ada4927 Analog Devices, Inc., ada4927 Datasheet - Page 19

• Manufacturer Part Number: ada4927
• Description: Ultralow Distortion Current Feedback Differential Adc Driver
• Manufacturer: Analog Devices, Inc.

For an unbalanced, single-ended input signal (see Figure 49),
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
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
formed by R
both input terminals due to negative voltage feedback and is in
phase with the input signal, thus reducing the effective voltage
across R
Terminating a Single-Ended Input
This section deals with how to properly terminate a single-
ended input to the ADA4927 with a gain of 1, R
R
output voltage of 1 V p-p and a source resistance of 50 Ω illustrates
the four simple steps that must be followed. Note that, because
the terminated output voltage of the source is 1 V p-p, the open
circuit output voltage of the source is 2 V p-p. The source shown
in Figure 50 indicates this open-circuit voltage.
1. The input impedance must be calculated using the following
G
formula:
= 348 Ω. An example using an input source with a terminated
R
R
Figure 49. The ADA4927 with Unbalanced (Single-Ended) Input
IN
IN
G
,
R
=
SE
in the upper loop and partially bootstrapping R
IN, SE
⎛
⎜
⎜
⎜
⎜
⎝
=
1
F
⎛
⎜
⎜
⎜
⎜
⎝
−
and R
1
2
−
V
R
R
×
OCM
G
G
2
(
R
G
×
R
G
R
in the lower loop. This voltage is present at
R
(
G
R
F
G
R
+
G
F
+
R
ADA4927
F
R
R
R
)
F
F
F
+V
–V
⎞
⎟
⎟
⎟
⎟
⎠
)
S
S
=
⎞
⎟
⎟
⎟
⎟
⎠
⎛
⎜
⎜
⎜
⎜
⎝
1
−
G
2
. The common-mode
×
(
R
348
L
348
V
348
OUT, dm
F
+
= 348 Ω, and
348
G
.
)
⎞
⎟
⎟
⎟
⎟
⎠
=
Rev. 0 | Page 19 of 24
464
Ω
3.
2.
2V p-p
2V p-p
V
S
To match the 50 Ω source resistance, the termination
resistor, R
closest standard 1% value for R
It can be seen from Figure 51 that the effective R
upper feedback loop is now greater than the R
lower loop due to the addition of the termination resistors.
To compensate for the imbalance of the gain resistors,
a correction resistor (R
lower loop. R
source resistance R
is equal to R
V
S
Figure 50. Calculating Single-Ended Input Impedance R
50Ω
R
S
2V p-p
50Ω
R
Figure 52. Calculating the Thevenin Equivalent
S
50Ω
V
R
T
Figure 51. Adding Termination Resistor R
S
IN
, is calculated using R
464Ω
56.2Ω
R
S
||R
TS
IN
R
50Ω
R
T
is equal to the Thevenin equivalent of the
T
S
.
348Ω
348Ω
V
R
R
S
OCM
G
G
348Ω
V
348Ω
and the termination resistance R
R
R
OCM
R
56.2Ω
G
G
T
ADA4927-1/ADA4927-2
TS
) is added in series with R
348Ω
348Ω
ADA4927
R
R
348Ω
ADA4927
348Ω
F
F
1.06V p-p
+V
–V
T
R
R
F
T
F
is 56.2 Ω.
+V
–V
S
S
||464 Ω = 50 Ω. The
S
S
V
TH
26.5Ω
R
TH
T
R
L
R
G
L
V
in the
G
OUT, dm
IN
in the
G
V
OUT, dm
in the
T
and