ADA4932-1_16 Datasheet, PDF (21/27 Pages) Analog Devices – Low Power, Differential ADC Driver

Data Sheet
ADA4932-1/ADA4932-2
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:
G_N

2
β₁  β₂ 
is the circuit noise gain.
β₁

R_G1
R_F1  R_G1
and
β₂

R_G2
R_F2  R_G2
are the feedback factors.
When the feedback factors are matched, RF1/RG1 = RF2/RG2, β1 =
β2 = β, and the noise gain becomes
G_N

1
β
1
R_F
R_G
Note that the output noise from VOCM goes to zero in this case.
The total differential output noise density, vnOD, is the root-sum-
square of the individual output noise terms.
8
 ^v_nOD ^
v
2
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.
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.
The gain from the VOCM/VOCMx pin to VOUT, 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 VOCM input (including
noise). The extreme case occurs when one loop is open and the
other has 100% feedback; in this case, the gain from VOCM input
to VOUT, 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
VOCM input are negligible. If the loops are intentionally mismatched
by a large amount, it is necessary to include the gain term from
VOCM to VOUT, dm and account for the extra noise. For example, if
β1 = 0.5 and β2 = 0.25, the gain from VOCM to VOUT, dm is 0.67. If
the VOCM/VOCMx 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 (9.6 nV/√Hz)(0.67) = 6.4 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 above 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 VOCM input, negligible VOCM 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 56, the input impedance (RIN, dm) between the inputs
(+DIN and −DIN) is RIN, dm = RG + RG = 2 × RG.
R_F
+V_S
+D_IN
–D_IN
R_G
+IN
V_{OCM ADA4932-1/}
ADA4932-2
R_G
–IN
V_{OUT, dm}
–V_S
R_F
Figure 56. ADA4932-1/ADA4932-2 Configured for Balanced (Differential) Inputs
For an unbalanced, single-ended input signal (see Figure 57),
the input impedance is


  ^{RIN, se}





1

2

R_G
R_F
R_G 
R_F




R_F
R_{IN, se}
R_G
+V_S
V_OCM
R_G
ADA4932-1/
ADA4932-2
R_L V_{OUT, dm}
–V_S
R_F
Figure 57. The ADA4932-1/ADA4932-2 with Unbalanced (Single-Ended) Input
The input impedance of the circuit is effectively higher than it is
for a conventional op amp connected as an inverter because a
fraction of the differential output voltage appears at the inputs
as a common-mode signal, partially bootstrapping the voltage
across the input resistor, RG. The common-mode voltage at the
amplifier input terminals can be easily determined by noting that
the voltage at the inverting input is equal to the noninverting
output voltage divided down by the voltage divider that is formed
by RF and RG in the lower loop. This voltage is present at both
input terminals due to negative voltage feedback and is in phase
Rev. E | Page 21 of 27