Datasheet

OFFSET ADJUSTMENT
e =A ´
O VMAX
2 (R i ) +e +2 4kTR´ ´ ´
S n n S
2 2
(11)
R
F
+V
IN
R
G+
R
G-
-V
IN
FB
R
G
R
S
e
O
VCA822
*
e
O
i
n
4kTR
S
R
S
*
i
n
4kTR
S
NOTE:R andR arenoiseless.
F G
NOISE
+V
IN
R
G+
R
G-
-V
IN
FB
R
G
50W
10kW
1kW
50W
V
IN
VCA822
R
F
0.1 Fm
0.1 Fm
10kW
4kW
+5V
-5V
V
OUT
+5V
-5V
InputStageandMultiplexerCore
OffsetCompensationCircuit
OutputStageOffset
CompensationCircuit
VCA822
SBOS343C SEPTEMBER 2007 REVISED DECEMBER 2008 .....................................................................................................................................
www.ti.com
This model is formulated in Equation 11 and
Figure 91 .
As a result of the internal architecture used on the
VCA822, the output offset voltage originates from the
output stage and from the input stage and multiplier
core. Figure 92 shows how to compensate both A more complete model is shown in Figure 93 . For
sources of the output offset voltage. Use this additional information on this model and the actual
procedure to compensate the output offset voltage: modeled noise terms, please contact the High-Speed
starting with the output stage compensation, set Product Application Support team at www.ti.com .
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.
The VCA822 offers 8.2nV/ Hz input-referred voltage
Figure 91. Simple Noise Model
noise density at a gain of +10V/V and 1.8 pA/ Hz
input-referred current noise density. The
input-referred voltage noise density considers that all
noise terms, except the input current noise but
including the thermal noise of both the feedback
resistor and the gain resistor, are expressed as one
term.
Figure 92. Adjusting the Input and Output Voltage Sources
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