Datasheet

1.2 Using External Operational Amplifier Supplies
2 Block Diagrams
2.1 System Block Diagram
3 Key Texas Instruments Components
3.1 VCA824
www.ti.com Block Diagrams
By default both amplifiers are set up to operate with a ± 5 V. This is adequate in most cases for evaluation
purposes; however, both the OPA656 and VCA824 can be operated at a maximum of ± 6-V supply. Ferrite
beads allow the use of a different ± Vamp supply for both amplifiers, if desired.
Figure 3 shows the functional blocks on the TSW7001 board. The Texas Instruments integrated circuits
(IC) are listed on board for reference.
Figure 3. TSW7001EVM Block Diagram
The VCA824 is a dc-coupled, ultra-wideband, voltage-controlled amplifier with a linear in V/V gain control
voltage input to adjust the gain down 40 dB from the nominal gain set by the gain resistor R291 (Rg) and
the feedback resistor R302 (Rf). The gain element is isolated from both inputs, permitting gain control
shaping techniques to be implemented easily. Both the inverting and noninverting inputs of the VCA824
are high impedance, allowing a simple interface to the prior stage.This EVM has a nominal gain of 100
V/V (40 dB). Typical applications that are well-suited to the VCA824 include differential line receivers,
differential equalizers, pulse amplitude compensation, and variable attenuators. More information for the
VCA824 is available in the data sheet (SBOS394 ). For a lower speed VCA, consider the VCA822. For a
linear in dB gain adjust range, consider the VCA820 and VCA821.
Figure 4. VCA824 Schematic
SLWU058 – August 2008 Voltage-Controlled Amplifier Evaluation Kit 3
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