Texas Instruments VCA8613EVM User Manual page 9

3 Signals
3.1 Input Signals
The input signals are applied to SMA connectors J19 to J26. The input signals
should be provided from a low-impedance source (less than 100Ω) as a result
of the 4.5kΩ input impedance of the VCA. The inputs are ac-coupled into the
LNA of the VCA8613 through 0.01µF capacitors, and do not attenuate the in-
put signal.
3.2 Output Signals
The outputs of the evaluation module are located at SMA connectors J28
through J35 for the TGC outputs, and J9 to J18 for the CW outputs. The out-
puts from the VCA8613 are buffered with operational amplifiers.
3.3 TGC Outputs
The differential TGC outputs are buffered by op amps (OPA690) in a differen-
tial configuration with a differential gain of 1. When testing the VCA8613EVM,
outputs J28 through J35 (TGC outputs) must be terminated into 50Ω loads
such as those of a spectrum analyzer. This termination will result in the proper
matching for the output amplifiers, and no loss will have to be taken into ac-
count.
3.4 CW Outputs
The following diagram 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 conver-
sion in order to view the outputs with an oscilloscope or spectrum analyzer.
The OPA690 op amp is used as the current-to-voltage converter. Provisions
have been made on the EVM board to monitor the current directly by the use
of JP1 and JP2. When monitoring of the current is needed, solder jumpers JP4
through JP13 should be removed to help alleviate any errors associated with
bias current requirements of the current-to-voltage converter.
+3V
220µH
VCA861x
CW
0.1µF
100Ω
Output
I
IN
+1.25V
Not
Populated
0Ω
6.3kΩ
0.01µF
50Ω SMA
OPA690
VCA8613EVM
Signals
3