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3.4
CW Outputs
THS4021
+2.5V
Not
Installed
0
W
100
W
6.3k
W
220
m
H
0.1
m
F
0.01
m
F
50
W
VCA85XX CW Output
+2.5V
I in
Not
Installed
0.1
m
F
4
Microcontroller
5
Modes of Operation
5.1
Default-Power-Up Mode
Microcontroller
shows the CW output circuit. This circuit is duplicated for each CW output.
Figure 1. CW Output Circuit
The CW outputs are current outputs and require a current-to-voltage conversion in order to view the
outputs with an oscilloscope or spectrum analyzer. The THS4021 operational amplifier is used as the
current-to-voltage converter.
When using the VCA8500, the CW outputs need a compliance voltage of +2.5 V. The circuit in
shows how the compliance voltage is applied to the VCA8500 device through a 220-
µ
H inductor. The CW
output circuit allows the noise of the CW path to be measured. This configuration is the reason for the
large feedback resistor in the output amplifier circuit. Before summing more than two CW channels,
ensure that the output amplifier does not exceed its maximum output capability. To increase the output
capability of the THS4021 ensure that W2 and W3 are open and supply
±
15 V to P3.
The VCA8500BOARD contains a microcontroller to access internal functions. The microcontroller provides
the evaluation module the ability to be controlled by software running on a PC. The microcontroller is run
from its own internal oscillator set to 12 MHz. The only external clock signals are those used to
communicate to the VCA8500 via the serial data clock line, and the embedded clock signal in the RS-232
communications circuit. The RS-232 clock operates only when data is being sent from the personal
computer (PC) to the microcontroller. The serial communication from the PC to the microcontroller is set to
19200 kbps. This baud rate is set internally on the microcontroller and is configured automatically in the
PC software.
The three modes of operation for the VCA8500BOARD are Default Power Up, PC Control, and
Direct-Control.
If no external source or PC control is available, the VCA8500 powers up in a default mode, which sets the
VCA8500 to TGC mode with these settings, PGA Gain 20 dB, Clamp Level Disabled, and Filter Bandwidth
15 MHz.
SLOU216 – January 2008
VCA8500BOARD
3
