Datasheet

ADA4927-1/ADA4927-2
Rev. A | Page 17 of 24
APPLICATIONS INFORMATION
ANALYZING AN APPLICATION CIRCUIT
The ADA4927 uses high open-loop transimpedance and negative
current feedback to control its differential output voltage in
such a way as to minimize the differential error currents. The
differential error currents are defined as the currents that flow
in and out of the differential inputs labeled +IN and −IN (see
Figure 46). For most purposes, these currents can be assumed
to be zero. The voltage between the +IN and −IN inputs is
internally bootstrapped to 0 V; therefore, the voltages at the
amplifier inputs are equal, and external analysis can be carried
out in a similar fashion to that of voltage feedback amplifiers.
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 application circuit
can be analyzed.
SETTING THE CLOSED-LOOP GAIN
Using the approach previously described, the differential gain of
the circuit in Figure 46 can be determined by
G
F
dmIN
RV
=
,
dmOUT
R
V
,
This presumes that the input resistors (R
G
) and feedback
resistors (R
F
) on each side are of equal value.
ESTIMATING THE OUTPUT NOISE VOLTAGE
The differential output noise of the ADA4927 can be estimated
using the noise model in Figure 47. 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). 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 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 each
gain resistor appears at the output multiplied by R
F
/R
G
. Table 11
summarizes the input noise sources, the multiplication factors,
and the output-referred noise density terms.
V
V
ADA4927
+
R
F2
V
nOD
V
nCM
V
OCM
V
nIN
R
F1
R
G2
R
G1
nRF1nRG1
V
nRF2
i
nIN+
i
nIN–
07574-047
V
nRG2
Figure 47. 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
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