Datasheet
ADA4927-1/ADA4927-2
Rev. A | Page 19 of 24
For an unbalanced, single-ended input signal (see Figure 49),
the input impedance is
()
⎟
⎟
⎟
⎟
⎞
⎜
⎛
⎠
⎜
⎜
⎜
⎝
+×
−
=
F
G
F
G
SEIN
RR
R
R
R
2
1
,
ADA4927
R
L
V
OUT, dm
+V
S
–V
S
R
G
R
G
R
F
R
F
V
OCM
R
IN, SE
0
7574-049
Figure 49. The ADA4927 with Unbalanced (Single-Ended) Input
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
G
. 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
formed by R
F
and R
G
in the lower loop. This voltage is present at
both input terminals due to negative voltage feedback and is in
phase with the input signal, thus reducing the effective voltage
across R
G
in the upper loop and partially bootstrapping R
G
.
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
F
= 348 Ω, and
R
G
= 348 Ω. An example using an input source with a terminated
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
formula:
Ω464
)348348(2
348
1
348
)(2
1
=
⎟
⎟
⎟
⎠
⎜
⎜
⎜
⎝
+×
−
=
⎟
⎟
⎟
⎠
⎜
⎜
⎜
⎝
+×
−
=
FG
F
G
IN
RR
R
R
R
⎟
⎞
⎜
⎛
⎟
⎞
⎜
⎛
R
S
50Ω
V
S
2V p-p
R
IN
464Ω
ADA4927
R
L
V
OUT, dm
+V
S
–V
S
R
F
348Ω
R
G
348Ω
R
G
348Ω
V
OCM
R
F
348Ω
07574-050
Figure 50. Calculating Single-Ended Input Impedance R
IN
2. To match the 50 Ω source resistance, the termination
resistor, R
T
, is calculated using R
T
||464 Ω = 50 Ω. The
closest standard 1% value for R
T
is 56.2 Ω.
ADA4927
R
L
V
OUT, dm
+V
S
–V
S
R
F
348Ω
R
S
50Ω
V
S
2V p-p
R
IN
50Ω
R
G
348Ω
R
G
348Ω
R
T
56.2Ω
V
OCM
R
F
348Ω
0
7574-051
Figure 51. Adding Termination Resistor R
T
3. It can be seen from Figure 51 that the effective R
G
in the
upper feedback loop is now greater than the R
G
in the
lower loop due to the addition of the termination resistors.
To compensate for the imbalance of the gain resistors,
a correction resistor (R
TS
) is added in series with R
G
in the
lower loop. R
TS
is equal to the Thevenin equivalent of the
source resistance R
S
and the termination resistance R
T
and
is equal to R
S
||R
T
.
R
S
50Ω
V
S
2
V p-
p
R
TH
26.5Ω
R
T
56.2Ω
V
TH
1.06V p-p
0
7574-052
Figure 52. Calculating the Thevenin Equivalent