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ADA4930-1YCP-EBZ

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24P25-P25
ADA4930-1/ADA4930-2 Data Sheet
Rev. B | Page 16 of 25
OPERATIONAL DESCRIPTION
DEFINITION OF TERMS
+IN
–IN
+OUT
–OUT
+D
IN
–
FB
+FB
–D
IN
V
OCM
R
G
R
F
R
G
V
OUT, dm
R
L, dm
R
F
ADA4930
09209-049
Figure 42. Circuit Definitions
Differential Voltage
Differential voltage refers to the difference between two
node voltages. For example, the output differential voltage (or,
equivalently, output differential-mode voltage) is defined as
V
OUT, dm
= (V
+OUT
− V
−OUT
)
where V
+OUT
and V
−OUT
refer to the voltages at the +OUT and
−OUT terminals with respect to a common reference.
Common-Mode Voltage
Common-mode voltage refers to the average of two node voltages.
The output common-mode voltage is defined as
V
OUT, cm
= (V
+OUT
+ V
−OUT
)/2
Balance
Output balance is a measure of how close the differential signals are
to being equal in amplitude and opposite in phase. Output balance
is most easily determined by placing a well-matched resistor
divider between the differential voltage nodes and comparing the
magnitude of the signal at the divider midpoint with the magnitude
of the differential signal (see Figure 39). By this definition, output
balance is the magnitude of the output common-mode voltage
divided by the magnitude of the output differential mode voltage.
dmOUT
cmOUT
V
V
ErrorBalanceOutput
,
,
Data Sheet ADA4930-1/ADA4930-2
Rev. B | Page 17 of 25
THEORY OF OPERATION
The ADA4930-1/ADA4930-2 differ from conventional op amps
in that they have two outputs whose voltages move in opposite
directions and an additional input, V
OCM
. Like an op amp, they rely
on high open-loop gain and negative feedback to force these
outputs to the desired voltages. The ADA4930-1/ADA4930-2
behave much like standard voltage feedback op amps and facilitate
single-ended-to-differential conversions, common-mode level
shifting, and amplifications of differential signals. Like op amps,
the ADA4930-1/ADA4930-2 have high input impedance and low
output impedance.
Two feedback loops control the differential and common-mode
output voltages. The differential feedback, set with external
resistors, controls the differential output voltage. The common-
mode feedback controls the common-mode output voltage. This
architecture makes it easy to set the output common-mode level
to any arbitrary value within the specified limits. The output
common-mode voltage is forced to be equal to the voltage applied
to the V
OCM
input by the internal common-mode feedback loop.
The internal common-mode feedback loop produces outputs
that are highly balanced over a wide frequency range without
requiring tightly matched external components. This results
in differential outputs that are very close to the ideal of being
identical in amplitude and are exactly 180° apart in phase.
ANALYZING AN APPLICATION CIRCUIT
The ADA4930-1/ADA4930-2 use high open-loop gain and
negative feedback to force their differential and common-mode
output voltages 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 42). 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 two assumptions,
any application circuit can be analyzed.
SETTING THE CLOSED-LOOP GAIN
The differential-mode gain of the circuit in Figure 42 is
determined by
G
F
dmIN
dmOUT
R
R
V
V
,
,
where the gain and feedback resistors,
R
G
and R
F
, on each side
are equal.
ESTIMATING THE OUTPUT NOISE VOLTAGE
The differential output noise of the ADA4930-1/ADA4930-2 can
be estimated using the noise model in Figure 43. The input-referred
noise voltage density, v
nIN
, is modeled as differential. The noise
currents, i
nIN−
and i
nIN+
, appear between each input and ground.
ADA4930
+
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–
09209-050
Figure 43. Noise Model
Similar to the case of conventional op amps, the output noise
voltage densities can be estimated by multiplying the input-
referred terms at +IN and −IN by an appropriate output factor.
The output voltage due to v
nIN
is obtained by multiplying v
nIN
by
the noise gain, G
N
.
The circuit noise gain is

