Chapter 4
Theory of Operation
© National Instruments Corporation
4-3
SCXI-1124 User Manual
Analog Output Circuitry
The SCXI-1124 has six independent isolated voltage or current control outputs. Each channel
has four pins on the front signal connector–a ground reference, a programmable voltage output, a
loop supply, and a programmable current sink.
Voltage Output Circuitry
The voltage output circuitry consists of a voltage reference, DAC, unipolar and bipolar selection
circuitry, range selection circuitry, and an output buffer. The DAC is a 12-bit monotonic DAC
with excellent DNL and INL specifications, making it ideal for process control applications.
You can program the polarity and range selection circuitry for ranges of 0 to 1 V, 0 to 5 V,
0 to 10 V,
±
1 V,
±
5 V, and
±
10 V. The DAC channels have been designed so that the actual
range is always slightly greater than the nominal range. The following table shows typical
ranges.
Table 4-1. Typical DAC Channel Ranges
Nominal Range
Typical Range
0 to 1 V
-9 mV to 1.006 V
0 to 5 V
-46 mV to 5.028 V
0 to 10 V
-92 mV to 10.055 V
±
1 V
-1.015 V to 1.015 V
±
5 V
-5.075 V to 5.073 V
±
10 V
-10.150 V to 10.145 V
In unipolar mode, the output voltage is defined by the following equation:
V
out
= V
ref
* 1.015 * (code - 38)/4,096
where 0
≤
code
≤
4,095.
In bipolar mode, the output voltage is defined by the following equation:
V
out
= V
ref
* 1.015 * (code - 2,048)/2,048
where 0
≤
code
≤
4,095.
In both formulas, V
ref
is 1 V for the 0 to 1 V or
±
1 V range, 5 V for the 0 to 5 V or
±
5 V range,
and 10 V for the 0 to 10 V or
±
10 V range.
All channels power up in the 0 to 1 V range with a DAC code of zero, producing a typical power
up state of -9 mV. The channels also go to this state if the chassis is reset.
In addition, there can be gain and offset errors in the voltage output circuitry. To account for
these errors, there are calibration constants stored in the EEPROM for each channel range.
These constants encode the values you write to the DAC to get the two endpoints for the nominal
