Keithley KPCI-3101 Series, Руководство пользователя

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2-6 Principles of Operation KPCI-3101 — KPCI-3104 Series User’s Manual
A/D sample clock sources
The KPCI-3101–4 Series board provides two clock sources for pacing analog input operations in
continuous mode:
• An internal A/D sample clock that uses the 24-bit A/D Counter on the board; and
•
An external A/D sample clock that you can connect to the screw terminal panel.
You use an A/D sample clock to pace the acquisition of each channel in the channel-gain list;
this clock is also called the A/D pacer clock.
NOTE
If you specify Digital Capabilities for channel 0, the A/D sample clock
(internal or external) also paces the acquisition of the 16 digital input
lines. Refer to the DriverLINX Analog I/O Programming Guide, pro-
vided with DriverLINX.
The following subsections describe the internal and external A/D sample clocks in more detail.
Internal A/D sample clock
The internal A/D sample clock uses a 20MHz time base. Conversions start on the falling edge of
the counter output; the output pulse is active low.
Using software, specify the clock source as internal and the clock frequency at which to pace the
operation. The minimum frequency supported is 1.2Hz (1.2 Samples/s); the maximum fre-
quency supported differs depending on the board type, as shown in
Table 2-3 .
According to sampling theory (Nyquist Theorem), specify a frequency that is at least twice as
fast as the input’s highest frequency component. For example, to accurately sample a 20kHz sig-
nal, specify a sampling frequency of at least 40kHz. Doing so avoids an error condition called
aliasing, in which high frequency input components erroneously appear as lower frequencies
after sampling.
NOTE If input channel 0 is programmed for Digital Capabilities, and is the
only channel programmed, the maximum frequency is 3MHz
(3MSamples/s). Refer to the DriverLINX Analog I/O Programming
Guide, provided with DriverLINX.
Table 2-3
Maximum frequency supported
Board Type Maximum Frequency
KPCI-3101 225kHz
KPCI-3102 225kHz
KPCI-3103 400kHz
KPCI-3104 400kHz