Datasheet

Data Sheet AD8476
Rev. B | Page 19 of 24
APPLICATIONS INFORMATION
TYPICAL CONFIGURATION
The AD8476 is designed to facilitate single-ended-to-differential
conversion, common-mode level shifting, and precision processing
of signals so that they are compatible with low voltage ADCs.
Figure 54 shows a typical connection diagram of the AD8476.
SINGLE-ENDED-TO-DIFFERENTIAL CONVERSION
Many industrial systems have single-ended inputs from input
sensors; however, the signals are frequently processed by high
performance differential input ADCs for higher precision. The
AD8476 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 54). Note that either input can be driven by the source
with the only effect being that the outputs have reversed polarity.
The AD8476 also accepts truly differential input signals in
precision systems with differential signal paths.
SETTING THE OUTPUT COMMON-MODE VOLTAGE
The VOCM pin of the AD8476 is internally biased by a
precision voltage divider comprising of two 1 MΩ 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 0.05% of the expected
value.
10195-102
10kΩ
10kΩ
10kΩ
10kΩ
INN
1
+V
S
2
VOCM
3
+OUT
4
INP
8
–V
S
7
NC
6
–OUT
5
AD8476
LOAD
INPUT
SIGNAL
SOURCE
+5V
–V
OUT
+V
OUT
+
10µF
0.1µF
+
10µF
0.1µF
–5V
Figure 54. Typical Configuration—8-Lead MSOP
In cases where control of the output common-mode level is
desired, an external source or resistor divider can be used to
drive the VOCM pin. If driven directly from a source, or with a
resistor divider of unequal resistor values, the resistance seen by
the VOCM pin should be less than 1 kΩ. 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 500 kΩ. If multiple AD8476
devices share one ADC reference output, a buffer may be neces-
sary to drive the parallel inputs.
Table 6. Differential Input ADCs
1
ADC Resolution Throughput Rate Power Dissipation
AD7674 16 Bits 100 kSPS 25 mW
AD7684 16 Bits 100 kSPS 6 mW
AD7687 16 Bits 250 kSPS 12.5 mW
AD7688
16 Bits
500 kSPS*
21.5 mW
1
Depending on measurement/application type, check that the AD8476 meets
settling time requirements.