4-6
Range, Units, Digits, Rate, and Filters
Model 6487 Reference Manual
Rate
The RATE key selects the integration time of the A/D converter. This is the period of time
the input signal is measured. 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 (NPLCs), where 1 PLC for 60Hz is
16.67msec (1/60) and 1 PLC for 50Hz (and 400Hz) is 20msec (1/50).
In general, the Model 6487 has a parabola-like shape for its speed vs. noise characteristics
and is shown in
. The Model 6487 is optimized for the 1 PLC to 10 PLC reading
rate. At these speeds (lowest noise region in the graph), The Model 6487 will make correc-
tions for its own internal drift and still be fast enough to settle a step response <100ms.
Figure 4-1
Speed vs. noise characteristics
The rate setting is global for all ranges. Therefore, it does not matter what range is pres-
ently selected when you set rate.
There are two ways to set rate. You can select slow, medium, or fast by using the RATE
key or you can set the number of power cycles from the NPLC menu that is accessed by
pressing CONFIG / LOCAL (while in LOCAL) and then RATE.
RATE Key
— The RATE key selections are explained as follows:
•
SLOW
— Selects the slowest preset integration time (6 PLC for 60Hz or 5 PLC for
50Hz). The SLOW rate provides better noise performance at the expense of speed.
•
MED
— Selects the medium integration time (1 PLC). Select the MED rate when
a compromise between noise performance and speed is acceptable.
•
FAST
— Selects the fastest preset integration time (0.1 PLC). Select the FAST rate if
speed is of primary importance (at the expense of increased reading noise).
Lowest
noise
region
Voltage
Noise
166.7
μ
s
16.67ms
166.67ms
Integration Time
