VCA821_12 Datasheet, PDF (25/37 Pages) Texas Instruments – Ultra-Wideband, > 40dB Gain Adjust Range, Linear in dB VARIABLE GAIN AMPLIFIER

VCA821
www.ti.com ....................................................................................................................................... SBOS407B – DECEMBER 2007 – REVISED DECEMBER 2008
OUTPUT VOLTAGE DYNAMIC RANGE
With its large output current capability and its wide
output voltage swing of ±3.9V typical on 100Ω load, it
is easy to forget other types of limitations that the
VCA821 can encounter. For these limitations, careful
analysis must be done to avoid input stage limitation:
either voltage or I_RG current. Note that if control pin
V_G varies, the gain limitation may affect other aspects
of the circuit.
BANDWIDTH
The output stage of the VCA821 is a wideband
current-feedback amplifier. As such, the feedback
resistance is the compensation of the last stage.
Reducing the feedback element and maintaining the
gain constant limits the useful range of I_RG, and
therefore, reduces the gain adjust range. For a given
gain, reducing the gain element limits the maximum
achievable output voltage swing.
OFFSET ADJUSTMENT
As a result of the internal architecture used on the
VCA821, the output offset voltage originates from the
output stage and from the input stage and multiplier
core. Figure 85 shows how to compensate both
sources of the output offset voltage. Use this
procedure to compensate the output offset voltage:
starting with the output stage compensation, set
V_G = –1V to eliminate all offset contribution of the
input stage and multiplier core. Adjust the output
stage offset compensation potentiometer. Finally, set
V_G = +1V to the maximum gain and adjust the input
stage and multiplier core potentiometer. This
procedure effectively eliminates all offset contribution
at the maximum gain. Because adjusting the gain
modifies the contribution of the input stage and the
multiplier core, some residual output offset voltage
remains.
+5V
Output Stage Offset
Compensation Circuit
10kW
0.1mF
4kW
-5V
R_F
V_IN
50W
+V_IN
R_{G+ FB}
R_G
VCA821
+5V
R_G-
-V_IN
1kW
50W
10kW
-5V
0.1mF
Input Stage and Multiplexer Core
Offset Compensation Circuit
V_OUT
Figure 85. Adjusting the Input and Output Voltage Sources
Copyright © 2007–2008, Texas Instruments Incorporated
Product Folder Link(s): VCA821
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