Digital-To-Analog Converter (Dac) Block - HP 5334B Service Manual

8-130. BUFFER STAGE. The Buffer stage translates the ECL signals from the Schmitt stage to the ap-
propriate levels for driving the MRC input and the front panel trigger light.
8-131. The major component in the buffer is quad ECL-to-TTL translator U21. In Channel A, U21C drives the
MRC input, and U21B is used as a bidirectional one-shot to drive the front panel trigger light through U24E.
8-132. The driver U21C receives the complementary ECL signals from the Schmitt stage and outputs a single
l T L signal. The resistor divider formed by R51 and R52, together with compensation capacitor C54, converts
the TTL output to an EECL signal riding on a dc level of approximately
+
3 volts for input to the MRC.
8-133. The trigger light one-shot driver U21B receives an ECL input signal from one of the Schmitt stage out-
puts. A reference voltage feeds into the other input from the U21 Vbb line, pin
1,
via resistor R53. A positive
feedback loop through C47 and R54 triggers the one-shot at a 10 Hz rate. This output goes to TTL buffer U24E,
which then drives the front panel trigger light.
8-134. DIGITAL-TO-ANALOG CONVERTER (DAC) BLOCK
8-135. The Digital-to-Analog (DAC) Block converts an &bit microcomputer (MCU) code to an analog voltage
to program the input amplifier trigger level or sensitivity setting. It also provides a dc reference voltage for the
READ LEVELS-Trigger levels and READ LEVELS-Peak Levels functions. The DAC Block is controlled by
the Measurement MCU.
8-136. The DAC Block contains two identical circuits, DAC A and DAC B. The Measurement MCU sends
data to both DAC circuits over the &line Measurement Data Bus. Additionally, there are nine other signals
from the Measurement MCU for control of different functions of the DAC Block.
8-137. Each DAC has two parts: A voltage section that performs the digital-to-analog conversion; and a switch-
ing section that passes the analog voltage to the appropriate circuit in the Input Amplifier Block, according to
the measurement function. Since the two DAC circuits are identical, the following detailed description will refer
only to the DAC A circuit but can be applied as well to the DAC B circuit.
8-138. DAC Block Circuit Description (DAC A)
8-139. VOLTAGE SELECTION SECTION. The first half of the DAC circuit, the voltage section, contains a
DAC Integrated Circuit (IC), two operational amplifiers (op-amps) feedback loops, and an analog CMOS
switch. The DAC IC, U33, outputs a current dependent upon the &bit code input from the Measurement MCU.
This current is converted into a nonpositive voltage by the first op-amp U25A. Polarity switch U26B and op-amp
U25D operate together to convert this nonpositive output to either a positive or negative level.
8-140. The core of this section is DAC U33. This CMOS IC receives an &bit data code from the Measurement
MCU at pins
4
through
1 1
over the Measurement Data Bus. Since this data bus is used for communicating with
many other devices, the DAC must be strobed by the MCU at the CHIP SELECT line, pin 12, and the WRITE
line at pin 13. These lines latch the valid &bit code into U33 when they both momentarily go low.
8-141. DAC U33 receives an input current at the VREF line, pin 15. The DAC has 8 CMOS switches to route
the input current through an internal resistor ladder network either to ground at pin 2 or to the output at pin
1.
The output current is dependent upon the binary code input to the 8 switches. With all data lines latched HIGH,
the current at pin
1
is equal to the input current at pin 15; with the data lines latched LOW, all input current is
shunted to ground at pin 2. There are 256 incremental steps of signal level that can appear at the output propor-
tional to the binary weighted input code.
8-142. Op-amp U25A operates in a feedback configuration to act as a current-to-voltage converter. This feed-
back loop keeps the output voltage of the DAC at zero volts. Schottky diode CR23 protects the DAC output
HP
5334B
-
Service
Manual
8-20