VCA810_16 Datasheet, PDF (18/40 Pages) Texas Instruments – VCA810 High Gain Adjust Range, Wideband and Variable Gain Amplifier

VCA810
SBOS275G – JUNE 2003 – REVISED DECEMBER 2015
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50
40
30
Maximum Error Band
20
10
Typical Devices
0
-10
-20
-30
-40
-50
-40 -30 -20 -10 0 10 20 30 40
Gain (dB)
Figure 29. Output Offset Error versus Gain
Figure 18 shows the distribution for the output offset voltage at maximum gain.
8.3.4 Offset Adjustment
Where desired, the offset of the VCA810 can be removed as shown in Figure 30. This circuit simply presents a
DC voltage to one of the amplifier inputs to counteract the offset error voltage. For best offset performance, the
trim adjustment should be made with the amplifier set at the maximum gain of the intended application. The
offset voltage of the VCA810 varies with gain as shown in Figure 29, limiting the complete offset cancellation to
one selected gain. Selecting the maximum gain optimizes offset performance for higher gains where high
amplification of the offset effects produces the greatest output offset. Two features minimize the offset control
circuit noise contribution to the amplifier input circuit. First, making the resistance of R₂ a low value minimizes the
noise directly introduced by the control circuit. This approach reduces both the thermal noise of the resistor and
the noise produced by the resistor with the amplifier input noise current. A second noise reduction results from
capacitive bypass of the potentiometer output. This reduction filters out power-supply noise that would otherwise
couple to the amplifier input.
V+
R_V
100kW
R₁ ^VIN
10kW
VCA810
V_O
1mF
V-
R₂
10W ^VC
Figure 30. Optional Offset Adjustment
This filtering action diminishes as the wiper position approaches either end of the potentiometer, but practical
conditions prevent such settings. Over its full adjustment range, the offset control circuit produces a ±5-mV input
offset correction for the values shown. However, the VCA810 only requires one-tenth of this range for offset
correction, assuring that the potentiometer wiper will always be near the potentiometer center. With this setting,
the resistance seen at the wiper remains high, which stabilizes the filtering function.
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