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4-8
Rate and bandwidth
Rate
The RATE key sets the integration time (measurement speed) of the A/D converter; the
period of time the input signal is measured (also known as aperture). The integration time
affects the amount of reading noise, as well as the ultimate reading rate of the instrument.
The integration time is specified in parameters based on a number of power line cycles
(NPLC), where 1 PLC for 60Hz is 16.67msec (1/60) and 1 PLC for 50Hz and 400Hz is
20msec (1/50).
In general, the fastest integration time (0.1 PLC from the front panel, 0.01 PLC from the
bus) results in increased reading noise and fewer usable digits, while the slowest integra-
tion time (5 PLC from the front panel, 50 PLC from the bus) provides the best common-
mode and normal-mode rejection. In-between settings are a compromise between speed
and noise.
The Model 2790 has a parabola-like shape for its speed vs. noise characteristics and is
shown in
At these rates (lowest noise region in graph), the Model 2790 will make corrections for its
own internal drift and still be fast enough to settle a step response <100ms.
Figure
4-1. The Model 2790 is optimized for the 1 PLC to 5 PLC reading rate.
Figure 4-1
Speed vs. noise characteristics
Voltage
Noise
166.7 s
16.67ms
(0.01 PLC)
(1 PLC)
Model 2790 SourceMeter
Lowest
Noise
Region
83.33ms
(5 PLC)
Aperture Time
®
Switch System Reference Manual
1s
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