Measurement Computing Data Translation DT9857, User Manual

Page: 77 / 192

Principles of Operation
77
IEPE Functions
Applications that require accelerometer, vibration, noise, or sonar measurements often use
IEPE sensors. IEPE conditioning is built-in to the analog input circuitry of the DT9857 and
DT9857E module. The modules support the following software-programmable IEPE functions
for each analog input channel:
• Excitation current source – The DT9857 and DT9857E modules provide an internal
excitation current source of 4 mA. You can enable or disable the use of a internal excitation
current source using software. By default, the excitation current source is disabled.
• Coupling type – You can select whether AC coupling or DC coupling is used. By default,
DC coupling is selected for the DT9857 and DT9857E modules.
DT9857 and DT9857E modules p24 V of compliance voltage.
For information on wiring IEPE inputs, refer to page 48 .
Note: If you enable the use of the internal excitation current source, it is recommended that
you choose AC coupling.
Input Resolution
The resolution of the analog input channels is fixed at 24 bits; you cannot specify the
resolution in software.
Input Clock Source
The DT9857 and DT9857E modules support a 48 MHz internal clock source, which is derived
from the 24 MHz USB crystal oscillator. The same internal 48 MHz clock signal is used for the
both the analog input and analog output subsystems.
For the analog input subsystem, use software to specify the internal clock source and the
frequency at which to pace the input operation and to start the sample clock. The sampling
frequency can range from 195.3125 Hz to 105.469 kHz.
Note: 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
20 kHz signal, specify a sampling frequency of at least 40 kHz to avoid aliasing.
The sample frequencies for the A/D and D/A subsystems are independently programmable
and are derived from the same 48 MHz reference clock. Therefore, it is possible to establish a
fixed relationship between the A/D and D/A subsystem sample frequencies, including
setting them to the same frequency (30 kHz to 105.469 kHz). However, the subsystems are not
designed to be synchronous with each other.