Output Amplifier - Wavetek 75 Instruction Manual

(pin 17) and taking the current output from VREF (pin 1).
The current output is proportional to the binary number
applied to the digital inputs (OFFI-OFF12) and the
voltage applied to pin 17. That reference voltage is taken
from the precision reference in the waveform DAC. The
resistor on pins 1 and 2 of RN2 provides extra current
drive capabilities for the 10V reference. As with the
amplitude data, the offset data (OFF1 -OFF1 2) originates
at the microprocessor and is interfaced to this circuit
by the control shift register (ref:schematic 01 03-00-1 456
sheet 2). Because of this the logic levels of the offset data
are
+
5V and the DAC requires the
+
10V and
-
5V
power supplies. The op amp (U4) converts the current
output of the DAC to a voltage. The feedback resistor
for the op amp is internal to the DAC to ensure better
accuracy. The inverting input of the op amp (pin 2) acts
as a summing node and is held at virtual ground. The out-
put current of the DACflows into the summing node and
through the feedback resistor to create a voltage whose
nominal swing is
-
2.5V to
-
7.5V as the offset is
changed from
-
5V to
+
5V. The same attenuator
ranging that applies to the amplitude control circuit also
applies to the offset. When the Calibration constant is
set during calibration, the microprocessor adds a digital
offset constant to the offset data (ref:Section 5,
Calibration). This constant allows the offset circuit to
compensate for the offset errors throughout the analog
signal path and eliminates the need for an internal
calibration adjustment.
4.2.1.9 Output Amplifier
Refer to schematic 01 03-00-1 456, sheet 4. The output
amplifier provides the final waveform amplification and
output drive for the instrument. It consists of a discrete
inverting amplifier for the high frequency component of
the waveformwith an op amp to provide DC stabilityand
a low frequency signal path. A simplified model of the
amplifier is shown in figure 4-5. Pin 5 of RN2 acts as a
current summing node fcr the amplifier. The waveform
is summed into the amplifier through pin 4 of RN2 with
a gain of 5.1 2. The offset control voltage is summed into
the amplifier through pin 3 of RN2 with a gain of 4.096.
The 10V reference is also summed into the amplifier
through pin 2 of RN2. This creates an current equivalent
to the mid scale current from the offset voltage, thus
making the offset control bipolar.
High frequency signals drive the symmetrical common
emitter stage of 0 7 and 0 8 through C44 and C45. DC
and low frequency signals drive this stage through the
Figure 4-5. Output Amplifier Simplified Diagram
4-7
Scans by Artekhledia O 2006
I
C50
I I
4
R38
!I
a
PIN 4
WAVEFORM
PIN 3
T
c43
a
OFFSET VOLTAGE
-
>
TO ATTENUATOR
(FROM OFFSET
-
CONTROL)
(FROM AMPLITUDE
-
CONTROL)
PIN 2 PIN 5
+
10V REFERENCE
-
: :