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AD8137YCP-R2 数据表(PDF) 19 Page - Analog Devices

部件名 AD8137YCP-R2
功能描述  Low Cost, Low Power 12-Bit Differential ADC Driver
PDF  24 Pages
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制造商  AD [Analog Devices]
网页  http://www.analog.com
标志 AD - Analog Devices

AD8137YCP-R2 数据表(HTML) 19 Page - Analog Devices

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AD8137
Rev. B | Page 19 of 24
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
⎟
⎠
⎞
⎜
⎝
⎛
+
=
G
F
n
R
R
v
Vo_n
1
1
, or equivalently,
vn/β
(7)
where
vn is defined as the input-referred differential voltage
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
( )
F
n R
i
Vo_n
=
2
(8)
where in is defined as the input noise current of one input. Each
input needs to be treated separately since the two input currents
are statistically independent processes.
The contribution from each RG is computed as
⎟
⎠
⎞
⎜
⎝
⎛
=
G
F
G
R
R
TR
Vo_n
k
4
3
(9)
This result can be intuitively viewed as the thermal noise of
each RG multiplied by the magnitude of the differential gain.
The contribution from each RF is computed as
F
TR
Vo_n
k
4
4
=
(10)
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, (CF = 0) and
setting VIN = 0 one can write:
F
ON
AP
G
AP
IP
R
V
V
R
V
V
−
=
−
(11)
⎥
⎦
⎤
⎢
⎣
⎡
+
=
=
G
F
G
OP
AP
AN
R
R
R
V
V
V
(12)
Solving the above two equations and setting VIP to Vi gives the
gain relationship for VO, dm/Vi.
i
G
F
dm
O,
ON
OP
V
R
R
V
V
V
=
=
−
(13)
An inverting configuration with the same gain magnitude can
be implemented by simply applying the input signal to VIN and
setting VIP = 0. For a balanced differential input, the gain from
VIN, dm to VO, dm is also equal to RF/RG, where VIN, dm = VIP − VIN.
Feedback Factor Notation
When working with differential drivers, it is convenient to
introduce the feedback factor β, which is defined as
G
F
G
R
R
R
+
≡
β
(14)
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 VAN and VAP terminals extends to within
approximately 1 V of either supply rail. Since VAN and VAP are
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 VAN and VAP for the connection diagram
in Figure 63 can be expressed as
=
=
=
ACM
AP
AN
V
V
V
⎟
⎠
⎞
⎜
⎝
⎛
×
+
+
⎟
⎠
⎞
⎜
⎝
⎛
+
×
+
OCM
G
F
G
IN
IP
G
F
F
V
R
R
R
V
V
R
R
R
2
)
(
(15)
where VACM is the common-mode voltage present at the
amplifier input terminals.
Using the β notation, Equation (15) can be written as
( )
ICM
OCM
ACM
V
V
V
β
−
+
β
=
1
(16)
or equivalently,
(
)
ICM
OCM
ICM
ACM
V
V
V
V
−
β
+
=
(17)
where VICM is the common-mode voltage of the input signal,
that is
2
IN
IP
ICM
V
V
V
+
≡
For proper operation, the voltages at VAN and VAP must stay
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 (RIN, dm) is simply
G
dm
IN,
R
R
2
=
(18)
For a single-ended signal (for example, when VIN is grounded,
and the input signal drives VIP), the input impedance becomes
)
(
2
1
F
G
F
G
IN
R
R
R
R
R
+
−
=
(19)



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