Datasheet

ADA4930-1/ADA4930-2
Rev. A | Page 18 of 28
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.