh? command). As described below, you
can store and access data from these two buffers separately.
Using :TRACe commands to store data
Use :TRAC:POIN <n> and :TRIG:COUN <n> followed by :TRAC:FEED:CONT NEXT to store data
(n = Number of readings; 3000 maximum). Use FORM:ELEM:TRAC CURR1 | CURR2 to select
the channel. Turn on the output with :OUTP1 ON or :OUTP2 ON and then send :INIT to take the
instrument out of idle and store readings. After data is stored, send :TRAC:DATA? to access it.
y
X
i
2
i n 1 – =
n
∑
1
n
---
X
i
i 1 =
n
∑
⎝ ⎠
⎜ ⎟
⎜ ⎟
⎛ ⎞
2
⎝ ⎠
⎜ ⎟
⎜ ⎟
⎛ ⎞
–
n 1 –
--------------------------------------------------------------- =