Datasheet
ADA4932-1/ADA4932-2 Data Sheet
Rev. B | Page 20 of 28
APPLICATIONS INFORMATION
ANALYZING AN APPLICATION CIRCUIT
The ADA4932-x uses high open-loop gain and negative feedback
to force its differential and common-mode output voltages in
such a way as to minimize the differential and common-mode
error voltages. The differential error voltage is defined as the
voltage between the differential inputs labeled +IN and −IN
(see Figure 55). For most purposes, this voltage can be assumed
to be zero. Similarly, the difference between the actual output
common-mode voltage and the voltage applied to V
OCM
can also
be assumed to be zero. Starting from these principles, any applica-
tion circuit can be analyzed.
SETTING THE CLOSED-LOOP GAIN
Using the approach described in the Analyzing an Application
Circuit section, the differential gain of the circuit in Figure 55
can be determined by
G
F
dmIN
dm
OUT
R
R
V
V
=
,
,
This presumes that the input resistors (R
G
) and feedback resistors
(R
F
) on each side are equal.
ESTIMATING THE OUTPUT NOISE VOLTAGE
The differential output noise of the ADA4932-x can be
estimated using the noise model in Figure 56. The input-
referred noise voltage density, v
nIN
, is modeled as a differential
input, and the noise currents, i
nIN−
and i
nIN+
, appear between
each input and ground. The output voltage due to v
nIN
is obtained
by multiplying v
nIN
by the noise gain, G
N
(defined in the G
N
equation that follows). The noise currents are uncorrelated with
the same mean-square value, and each produces an output voltage
that is equal to the noise current multiplied by the associated
feedback resistance. The noise voltage density at the V
OCM
pin is
v
nCM
. When the feedback networks have the same feedback factor,
as is true in most cases, the output noise due to v
nCM
is common
mode. Each of the four resistors contributes (4kTR
xx
)
1/2
. The
noise from the feedback resistors appears directly at the output,
and the noise from the gain resistors appears at the output multip-
lied by R
F
/R
G
. Table 11 summarizes the input noise sources, the
multiplication factors, and the output-referred noise density terms.
ADA4932-x
+
R
F2
V
nOD
V
nCM
V
OCM
V
nIN
R
F1
R
G2
R
G1
V
nRF1
V
nRF2
V
nRG1
V
nRG2
i
nIN+
i
nIN–
07752-047
Figure 56. Noise Model
Table 11. Output Noise Voltage Density Calculations for Matched Feedback Networks
Input Noise Contribution Input Noise Term
Input Noise
Voltage Density
Output
Multiplication Factor
Differential Output Noise
Voltage Density Term
Differential Input v
nIN
v
nIN
G
N
v
nO1
= G
N
(v
nIN
)
Inverting Input i
nIN−
i
nIN−
× (R
F2
) 1 v
nO2
= (i
nIN−
)(R
F2
)
Noninverting Input
i
nIN+
i
nIN+
× (R
F1
)
1
v
nO3
= (i
nIN+
)(R
F1
)
V
OCM
Input v
nCM
v
nCM
0 v
nO4
= 0 V
Gain Resistor, R
G1
v
nRG1
(4kTR
G1
)
1/2
R
F1
/R
G1
v
nO5
= (R
F1
/R
G1
)(4kTR
G1
)
1/2
Gain Resistor, R
G2
v
nRG2
(4kTR
G2
)
1/2
R
F2
/R
G2
v
nO6
= (R
F2
/R
G2
)(4kTR
G2
)
1/2
Feedback Resistor, R
F1
v
nRF1
(4kTR
F1
)
1/2
1 v
nO7
= (4kTR
F1
)
1/2
Feedback Resistor, R
F2
v
nRF2
(4kTR
F2
)
1/2
1 v
nO8
= (4kTR
F2
)
1/2
Table 12. Differential Input, DC-Coupled
Nominal Gain (dB) R
F
(Ω) R
G
(Ω) R
IN, dm
(Ω) Differential Output Noise Density (nV/√Hz)
0 499 499 998 9.25
6
499
249
498
12.9
10 768 243 486 18.2
Table 13. Single-Ended Ground-Referenced Input, DC-Coupled, R
S
= 50 Ω
Nominal Gain (dB) R
F
(Ω) R
G1
(Ω) R
T
(Ω) (Std 1%) R
IN, cm
(Ω) R
G2
(Ω)
1
Differential Output Noise Density (nV/√Hz)
0 511 499 53.6 665 525 9.19
6 523 249 57.6 374 276 12.6
10 806 243 57.6 392 270 17.7
1
R
G2
= R
G1
+ (R
S
||R
T
).