AD8476 AD [Analog Devices], AD8476 Datasheet - Page 17

• Manufacturer Part Number: AD8476
• Description: Low Power, Unity Gain, Fully
• Manufacturer: AD [Analog Devices]

Data Sheet
THEORY OF OPERATION
OVERVIEW
The
trimmed resistors, that provides a precision gain of 1. The
internal differential amplifier of the
conventional operational amplifiers in that it has two outputs
whose voltages are equal in magnitude, but move in opposite
directions (180° out of phase).
The
differential conversion, common-mode level shifting and
precision driving of differential signals into low power,
differential input ADCs. The VOCM input allows the user to
set the output common-mode voltage to match with the input
range of the ADC. Like an operational amplifier, the VOCM
function relies on high open-loop gain and negative feedback to
force the output nodes to the desired voltages.
CIRCUIT INFORMATION
The
therefore, the amplifier exhibits a nominally constant gain
bandwidth product. Like a voltage feedback operational
amplifier, the
internal input terminals (the summing nodes of the internal
amplifier) and low output impedance.
The
the differential and common-mode output voltages. The differen-
tial feedback loop, which is fixed with precision laser-trimmed
on-chip resistors, controls the differential output voltage.
Output Common-Mode Voltage (VOCM)
The internal common-mode feedback controls the common-
mode output voltage. This architecture makes it easy for the
user to set the output common-mode level to any arbitrary
value independent of the input voltage. The output common-
mode voltage is forced by the internal common-mode feedback
loop to be equal to the voltage applied to the VOCM input. The
VOCM pin can be left unconnected, and the output common-
mode voltage self-biases to midsupply by the internal feedback
control.
AD8476
AD8476
AD8476
AD8476
is a fully differential amplifier, with integrated laser-
is designed to greatly simplify single-ended-to-
amplifier uses a voltage feedback topology;
employs two feedback loops, one each to control
AD8476
NOTES
1. NC = NO CONNECT.
DO NOT CONNECT TO THIS PIN.
Figure 51. Block Diagram
10kΩ
10kΩ
also has high input impedance at its
AD8476
AD8476
10kΩ
10kΩ
differs from
Rev. B | Page 17 of 24
Due to the internal common-mode feedback loop and the fully
differential topology of the amplifier, the
precisely balanced over a wide frequency range. This means that
the amplifier’s differential outputs are very close to the ideal of
being identical in amplitude and exactly 180° out of phase.
DC PRECISION
The dc precision of the
accuracy of its integrated gain resistors. Using superposition to
analyze the circuit shown in Figure 52, the following equation
shows the relationship between the input and output voltages of
the amplifier:
where:
The differential closed-loop gain of the amplifier is
and the common rejection of the amplifier is
The preceding equations show that the gain accuracy and the
common-mode rejection (CMRR) of the
mined primarily by the matching of the feedback networks
(resistor ratios). If the two networks are perfectly matched, that
is, if R
generate any CMRR errors and the differential closed loop gain
of the amplifier reduces to
Figure 52. Functional Circuit Diagram of the
V
V
V
=
R
V
V
v
V
V
P
v
OUT
IN
P
IN
IN
V
OUT
OUT
IN
and R
IN
IN
,
OUT
=
,
,
cm
dm
cm
,
,
,
dm
dm
cm
,
,
,
dm
RGP
dm
dm
(
RFP
=
R
,
=
cm
P
N
V
1
2
=
=
=
(
−
equal RF/RG, then the resistor network does not
R
(
P
2
2
RG
V
RF
2
,
V
V
P
R
−
R
(
+
P
N
P
R
R
N
−
V
P
2
+
R
N
R
P
VOCM
)
R
N
+
V
P
+
N
−
N
=
R
+
V
N
RGP
RGN
R
)
+
AD8476
P
RGN
RFN
)
IN
R
+
N
R
+
N
V
,
dm
)
P
R
OUT
+
N
1
2
R
RFN
(
,
RFP
dm
2
N
is highly dependent on the
R
1
2
P
R
(
2
N
+
+
R
AD8476
R
AD8476
AD8476
V
V
P
ON
OP
P
+
+
R
R
N
N
at a Given Gain
)
)
outputs are
are deter-
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