ad8475 Analog Devices, Inc., ad8475 Datasheet - Page 17

• Manufacturer Part Number: ad8475
• Description: Precision, Selectable Gain, Fully Differential Funnel Amplifier Ad8475
• Manufacturer: Analog Devices, Inc.

THEORY OF OPERATION
OVERVIEW
The AD8475 is a fully differential amplifier, with integrated laser-
trimmed resistors, that provides precision attenuating gains of
0.4 and 0.8. The internal differential amplifier of the AD8475
differs from conventional operational amplifiers in that it has
two outputs whose voltages are equal in magnitude, but move in
opposite directions (180° out of phase). An additional input,
VOCM, sets the output common-mode voltage. Like an opera-
tional amplifier, it relies on high open-loop gain and negative
feedback to force the output nodes to the desired voltages. The
AD8475 is designed to greatly simplify single-ended-to-differential
conversion, common-mode level shifting and precision attenua-
tion of large signals so that they are compatible with low voltage,
differential input ADCs.
CIRCUIT INFORMATION
The AD8475 amplifier uses a voltage feedback topology;
therefore, the amplifier exhibits a nominally constant gain
bandwidth product. Like a voltage feedback operational
amplifier, the AD8475 also has high input impedance at its
internal input terminals (the summing nodes of the internal
amplifier) and low output impedance.
The AD8475 employs two feedback loops, one each to control
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 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.
Due to the internal common-mode feedback loop and the fully
differential topology of the amplifier, the AD8475 outputs are
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.
–IN 0.8x –IN 0.4x
+IN 0.8x +IN 0.4x
1.25kΩ
1.25kΩ
Figure 49. Block Diagram
1.25kΩ
1.25kΩ
–V
+V
S
S
VOCM
AD8475
NC
1kΩ
1kΩ
–OUT
+OUT
Rev. A | Page 17 of 24
DC PRECISION
The dc precision of the AD8475 is highly dependent on the
accuracy of its internal resistors. Using superposition to analyze
the circuit shown in Figure 50, 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 AD8475 are deter-
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
The AD8475’s integrated resistors are precision wafer-laser-
trimmed to guarantee a minimum CMRR of 86dB (50μV/V),
and gain error of less that 0.05%. To achieve equivalent precision
and performance using a discrete solution, resistors must be
matched to 0.01% or better.
Figure 50. Functional Circuit Diagram of the AD8475 at a Given Gain
V
V
V
=
R
V
V
v
V
V
P
v
IN
IN
OUT
P
IN
OUT
V
OUT
IN
and R
IN
IN
,
OUT
=
,
,
cm
cm
dm
,
,
,
dm
dm
cm
,
,
,
dm
dm
dm
RGP
(
RFP
=
R
=
,
cm
P
N
V
1
2
=
=
=
(
−
R
equal RF/RG, then the resistor network does not
(
P
2
2
RG
V
RF
2
,
P
R
−
R
V
V
(
+
P
R
R
P
N
N
−
V
P
2
+
R
N
R
P
)
R
N
+
P
VOCM
V
+
N
−
N
=
R
+
V
N
R
)
+
RGP
RGN
P
RGN
RFN
)
IN
+
R
N
R
+
N
V
,
)
dm
P
R
OUT
+
N
1
2
R
(
,
dm
2
N
RFN
RFP
R
1
2
P
R
(
2
N
+
+
R
R
P
V
V
P
+
ON
OP
+
R
R
N
N
)
)
AD8475