Datasheet

ADA4927-1/ADA4927-2
Rev. A | Page 18 of 24
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)
0 301 301 602 8.0
20 442 44.2 88.4 21.8
26 604 30.1 60.2 37.9
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) R
IN, cm
(Ω) R
G2
(Ω)
1
Differential Output Noise Density (nV/√Hz)
0 309 301 56.2 401 328 8.1
20 511 39.2 158 73.2 77.2 18.6
26 806 28 649 54.2 74.4 29.1
1
R
G2
= R
G1
+ (R
S
||R
T
).
Similar to the case of a conventional op amp, the output noise
voltage densities can be estimated by multiplying the input-
referred terms at +IN and −IN by the appropriate output factor,
where:
()
21
N
ββ
G
+
=
2
is the circuit noise gain.
G1
F1
G1
1
RR
β
+
=
R
and
G2
F2
G2
2
RR
β
+
=
R
are the feedback factors.
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
Rβ
G +== 1
R1
Note that the output noise from V
OCM
goes to zero in this case.
The total differential output noise density, v
nOD
, is the root-sum-
square of the individual output noise terms.
∑
=
=
1i
2
nOinOD
vv
8
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 2.5 V, a differential offset voltage is present at the
output of (2.5 V)(0.67) = 1.67 V. The differential output noise
contribution is (15 nV/√Hz)(0.67) = 10 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 25 mV
due to a 2.5 V V
OCM
input, negligible V
OCM
noise contribution,
and no significant degradation in output balance error.
CALCULATING THE INPUT IMPEDANCE FOR AN
APPLICATION CIRCUIT
The effective input impedance of a circuit depends on whether
the amplifier is being driven by a single-ended or differential
signal source. For balanced differential input signals, as shown
in Figure 48, the input impedance (R
IN, dm
) between the inputs
(+D
IN
and −D
IN
) is simply R
IN, dm
= R
G
+ R
G
= 2 × R
G
.
+V
S
–V
S
ADA4927
+IN
–IN
R
F
R
F
+D
IN
–D
IN
V
OCM
R
G
R
G
V
OUT, dm
07574-048
Figure 48. The ADA4927 Configured for Balanced (Differential) Inputs