Setting Digits; Rate; Figure 4-1: Speed Versus Noise Characteristics - Keithley 3700 series Reference Manual

Section 4: Range, Digits, Rate, Bandwidth, and Filter

Setting digits

Even though the parameters for the dmm.displaydigits attribute (see dmm.displaydigits for
remote control or DIGITS under the function attribute menu on front panel) are expressed as
integers (3 to 7), you can specify resolution using a real number. For example, to select 3½ digit
resolution, let <n> = 3.5. Internally the instrument rounds the entered parameter value to the
nearest integer. Each mainframe input function can have its own unique digits setting.

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, 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
then the DMM key to bring up the function attribute menu. From these, 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 still be fast
enough to settle a step response <100ms.

Figure 4-1: Speed versus noise characteristics

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Series 3700 System Switch/Multimeter Reference Manual
Document Number: 3700S-901-01 Rev. A / August 2007