ADLINK Technology USB-1901, User Manual

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38 Operation
4.2.3 Software Conversion with Polling Data Transfer
Acquisition Mode (Software Polling)
Generally the most convenient way to acquire a single A/D data
sample, the A/D converter starts a conversion when the dedicated
software command is executed. The software then polls the
conversion status and reads back the A/D data when it is
available.
This method is indicated when there is a need to process A/D data
in real time or instant closed-loop control. In this mode, the timing
of the A/D conversion is fully controlled by the software.
In Software Polling, the channel, gain, and input configuration
(RSE, NRSE, or DIFF) may be specified for each single-point
measurement. For example:
Ch2 with ± 5 V input range and single-end NRSE connection
4.2.4 Continuous Acquisition (Scanning) Mode
Continuous A/D Conversion Clock Source
When the onboard ADC receives a conversion clock signal, A/
D conversion is triggered. The USB-1900 Series conversion
clock may originate with the internal hardware timer or ECLK
(external A/D conversion clock) pin. While the conversion clock
source can be chosen by setting AI source configuration, if pre-
cision acquisition is required, use of the internal hardware timer
is recommended.
WARNING:
For current input (USB-1903), the current signal will be con-
verted to voltage by a precision resistor, and the input gain
fixed to 1(input range = ±10V), with transfer formula:
I (mA) = V (mV) / 24.89353693
Ÿ
WARNING:
The A/D conversion rate is determined by the software timer
and may not be precise.