Speed; Setting Speed - Keithley 2657A Reference Manual

Section 2: General operation

Speed

The Model 2657A has two analog-to-digital converters (ADC): 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).

Setting speed

Speed is set from the SPEED configuration menu and is structured as follows.
Front-panel speed configuration
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
Selecting the FAST menu item selects the fast analog-to-digital converter (ADC). All other menu
items on the speed menu select the integrating ADC.
The SPEED setting affects all measurement functions. After setting speed, display resolution can be
changed using the DIGITS key.
2-78
Model 2657A High Power System SourceMeter® Instrument Reference Manual
NOTE
NOTE
2657A-901-01 Rev. B/December 2012