Datasheet

Data Sheet AD8475
Rev. C | Page 19 of 24
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 voltages in the signal
path; 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.
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 with output 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 gain error 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 voltage
reference of an ADC via a resistor divider as shown in Figure 55.
Connecting the VOCM input in this manner reduces power
supply noise and optimizes the output common mode voltage
of the AD8475 to utilize the entire differential input voltage
range of the ADC. If AD8475 is used with a single supply that
is the same voltage as the voltage reference, two 10 kΩ resistors
connected to the VOCM pin is sufficient to override the inter-
nal resistors. Otherwise, a voltage follower should be used to
drive VOCM.
09432-200
V
OUT
= (V
+OUT
– V
–OUT
)
0.1µF
REF
+V
S
+
0.1µF
10µF
LOW
IMPEDANCE
INPUT SOURCE
–V
S
+
0.1µF
10µF
1kΩ
1kΩ
1.25kΩ
1.25kΩ
AD8475
–IN 0.8x –IN 0.4x +V
S
VOCM +OUT
+IN 0.8x +IN 0.4x
–V
S
NC –OUT
1.25kΩ
1.25kΩ
Figure 52. Typical Configuration—10-Lead MSOP