Data Translation DT9820 Series, User Manual

Page: 31 / 112

Principles of Operation
19
2
2
2
2
2
2
2
2
2
Code
Offset Binary
= C00000h XOR 2
23
= 400000h
Vin = 400000h * 0.000000
−
2.5 V
Vin =
−1.
25 V
Data Transfer
The DT9820 module transfers data to a user buffer that you allocate in
the host computer. Keep the following recommendations in mind
when allocating user buffers for continuous analog input operations
on the DT9820 Series:
• Allocate a minimum of three user buffers.
• Specify a buffer size at least as large as the sampling rate; for
example, if you are using a sampling rate of 960 Samples/s
(960 Hz), specify a buffer size of 960. For sampling rates less than
120 Samples/s, the minimum buffer size that you should specify
is 16 samples. For sampling rates greater than 240 Samples/s, the
minimum buffer size that you should specify is 128 samples.
• Specify the width of the buffer as 4 bytes.
• Specify the buffer wrap mode of either none (for gap-free data) or
multiple in software. The buffer wrap modes are defined as
follows:
−
If the wrap mode is none, data is written to the allocated
buffers until no more empty buffers are available; at that
point, the operation stops.
−
If wrap mode is multiple, data is written to the allocated
multiple buffers continuously; when no more empty buffers
are available, the module overwrites the data in the filled
buffers starting with the first location of the first buffer. This
process continues indefinitely until you stop it.