ADA4938-1_15 Datasheet, PDF (19/28 Pages) Analog Devices – Ultralow Distortion Differential ADC Driver

ADA4938-1/ADA4938-2
THEORY OF OPERATION
The ADA4938-x differs from conventional op amps in that it
has two outputs whose voltages move in opposite directions.
Like an op amp, it relies on open-loop gain and negative
feedback to force these outputs to the desired voltages. The
ADA4938-x behaves much like a standard voltage feedback op
amp and makes it easier to perform single-ended-to-differential
conversions, common-mode level shifting, and amplifications of
differential signals. Also like an op amp, the ADA4938-x has
high input impedance and low output impedance.
Two feedback loops are employed to control the differential and
common-mode output voltages. The differential feedback, set
with external resistors, controls only the differential output
voltage. The common-mode feedback controls only the common-
mode output voltage. This architecture makes it easy to set the
output common-mode level to any arbitrary value. It is forced,
by internal common-mode feedback, to be equal to the voltage
applied to the VOCM input, without affecting the differential
output voltage.
The ADA4938-x architecture results in outputs that are highly
balanced over a wide frequency range without requiring tightly
matched external components. The common-mode feedback
loop forces the signal component of the output common-
mode voltage to zero, which results in nearly perfectly balanced
differential outputs that are identical in amplitude and are
exactly 180° apart in phase.
ANALYZING AN APPLICATION CIRCUIT
The ADA4938-x uses open-loop gain and negative feedback to
force its differential and common-mode output voltages in such
a way as 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 57). 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 VOCM 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 57 can be
determined by
V_{OUT, dm = RF}
V_{IN , dm}
R_G
This assumes the input resistors (RG) and feedback resistors (RF)
on each side are equal.
ESTIMATING THE OUTPUT NOISE VOLTAGE
The differential output noise of the ADA4938 can be estimated
using the noise model in Figure 58. The input-referred noise
voltage density, vnIN, is modeled as a differential input, and the
noise currents, inIN− and inIN+, appear between each input and
ground. The noise currents are assumed to be equal and produce a
voltage across the parallel combination of the gain and feedback
resistances. vn, cm is the noise voltage density at the VOCM pin.
Each of the four resistors contributes (4kTR)^1/2. Table 9 summarizes
the input noise sources, the multiplication factors, and the output-
referred noise density terms.
V_{nRG1 RG1}
R_F1
V_nRF1
i_nIN+
i_nIN–
+
V_{nIN ADA4938}
V_nOD
V_OCM
V_nRG2 ^RG2
R_F2
V_nRF2
V_nCM
Figure 58. ADA4938 Noise Model
Table 9. Output Noise Voltage Density Calculations
Input Noise Contribution
Differential Input
Inverting Input
Noninverting Input
VOCM Input
Gain Resistor, RG1
Gain Resistor, RG2
Feedback Resistor, RF1
Feedback Resistor, RF2
Input Noise Term
vnIN
inIN−
inIN+
vn, cm
vnRG1
vnRG2
vnRF1
vnRF2
Input Noise
Voltage Density
vnIN
inIN− × (RG2||RF2)
inIN+ × (RG1||RF1)
vn, cm
(4kTRG1)^1/2
(4kTRG2)^1/2
(4kTRF1)^1/2
(4kTRF2)^1/2
Output
Multiplication Factor
GN
GN
GN
GN(β1 − β2)
GN(1 − β1)
GN(1 − β2)
1
1
Output Noise
Voltage Density Term
vnO1 = GN(vnIN)
vnO2 = GN[inIN− × (RG2||RF2)]
vnO3 = GN[inIN+ × (RG1||RF1)]
vnO4 = GN(β1 − β2)(vnCM)
vnO5 = GN(1 − β1)(4kTRG1)^1/2
vnO6 = GN(1 − β2)(4kTRG2)^1/2
vnO7 = (4kTRF1)^1/2
vnO8 = (4kTRF2)^1/2
Rev. A | Page 19 of 28