ADA4927-1/ADA4927-2
Table 12. Differential Input, DC-Coupled
Nominal Gain (dB)
RF (Ω)
RG (Ω)
RIN, dm (Ω)
602
Differential Output Noise Density (nV/√Hz)
0
301
301
8.0
20
26
442
604
44.2
30.1
88.4
60.2
21.8
37.9
Table 13. Single-Ended Ground-Referenced Input, DC-Coupled, RS = 50 Ω
Nominal Gain (dB)
RF (Ω) RG1 (Ω)
RT (Ω) RIN, cm (Ω)
RG2 (Ω)1
328
77.2
74.4
Differential Output Noise Density (nV/√Hz)
0
20
26
309
511
806
301
39.2
28
56.2
158
649
401
73.2
54.2
8.1
18.6
29.1
1 RG2 = RG1 + (RS||RT).
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,
The feedback loops are nominally matched to within 1% in
most applications, and the output noise and offsets due to the
VꢀCM input are negligible. If the loops are intentionally mismatched
by a large amount, it is necessary to include the gain term from
where:
2
VꢀCM to Vꢀ, dm and account for the extra noise. For example, if
GN
=
is the circuit noise gain.
β1 = 0.5 and β2 = 0.25, the gain from VꢀCM to Vꢀ, dm is 0.67. If the
VꢀCM 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.
(
β1 + β2
RG1
)
RG2
β1 =
and β2 =
are the feedback factors.
RF1 + RG1
RF2 + RG2
When the feedback factors are matched, RF1/RG1 = RF2/RG2,
β1 = β2 = β, and the noise gain becomes
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.
1
β
RF
RG
GN
=
=1+
Note that the output noise from VꢀCM goes to zero in this case.
The total differential output noise density, vnꢀD, is the root-sum-
square of the individual output noise terms.
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 VꢀCM input, negligible VꢀCM noise contribution,
and no significant degradation in output balance error.
8
vnOD
=
v2
nOi
∑
i=1
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.
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 (RIN, dm) between the inputs
(+DIN and −DIN) is simply RIN, dm = RG + RG = 2 × RG.
IMPACT OF MISMATCHES IN THE FEEDBACK
NETWORKS
As previously mentioned, even if the external feedback networks
(RF/RG) 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.
R
F
+V
S
R
G
G
+IN
+D
–D
IN
V
OCM
V
ADA4927
The gain from the VꢀCM pin to Vꢀ, dm is equal to
2(β1 − β2)/(β1 + β2)
OUT, dm
IN
–IN
R
–V
S
When β1 = β2, this term goes to zero and there is no differential
output voltage due to the voltage on the VꢀCM input (including
noise). The extreme case occurs when one loop is open and the
other has 100% feedback; in this case, the gain from VꢀCM input
to Vꢀ, dm is either +2 or −2, depending on which loop is closed.
R
F
Figure 48. The ADA4927 Configured for Balanced (Differential) Inputs
Rev. 0 | Page 18 of 24