ad8475 Analog Devices, Inc., ad8475 Datasheet - Page 19

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

APPLICATIONS INFORMATION
TYPICAL CONFIGURATION
The AD8475 is designed to facilitate single-ended-to-differential
conversion, common-mode level shifting, and precision attenuation
of large signals so that they are compatible with low voltage ADCs.
Figure 53 shows a typical connection diagram of the AD8475
in a gain of 0.4. To use the AD8475 in a gain of 0.8, drive the
±IN 0.8x inputs with a low impedance source.
SINGLE-ENDED TO DIFFERENTIAL CONVERSION
Many industrial systems use single-ended; however, the signals
are frequently processed by high performance differential input
ADCs for higher precision. The AD8475 performs the critical
function of precisely converting single-ended signals to the
differential inputs of precision ADCs, and it does so with no
need for external components.
To convert a single-ended signal to a differential signal, connect
one input to the signal source and the other input to ground (see
Figure 55). Note that either input can be driven by the source
with the only effect being that the outputs have reversed polarity.
The AD8475 also accepts truly differential input signals in
precision systems with differential signal paths.
INPUT SOURCE
IMPEDANCE
LOW
+IN 0.8x +IN 0.4x
–IN 0.8x –IN 0.4x
Figure 52. Typical Configuration—10-Lead MSOP
10µF
1.25kΩ
1.25kΩ
+
Rev. A | Page 19 of 24
0.1µF
1.25kΩ
1.25kΩ
–V
+V
–V
S
+V
S
S
S
0.1µF
VOCM
NC
AD8475
0.1µF
SETTING THE OUTPUT COMMON-MODE VOLTAGE
The VOCM pin of the AD8475 is internally biased with a
precision voltage divider comprising two 200 kΩ resistors between
the supplies. This divider level shifts the output to midsupply.
Relying on the internal bias results in an output common-mode
voltage that is within 0.01% of the expected value.
In cases where control of the output common-mode level is
desired, an external source or resistor divider with source
resistance less than 100 Ω can be used to drive the VOCM pin.
If an external voltage divider consisting of equal resistor values
is used to set VOCM to midsupply, higher values can be used
because the external resistors are placed in parallel with the
internal resistors. The output common-mode offset listed in the
Specifications section assumes that the VOCM input is driven by a
low impedance voltage source.
Because of the internal divider, the VOCM pin sources and sinks
current, depending on the externally applied voltage and its
associated source resistance.
It is also possible to connect the VOCM input to the common-
mode level output of an ADC; however, care must be taken to
ensure that the output has sufficient drive capability. The input
impedance of the VOCM pin is 100 kΩ. If multiple AD8475
devices share one ADC reference output, a buffer may be neces-
sary to drive the parallel inputs.
1kΩ
1kΩ
–OUT
+OUT
+
10µF
REF
V
OUT
= (V
+OUT
– V
–OUT
)
AD8475