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Zeta User Manual Rev 1.0
Page 43
14.3
Digital-to-Analog Circuit
14.3.1
Overview
The Zeta base board data acquisition circuit uses a 16-bit D/A converter. This means that all output voltages are
defined by a 16-bit binary number, or D/A code, ranging from 0 to 65535 (2
16
-1), or binary 0000 0000 0000 0000
to 1111 1111 1111 1111. The Zeta base board D/A circuit supports only unipolar output voltage ranges (positive
voltages only). For a unipolar output range, the D/A codes are straight binary and will range from 0 (binary 0000
0000 0000 0000) for 0V to 65535 (binary 1111 1111 1111 1111) for the top end of the range.
The tables below summarize the relationship between D/A codes and output voltages. Note that the nominal
upper limit of the output range (5.0000V or 2.5000V) is not achievable, since this voltage would require a 17-bit
number (2
16
or 1 0000 0000 0000 0000). V
FS
means the full-scale output voltage, either 5V or 2.5V.
D/A Output Codes to Output Voltages
D/A Code
D/A code binary
Formula
0-5V output range
0-2.5V output range
0
0000 0000 0000 0000
0V
0.0000V
0.00000V
1
0000 0000 0000 0001
1 LSB (V
FS
/ 65536)
0.0763mV
0.0381mV
32767
0111 1111 1111 1111
V
FS
/ 2 - 1 LSB
2.4999V
1.24996V
32768
1000 0000 0000 0000
V
FS
/ 2
2.5000V
1.25000V
32769
1000 0000 0000 0001
V
FS
/ 2 + 1 LSB
2.5001V
1.25004V
65535
1111 1111 1111 1111
V
FS
- 1 LSB
4.9999V
2.49996V
14.3.2
D/A Resolution
The smallest change in output voltage that can be obtained is 1/(2
16
), or 1/65536, of the full-scale output range.
This smallest change results from an increase or decrease of 1 in the D/A code and is therefore referred to as 1
LSB (1 Least Significant Bit). The D/A resolution is always 16 bits, but the value of 1 LSB will vary with the output
range.
Polarity
Output Range
Resolution (1LSB)
Unipolar
0 - 5V
76.3µV
Unipolar
0
–
2.5V
38.1µV
14.3.3
Output Range Selection
The output range of the D/A circuit is programmable in software and can be selected from the 2 values shown in
the table above. Refer to the Universal Driver user manual D/A functions for details. The output range can be
changed anytime. However, unlike the A/D circuit, changing the output range affects all output channels
simultaneously. In general, you should select the smallest output range that allows the D/A converter to cover the
full range of output voltages you need for your application. For example, if you need the output to vary from 0V
minimum to 2V maximum, use the 0-2.5V output range for best resolution. Note that the Zeta base board D/A
circuit cannot output negative voltages.
