Datasheet
V = V + 10 V
OS OSO IOS
·
G
20
dB
(
(
VCA810
1 Fm
V
C
V-
V
IN
V
O
R
V
100kW
R
2
10W
V+
R
1
10kW
50
40
30
20
10
0
10
20
30
40
50
-
-
-
-
-
Output Offset Error (mV)
Gain (dB)
-40 -30 -20 -10 0 10 20 30 40
Maximum Error Band
Typical Devices
VCA810
SBOS275F –JUNE 2003–REVISED DECEMBER 2010
www.ti.com
OUTPUT OFFSET ERROR OFFSET ADJUSTMENT
Several elements contribute to the output offset Where desired, the offset of the VCA810 can be
voltage error; among them are the input offset removed as shown in Figure 46. This circuit simply
voltage, the output offset voltage, the input bias presents a dc voltage to one of the amplifier inputs to
current and the input offset current. To simplify the counteract the offset error voltage. For best offset
following analysis, the output offset voltage error is performance, the trim adjustment should be made
dependent only on the output-offset voltage of the with the amplifier set at the maximum gain of the
VCA810 and the input offset voltage. The output intended application. The offset voltage of the
offset error can then be expressed as Equation 7: VCA810 varies with gain as shown in Figure 45,
limiting the complete offset cancellation to one
selected gain. Selecting the maximum gain optimizes
(7)
offset performance for higher gains where high
Where: amplification of the offset effects produces the
greatest output offset. Two features minimize the
• V
OS
= Output offset error
offset control circuit noise contribution to the amplifier
• V
OSO
= Output offset voltage
input circuit. First, making the resistance of R
2
a low
• G
dB
= VCA810 gain in dB
value minimizes the noise directly introduced by the
• V
IOS
= Input offset voltage
control circuit. This approach reduces both the
thermal noise of the resistor and the noise produced
This is shown in Figure 45.
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.
Figure 46. Optional Offset Adjustment
Figure 45. Output Offset Error versus Gain
This filtering action diminishes as the wiper position
approaches either end of the potentiometer, but
The histogram Output Offset Voltage at Maximum
practical conditions prevent such settings. Over its full
Gain (Figure 18) in the Typical Characteristics curves
adjustment range, the offset control circuit produces a
shows the distribution for the output offset voltage at
±5mV input offset correction for the values shown.
maximum gain.
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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