Chapter 4
Theory of Operation
© National Instruments Corporation
4-5
SCXI-1124 User Manual
load
GND
VOUT
R
C
f
c
= 1
2
π
RC
SCXI-1124
Adding a lowpass filter for noise reduction
I
Figure 4-2. RC Filter
Changing Ranges
The voltage output circuitry has three operational amplifiers. When you change the value in the
DAC or a range, all of these operational amplifiers will change to a new value. If you try to
change a range but also change the DAC code so that the final output voltage stays the same, the
output typically ramps up and backs down. This happens because the internal operational
amplifiers are slewing to new values, and the output operational amplifier is also slewing, trying
to catch up. The duration of this spike can be up to 100
µ
s long.
Current Output
Each analog output channel has a voltage-to-current converter for creating a 0 to 20 mA sink,
and a loop supply for sourcing current. The current sink is an N-channel power MOSFET sink to
ground. The current sink converts the voltage from the output of the unipolar or bipolar selection
circuit. In order for the current sink to operate properly, you must put this circuit in unipolar
mode. You can use the current output with industry standard 0 to 20 or 4 to 20 mA current
loops. The output sink current is defined by the following equation:
ISINK = 20 mA * 1.017 * (code - 38)/4,096 where 0
≤
code
≤
4,095.
This equation works when the result is non-negative. The current sink does not source current.
If the formula gives a negative result, the current sink will sink 0 mA. On power up or chassis
reset, each DAC channel is in unipolar mode and the DAC code is zero, producing a default state
of 0 mA.
In addition, there can be gain and offset errors in the current output circuitry. To account for
these errors, there are calibration constants stored in the EEPROM for each channel current
range. These constants encode the values that need to be written to the DAC to get zero and
20 mA.