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 VOCM 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
dm
IN
R
V
=
,
dm
OUT
R
V
,
This presumes that the input resistors (RG) and feedback
resistors (RF) 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, vnIN, is modeled as a differential input, and the
noise currents, inIN and inIN+, appear between each input and
ground. The output voltage due to vnIN is obtained by multiplying
vnIN by the noise gain, GN (defined in the GN 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 VOCM pin is vnCM. When the feedback
networks have the same feedback factor, as in most cases, the
output noise due to vnCM is common mode. Each of the four
resistors contributes (4kTRxx)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 RF/RG. Table 11 summarizes the input noise sources, the multiplication factors,
and the output-referred noise density terms.
V
ADA4927
+
RF2
VnOD
VnCM
VOCM
VnIN
RF1
RG2
RG1
nRF1
nRG1
VnRF2
inIN+
inIN–
07
57
4-
0
47
VnRG2
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
vnIN
GN
vnO1 = GN(vnIN)
Inverting Input
inIN
inIN × (RF2)
1
vnO2 = (inIN)(RF2)
Noninverting Input
inIN
inIN × (RF1)
1
vnO3 = (inIN)(RF1)
VOCM Input
vnCM
0
vnO4 = 0
Gain Resistor, RG1
vnRG1
(4kTRG1)1/2
RF1/RG1
vnO5 = (RF1/RG1)(4kTRG1)1/2
Gain Resistor, RG2
vnRG2
(4kTRG2)1/2
RF2/RG2
vnO6 = (RF2/RG2)(4kTRG2)1/2
Feedback Resistor, RF1
vnRF1
(4kTRF1)1/2
1
vnO7 = (4kTRF1)1/2
Feedback Resistor, RF2
vnRF2
(4kTRF2)1/2
1
vnO8 = (4kTRF2)1/2