Datasheet

Data Sheet AD8137
Rev. E | Page 23 of 32
APPLICATIONS INFORMATION
ANALYZING A TYPICAL APPLICATION WITH
MATCHED R
F
AND R
G
NETWORKS
Typical Connection and Definition of Terms
Figure 64 shows a typical connection for the AD8137, using
matched external R
F
/R
G
networks. The differential input
terminals of the AD8137, V
AP
and V
AN
, are used as summing
junctions. An external reference voltage applied to the V
OCM
terminal sets the output common-mode voltage. The two
output terminals, V
OP
and V
ON
, move in opposite directions
in a balanced fashion in response to an input signal.
04771-0-055
+
–
V
AP
V
AN
V
ON
V
OP
–
+
V
O, dm
R
L, dm
AD8137
C
F
R
F
R
G
R
G
C
F
R
F
V
IP
V
OCM
V
IN
Figure 64. Typical Connection
The differential output voltage is defined as
V
O, dm
= V
OP
− V
ON
(1)
Common-mode voltage is the average of two voltages. The
output common-mode voltage is defined as
2
,
ONOP
cmO
VV
V
+
=
(2)
Output Balance
Output balance is a measure of how well V
OP
and V
ON
are
matched in amplitude and how precisely they are 180° out of
phase with each other. It is the internal common-mode feedback
loop that forces the signal component of the output common-
mode toward zero, resulting in the near perfectly balanced
differential outputs of identical amplitude and are exactly 180°
out of phase. The output balance performance does not require
tightly matched external components, nor does it require that
the feedback factors of each loop be equal to each other. Low
frequency output balance is ultimately limited by the mismatch
of an on-chip voltage divider.
Output balance is measured by placing a well-matched resistor
divider across the differential voltage outputs and comparing
the signal at the divider’s midpoint with the magnitude of the
differential output. By this definition, output balance is equal to
the magnitude of the change in output common-mode voltage
divided by the magnitude of the change in output differential
mode voltage:
dmO
cmO
V
V
BalanceOutput
,
,
∆
∆
=
(3)
The differential negative feedback drives the voltages at the summing
junctions V
AN
and V
AP
to be essentially equal to each other.
V
AN
= V
AP
(4)
The common-mode feedback loop drives the output common-
mode voltage, sampled at the midpoint of the two internal
common-mode tap resistors in Figure 62, to equal the voltage
set at the V
OCM
terminal. This ensures that
2
, dmO
OCMOP
V
VV +=
(5)
and
2
, dmO
OCMON
V
VV −=
(6)
ESTIMATING NOISE, GAIN, AND BANDWITH WITH
MATCHED FEEDBACK NETWORKS
Estimating Output Noise Voltage and Bandwidth
The total output noise is the root-sum-squared total of several
statistically independent sources. Because the sources are
statistically independent, the contributions of each must be
individually included in the root-sum-square calculation. Table 7
lists recommended resistor values and estimates of bandwidth
and output differential voltage noise for various closed-loop
gains. For most applications, 1% resistors are sufficient.
Table 7. Recommended Values of Gain-Setting Resistors and
Voltage Gain for Various Closed-Loop Gains
Gain R
G
(Ω) R
F
(Ω)
3 dB Bandwidth
(MHz)
Total Output
Noise (nV/√Hz)
1 1 k 1 k 72 18.6
2 1 k 2 k 40 28.9
5 1 k 5 k 12 60.1
10 1 k 10 k 6 112.0