AD8137 Analog Devices, AD8137 Datasheet - Page 19

• Manufacturer Part Number: AD8137
• Description: Low Cost, Low Power 12-Bit Differential ADC Driver
• Manufacturer: Analog Devices

The differential output voltage noise contains contributions
from the AD8137’s input voltage noise and input current noise
as well as those from the external feedback networks.
The contribution from the input voltage noise spectral density
is computed as
where v
noise. This equation is the same as that of traditional op amps.
The contribution from the input current noise of each input is
computed as
where i
input needs to be treated separately since the two input currents
are statistically independent processes.
The contribution from each R
This result can be intuitively viewed as the thermal noise of
each R
The contribution from each R
Voltage Gain
The behavior of the node voltages of the single-ended-to-
differential output topology can be deduced from the signal
definitions and Figure 63. Referring to Figure 63, (C
setting V
Solving the above two equations and setting V
gain relationship for V
An inverting configuration with the same gain magnitude can
be implemented by simply applying the input signal to V
setting V
V
IN, dm
Vo_n
Vo_n =
Vo_n
Vo_n
V
V
V
to V
OP
G
AN
IP
n
n
multiplied by the magnitude of the differential gain.
is defined as the input noise current of one input. Each
R
IN
IP
is defined as the input-referred differential voltage
−
−
=
G
O, dm
1
V
2
3
4 =
= 0. For a balanced differential input, the gain from
= 0 one can write:
V
V
=
AP
=
ON
AP
v
i
is also equal to R
n
n
=
k 4
=
=
( )
k 4
⎛
⎜
⎝
R
V
1
V
V
F
TR
TR
+
AP
O,
OP
R
R
R
G
dm
F
−
⎡
⎢
⎣
O, dm
F
G
F
V
⎛
⎜
⎝
R
⎞
⎟
⎠
ON
F
=
R
R
/V
, or equivalently, v
R
G
F
+
R
R
G
G
⎞
⎟
⎠
i
F
R
.
G
F
G
V
is computed as
is computed as
F
⎤
⎥
⎦
i
/R
G
, where V
n
/β
IN, dm
IP
to V
= V
F
i
= 0) and
gives the
IP
− V
IN
and
IN
(10)
(11)
(12)
(13)
Rev. B | Page 19 of 24
.
(7)
(8)
(9)
Feedback Factor Notation
When working with differential drivers, it is convenient to
introduce the feedback factor β, which is defined as
This notation is consistent with conventional feedback analysis
and is very useful, particularly when the two feedback loops are
not matched.
Input Common-Mode Voltage
The linear range of the V
approximately 1 V of either supply rail. Since V
essentially equal to each other, they are both equal to the
amplifier’s input common-mode voltage. Their range is
indicated in the specifications tables as input common-mode
range. The voltage at V
in Figure 63 can be expressed as
where V
amplifier input terminals.
Using the β notation, Equation (15) can be written as
or equivalently,
where V
that is
For proper operation, the voltages at V
within their respective linear ranges.
Calculating Input Impedance
The input impedance of the circuit in Figure 63 depends on
whether the amplifier is being driven by a single-ended or a
differential signal source. For balanced differential input
signals, the differential input impedance (R
For a single-ended signal (for example, when V
and the input signal drives V
β
V
⎛
⎜
⎝
V
V
V
R
R
R
ICM
IN
AN
ACM
ACM
IN,
≡
F
ACM
ICM
R
R
dm
=
=
+
≡
F
F
=
=
1
is the common-mode voltage of the input signal,
V
R
R
is the common-mode voltage present at the
=
+
V
−
β
V
G
G
AP
IP
V
R
ICM
2
( 2
×
G
OCM
R
=
R
+
2
(
R
G
V
G
V
V
G
+
R
IP
ACM
F
+
IN
β
+
R
+
(
2
V
(
1
F
AN
V
OCM
=
)
−
IN
AN
and V
β
)
and V
)
V
⎞
⎟
⎠
−
IP
+
ICM
V
), the input impedance becomes
⎛
⎜
⎝
AP
ICM
R
AP
for the connection diagram
F
)
R
+
terminals extends to within
G
R
AN
G
and V
×
V
IN, dm
OCM
AN
AP
) is simply
IN
⎞
⎟
⎠
is grounded,
must stay
and V
AD8137
AP
are
(14)
(15)
(16)
(17)
(18)
(19)