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5790B
Service Manual
DAC Assembly (A16)
The DAC (digital-to-analog converter) provides a digitally adjustable precision dc
voltage from 0 V to 11 V. The DAC uses a pulse-width-modulated scheme to
vary its output. The block diagram of the DAC assembly is shown in Figure 2-6.
The two inputs of the five-pole filter are two precision square waves with different
fixed amplitudes and independently variable duty cycles controlled by software.
The filter's first input square wave is called the first channel. It is switched
between the reference voltage (13 V) and 0 V.
The filter's second input square wave is called the second channel. It is switched
between approximately 0.78 mV and 0 V. Its amplitude is derived by resistively
dividing the 13 V reference. This second channel is used for extra resolution.
The filter rejects all AC components of the waveforms above 30 Hz. Since the
frequency of the square waves is 190 Hz, the output of the filter is a dc voltage
which is the sum of the average voltages of the two waveforms. The Output
Stage, which consists of the dc amplifier hybrid and the output buffer, isolates the
filter output from the DAC output and gives current drive to the DAC output.
To change the DAC voltage, the average value of the two square waves must be
varied. To determine the average value, multiply the waveforms amplitude by its
duty cycle. Vary the duty cycle and keep the amplitude fixed to change the DAC
voltage.
For example, if the duty cycle of the first channel is 10 % and the second channel
50 %, the overall average voltage would be:
The duty cycle resolution is 0.0024 %, which gives a first channel resolution of
0.309 mV and second channel resolution of 18.5 nV.
The duty cycle control circuitry creates the two digital square waves for the first
and second channels. These two waveforms are first run through the
optocouplers for isolation and then into analog switching and level shifting
circuits. These circuits derive the proper signals to switch the input of the filter at
the levels explained above.
2-10
(
) (
×
+
×
0
1 .
13
V
0
5 .
. 0
78
)
=
mV
. 1
300390
V
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