AD8476_16 Datasheet, PDF (19/25 Pages) Analog Devices – Low Power, Unity Gain, Fully Differential Amplifier and ADC Driver

AD8476
The AD8476 integrated resistors are precision wafer-laser-
trimmed to guarantee a minimum CMRR of 90 dB (32 μV/V),
and gain error of less that 0.02%. To achieve equivalent precision
and performance using a discrete solution, resistors must be
matched to 0.01% or better.
INPUT VOLTAGE RANGE
The AD8476 can measure input voltages as large as 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
properly is constrained by the output range of the amplifier and
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 53 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 reduces to
V_PLUS
= V_MINUS
=
RG
RF + RG
^VOCM
+
1
2
RF
RG
V_P

The internal amplifier of the AD8476 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.
The AD8476 provides overvoltage protection for excessive input
voltages beyond the supply rails. Integrated ESD protection diodes
at the inputs prevent damage to the AD8476 up to +VS + 18 V
and −VS − 18 V.
Data Sheet
DRIVING THE AD8476
Care should be taken to drive the AD8476 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 AD8476. For the best performance, source
impedance to the AD8476 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 AD8476.
POWER SUPPLIES
The AD8476 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
18 V. The AD8476 can also operate on dual supplies from
±1.5 V to ±9 V
A stable dc voltage should be used to power the AD8476. Note
that noise on the supply pins can adversely affect performance.
For more information, see the PSRR performance curve in
Figure 10.
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.
RG
RF
V_P
RG
1 RF
RF
VOCM +
V_P − V_N +
V_N
RF + RG
2 RG
RF + RG
VOCM
V_ON
V_OP
V_N
RG
RF
Figure 53. Voltages at the Internal Op Amp Inputs of the AD8476
Rev. B | Page 18 of 24