Speed - Tektronix Keithley 2651A Reference Manual

Section 2: General operation
Digits programming example
-- Select 5.5 digits.
display.smua.digits = display.DIGITS_5_5

Speed

The Model 2651A has two analog-to-digital converters (ADCs): one integrating, one high speed.
Fastest reading rates are achieved by selecting the fast ADC. You can select the fast ADC by
pressing the SPEED key and choosing the FAST option.
You can also use the SPEED key to select the integrating ADC and set the period of time the input
signal is measured (this is called the integration time or measurement aperture). The integration time
affects the usable digits, the amount of reading noise, and the reading rate of the instrument. The
integration time is specified in parameters based on the number of power line cycles (NPLC), where 1
PLC for 60 Hz is 16.67 ms (1/60) and 1 PLC for 50 Hz is 20 ms (1/50).
In general, the fastest integration time (0.001 PLC) results in the fastest reading rate for the
integrating ADC, but also causes increased reading noise and fewer usable digits. The slowest
integration time (25 PLC) provides the best common-mode and normal-mode noise rejection, but has
the slowest reading rate. Settings between the fastest and slowest integration times are a
compromise between speed and noise. The default power-on speed setting is NORMAL (1 PLC).
The SPEED setting affects all measurement functions. After setting speed, display resolution can be
changed using the DIGITS key.
Setting the speed from the front panel
Press SPEED (or use the CONFIG menu) to display the following menu items:
•
FAST: Selects the fast analog-to-digital converter (ADC) (fast performance, but accuracy is
reduced)
•
MED: Selects the integrating ADC and sets its measurement aperture to 0.10 PLC (speed and
accuracy are balanced)
•
NORMAL: Selects the integrating ADC and sets its measurement aperture to 1.00 PLC (speed
and accuracy are balanced)
•
HI-ACCURACY: Selects the integrating ADC and sets its measurement aperture to 10.00 PLC
(high accuracy, but speed is reduced)
•
OTHER: Selects the integrating ADC and sets its measurement aperture to any PLC value from
0.001 to 25
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Model 2651A High Power System SourceMeter® Instrument Reference Manual
2651A-901-01 Rev. C October 2021