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VCA824IDG4 Datasheet(PDF) 19 Page - Texas Instruments

Part No. VCA824IDG4
Description  Ultra-Wideband, > 40-dB Gain Adjust Range, Linear in V/V Variable Gain Amplifier
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Maker  TI1 [Texas Instruments]
Homepage  http://www.ti.com
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VCA824IDG4 Datasheet(HTML) 19 Page - Texas Instruments

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VCA824
www.ti.com
SBOS394D – NOVEMBER 2007 – REVISED JANUARY 2016
8 Detailed Description
8.1 Overview
The VCA824 is a voltage controlled variable gain amplifier with differential inputs and a single ended output. The
maximum gain is set by external resistors while the gain range is controlled by an external analog voltage. The
maximum gain is designed for gains of 2 V/V up to 100 V/V and the analog control allows a gain range of over
40 dB. The VCA824 Input consists of two buffers, which together create a fully symmetrical, high impedance
differential input with a typical common mode rejection of 80 dB. The gain set resistor is connected between the
two input buffer output pins, so that the input impedance is independent of the gain settings. The bipolar inputs
have a input voltage range of 1.6 and –2.1 V on ±5 V supplies. The amplifier maximum gain is set by external
resistors, but the internal gain control circuit is controlled by a continuously variable, analog voltage. The gain
control is a multiplier stage which is linear in V/V. The gain control input pin operates over a voltage range of –1
V to 1 V. The VCA824 contains a high-speed, high-current output buffer. The output stage can typically swing
±3.9 V and source/sink ±90 mA. The VCA824 can be operated over a voltage range of ±3.5 V to ±6 V.
8.2 Functional Block Diagram
8.3 Feature Description
The VCA824 can be operated with both single ended or differential input signals. The inputs present consistently
high impedance across all gain configurations. By using an analog control signal the amplifier gain is
continuously variable for smooth, glitch free gain changes. With a large signal bandwidth of 320 Mhz and a slew
rate of 2500 V/us the VCA824 offers linear performance over a wide range of signal amplitudes and gain
settings. The low impedance/high current output buffer can drive loads ranging from low impedance transmission
lines to high-impedance, switched-capacitor analog-to-digital converters. By using closely matched internal
components, the VCA824 offers gain accuracy of ±0.3 dB.
8.4 Device Functional Modes
The VCA824 functions as a differential input, single maximum gain of operation-ended output variable gain
amplifier. This functional mode is enabled by applying power to the amplifier supply pins and is disabled by
turning the power off. The gain is continuously variable through the analog gain control input. While the gain
range is fixed, the maximum gain is set by two external components, Rf and Rg, as shown in the Functional
Block Diagram. The maximum gain is equal to 2x (Rf / Rg). This gain is achieved with a 1-V voltage on the gain
adjust pin VG. As the voltage decreases on the VG pin, the gain decreases in a linear in dB fashion with over 40
dB of gain range from 1-V to –1-V control voltage. As with most other differential input amplifiers, inputs can be
applied to either one or both of the amplifier inputs. The amplifier gain is controlled through the gain control pin.
8.4.1 Maximum Gain Of Operation
This section describes the use of the VCA824 in a fixed-gain application in which the VG control pin is set at
VG = 1 V. The tradeoffs described here are with bandwidth, gain, and output voltage range.
Copyright © 2007–2016, Texas Instruments Incorporated
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