User manual

Series 3700 System Switch/Multimeter Reference Manual Section 5: Range, Digits, Rate, Bandwidth, and Filter
3700S-901-01 Rev. C / July 2008 5-5
Rate
The RATE key sets the integration time (measurement speed) of the A/D
converter. This controls how long 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
is 20msec (1/50).
In general, the fastest integration time (0.1 PLC using the front panel RATE key,
or 0.0005 PLC from the bus or through the DMM > CONFIG NPLC menu)
results in increased reading noise and fewer usable digits, while the slowest
integration time (5 PLC using the front panel RATE key, or 15 PLC from the bus
or through the DMM > CONFIG NPLC menu) provides the best common-mode
and normal-mode rejection. In-between settings are a compromise between
speed and noise. To set the NPLC from 0.0005 to 15 on the front panel, press
the CONFIG key, and then press the DMM key to open the function attribute
menu. From the function attribute menu, select NPLC to dial in a specific value
for NPLC.
The Series 3700 has a parabola-like shape for its speed versus noise
characteristics. The Series 3700 is optimized for the 1 PLC to 5 PLC reading
rate. At these rates (lowest noise region in graph), the Series 3700 will make
corrections for its own internal drift and will still be fast enough to settle a step
response <100ms.
Figure 5-1: Speed versus noise characteristics