Datasheet

THS4021
+2.5V
Not
Installed
0W
100W
6.3kW
220mH
0.1mF
0.01mF
50W
VCA85XXCWOutput
+2.5V
Iin
Not
Installed
0.1mF
www.ti.com Microcontroller
3.3 TGC Outputs
The differential TGC outputs are buffered by operational amplifiers (OPA842) in a differential configuration
with a differential gain of 1. When testing the VCA8500BOARD, outputs J21 through J28 (TGC outputs)
must be terminated into 50-Ω loads such as those of a spectrum analyzer. This termination results in the
proper matching for the output amplifiers, and no loss has to be taken into account.
3.4 CW Outputs
Figure 1 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 Figure 1
shows how the compliance voltage is applied to the VCA8500 device through a 220-μH 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.
4 Microcontroller
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 (baud rate). This baud rate is set internally on the microcontroller and is configured
automatically in the PC software.
5 Modes of Operation
The three modes of operation for the VCA8500BOARD are Default Power Up, PC Control, and Direct-
Control.
3
SLOU216B–January 2008–Revised January 2013 VCA8500BOARD
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