21
N
ββ
G
2
where the feedback factors are
G1
F1
G1
1
RR
R
β
and
G2
F2
G2
2
RR
R
β
.
When the feedback factors are matched, R
F1
/R
G1
= R
F2
/R
G2
,
β1 = β2 = β, and the noise gain becomes
G
F
N
R
R
β
G 1
1
.
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 and the
output noise from V
OCM
is zero.
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 multiplied
by R
F
/R
G
.
The total differential output noise density, v
nOD
, is the root-sum-
square of the individual output noise terms.
8
1i
2
)(
nODinOD
vv
ADA4930-1/ADA4930-2 Data Sheet
Rev. B | Page 18 of 25
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 Terms
Differential Input v
nIN
v
nIN
G
N
v
nOD1
= G
N
(v
nIN
)
Inverting Input i
nIN+
i
nIN+
× (R
F2
) 1 v
nOD2
= (i
nIN+
)(R
F2
)
Noninverting Input i
nIN−
i
nIN−
× (R
F1
) 1 v
nOD3
= (i
nIN−
)(R
F1
)
V
OCM
Input v
nCM
v
nCM
0 v
nOD4
= 0
Gain Resistor R
G1
v
nRG1
(4kTR
G1
)
1/2
R
F1
/R
G1
v
nOD5
= (R
F1
/R
G1
)(4kTR
G1
)
1/2
Gain Resistor R
G2
v
nRG2
(4kTR
G2
)
1/2
R
F2
/R
G2
v
nOD6
= (R
F2
/R
G2
)(4kTR
G2
)
1/2
Feedback Resistor R
F1
v
nRF1
(4kTR
F1
)
1/2
1 v
nOD7
= (4kTR
F1
)
1/2
Feedback Resistor R
F2
v
nRF2
(4kTR
F2
)
1/2
1 v
nOD8
= (4kTR
F2
)
1/2
Table 12. Differential Input, DC-Coupled, V
S
= 5 V
Nominal Gain (dB) R
F1
, R
F2
(Ω) R
G1
, R
G2
(Ω) R
IN, dm
(Ω) Differential Output Noise Density (nV/√Hz)
0 301 301 602 4.9
6 301 150 300 6.2
10 301 95.3 190.6 7.8
14 301 60.4 120.4 10.1
Table 13. Single-Ended Ground-Referenced Input, DC-Coupled, R
S
= 50 Ω, V
S
= 5 V
Nominal Gain (dB) R
F1
, R
F2
(Ω) R
G1
(Ω) R
T
(Ω) R
IN, cm
(Ω) R
G2
(Ω)
1
Differential Output Noise Density (nV/√Hz)
0 301 142 64.2 190.67 170 5.9
6 301 63.4 84.5 95.06 95 7.8
10 301 33.2 1 k 53.54 69.3 9.3
14 301 10.2 1.15 k 17.5 57.7 10.4
1
R
G2
= R
G1
+ (R
S
||R
T
).
Table 11 summarizes the input noise sources, the multiplication
factors, and the output-referred noise density terms.
Table 12 and Table 13 list several common gain settings, associated
resistor values, input impedance, and output noise density for both
balanced and unbalanced input configurations.
IMPACT OF MISMATCHES IN THE FEEDBACK
NETWORKS
As previously mentioned, even if the external feedback networks
(R
F
/R
G
) are mismatched, the internal common-mode feedback
loop still forces the outputs to remain balanced. The amplitudes
of the signals at each output remain equal and 180° out of phase.
The input-to-output differential mode gain varies proportionately
to the feedback mismatch, but the output balance is unaffected.
The gain from the V
OCM
pin to V
O, dm
is equal to
2(β1 − β2)/(β1 + β2)
When β1 = β2, this term goes to zero and there is no differential
output voltage due to the voltage on the V
OCM
input (including
noise). The extreme case occurs when one loop is open and the
other has 100% feedback; in this case, the gain from V
OCM
input
to V
O, dm
is either +2 or −2, depending on which loop is closed. The
feedback loops are nominally matched to within 1% in most
applications, and the output noise and offsets due to the V
OCM
input are negligible. If the loops are intentionally mismatched by a
large amount, it is necessary to include the gain term from V
OCM
to V
O, dm
and account for the extra noise. For example, if β1 = 0.5
and β2 = 0.25, the gain from V
OCM
to V
O, dm
is 0.67. If the V
OCM
pin
is set to 0.9 V, a differential offset voltage is present at the output of
(0.9 V)(0.67) = 0.6 V. The differential output noise contribution is
(5 nV/√Hz)(0.67) = 3.35 nV/√Hz. Both of these results are
undesirable in most applications; therefore, it is best to use
nominally matched feedback factors.
Mismatched feedback networks also result in a degradation of
the ability of the circuit to reject input common-mode signals,
much the same as for a four-resistor difference amplifier made
from a conventional op amp.
As a practical summarization of the previous issues, resistors of
1% tolerance produce a worst-case input CMRR of approximately
40 dB, a worst-case differential-mode output offset of 9 mV due
to a 0.9 V V
OCM
input, negligible V
OCM
noise contribution, and
no significant degradation in output balance error.
INPUT COMMON-MODE VOLTAGE RANGE
The input common-mode range at the summing nodes of the
ADA4930-1/ADA4930-2 is specified as 0.3 V to 1.5 V at V
S
= 3.3 V.
To avoid nonlinearities, the voltage swing at the +IN and −IN
terminals must be confined to these ranges.
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ADA4930-1YCP-EBZ

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Amplifier IC Development Tools ADA4930-1 Eval Brd
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