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AD7982 Datasheet(PDF) 18 Page - Analog Devices

Part No. AD7982
Description  Precision, Selectable Gain, Fully Differential Funnel Amplifier
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
Logo AD - Analog Devices

AD7982 Datasheet(HTML) 18 Page - Analog Devices

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AD8475
Rev. B | Page 18 of 24
INPUT VOLTAGE RANGE
The AD8475 can measure input voltages that are larger than the
supply rails. The internal gain and feedback resistors form a
divider, which reduces the input voltage seen by the internal
input nodes of the amplifier. The largest voltage that can be
measured is constrained by the capability of the amplifier’s
internal summing nodes. This voltage is defined by the input
voltage and the ratio between the feedback and the gain resistors.
Figure 51 shows the voltage at the internal summing nodes of
the amplifier, defined by the input voltage and internal resistor
network. If VN is grounded, the expression shown in the figure
reduces to
+
+
=
=
P
MINUS
PLUS
V
RG
RF
VOCM
RG
RF
RG
V
V
2
1
The internal amplifier of the AD8475 has rail-to-rail inputs. To
obtain accurate measurements with minimal distortion, the
voltage at the internal inputs of the amplifier must stay below
+VS − 1 V and above −VS.
For example, with VS = 5 V in a G = 0.4 configuration, the
AD8475 can measure an input as high as ±12.5 V and maintain
its excellent distortion performance.
The AD8475 provides overvoltage protection for excessive input
voltages beyond the supply rails. Integrated ESD protection diodes
at the inputs prevent damage to the AD8475 up to +VS + 10.5 V
and −VS − 16 V.
DRIVING THE AD8475
Care should be taken to drive the AD8475 with a low
impedance source: for example, another amplifier. Source
resistance can unbalance the resistor ratios and, therefore,
significantly degrade the gain accuracy and common-mode
rejection of the AD8475. For the best performance, source
impedance to the AD8475 input terminals should be kept
below 0.1 Ω. Refer to the DC Precision section for details on
the critical role of resistor ratios in the precision of the AD8475.
POWER SUPPLIES
The AD8475 operates over a wide range of supply voltages. It
can be powered on a single supply as low as 3 V and as high as
10 V. The AD8475 can also operate on dual supplies from
±1.5 V up to ±5 V
A stable dc voltage should be used to power the AD8475. Note
that noise on the supply pins can adversely affect performance.
For more information, see the PSRR performance curve in
Figure 13.
Place a bypass capacitor of 0.1 μF between each supply pin and
ground, as close as possible to each supply pin. Use a tantalum
capacitor of 10 μF between each supply and ground. It can be
farther away from the supply pins and, typically, it can be
shared by other precision integrated circuits.
RF
RF
RG
RG
VON
VOP
VOCM
VP
VN
VN
RF + RG
RF
VP − VN
RG
RF
VOCM
RF + RG
RG
+
+
2
1
Figure 51. Voltages at the Internal Op Amp Inputs of the AD8475


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