ada4937-2 Analog Devices, Inc., ada4937-2 Datasheet - Page 19

• Manufacturer Part Number: ada4937-2
• Description: Ultralow Distortion Differential Adc Driver
• Manufacturer: Analog Devices, Inc.

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:
When R
becomes
Note that the output noise from V
The total differential output noise density, v
square of the individual output noise terms.
IMPACT OF MISMATCHES IN THE FEEDBACK
NETWORKS
As previously mentioned, even if the external feedback networks
(R
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.
As well as causing a noise contribution from V
matching errors in the external resistors 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.
In addition, if the dc levels of the input and output common-
mode voltages are different, matching errors result in a small
differential-mode output offset voltage. When G = 1, with a
ground referenced input signal and the output common-mode
level set to 2.5 V, an output offset of as much as 25 mV (1% of
the difference in common-mode levels) can result if 1% tolerance
resistors are used. Resistors of 1% tolerance result in a worst-
case input CMRR of about 40 dB, a worst-case differential-
mode output offset of 25 mV due to 2.5 V level-shift, 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 51, the input impedance (R
(+D
G
β
1
N
F
=
/R
=
IN
G
v
R
G
(
nOD
and −D
) are mismatched, the internal common-mode feedback
F1
N
β
R
1
F1
+
=
+
G1
2
/R
=
R
1
β
β
G1
G1
2
=
∑
i
)
IN
= R
=
8
1
1
and
) is simply R
is the circuit noise gain.
v
+
nOi
2
F2
R
R
/R
G
F
β
2
G2
=
, then β1 = β2 = β, and the noise gain
R
F2
IN, dm
R
+
G2
R
G2
= 2 × R
OCM
are the feedback factors.
IN, dm
goes to zero in this case.
G
) between the inputs
.
nOD
, is the root-sum-
OCM
, ratio
Rev. A | Page 19 of 28
For an unbalanced, single-ended input signal (see Figure 52),
the input impedance is
The input impedance of the circuit is effectively higher than it
would be 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 R
Terminating a Single-Ended Input
This section deals with how to properly terminate a single-
ended input to the ADA4937. Using a simple example with an
input source of 2 V and a source resistor of 50 Ω, four simple
steps must be followed.
1.
Figure 52. ADA4937 Configured for Unbalanced (Single-Ended) Input
The input impedance must be calculated using the formula
Figure 51. ADA4937 Configured for Balanced (Differential) Inputs
R
R
V
2V
IN
IN
S
,
=
cm
R
⎛
⎜
⎜
⎜
⎜ ⎜
⎝
50Ω
+D
–D
R
=
S
1
S
Figure 53. Single-Ended Input Impedance R
IN
IN
−
⎛
⎜
⎜
⎜
⎜ ⎜
⎝
1
267Ω
2
R
R
R
−
T
IN
S
×
R
R
2
G
G
(
R
×
R
G
R
G
R
(
200Ω
V
200Ω
R
V
R
R
F
R
R
OCM
G
G
R
OCM
R
+
T
G
G
+IN
–IN
G
G
F
V
ADA4937-1/ADA4937-2
R
+
ADA4937
OCM
F
R
)
+V
F
R
R
⎞
⎟
⎟
⎟
⎟ ⎟
⎠
F
S
F
)
=
R
R
ADA4937
200Ω
200Ω
ADA4937
⎞
⎟
⎟
⎟
⎟ ⎟
⎠
F
F
R
R
⎛
⎜
⎜
⎜
⎜
⎝
+V
F
F
+V
–V
1
G
S
.
S
S
−
2
×
V
OUT, dm
(
200
200
200
V
OUT, dm
IN
+
R
L
200
V
)
O
⎞
⎟
⎟
⎟
⎟
⎠
=
267
Ω