Datasheet
AD8475 Data Sheet
Rev. C | Page 20 of 24
12
11
10
1
3
4
NC
–OUT
+OUT
9
VOCM
+IN 0.4x
–IN 0.8x
2
+IN 0.8x
–IN 0.4x
6
+V
S
5
7
+V
S
8
+V
S
16
+IN 0.4x
15
–V
S
14
–V
S
13
–V
S
1kΩ
1.25kΩ
1.25kΩ
1kΩ
AD8475
1.25kΩ
1.25kΩ
–IN 0.4x
+V
S
–V
S
0.1µF
V
OUT
= (V
+OUT
– V
–OUT
)
REF
+
0.1µF10µF
LOW
IMPEDANCE
INPUT SOURCE
V
IN
+
0.1µF 10µF
09432-165
Figure 53. Typical Configuration—16-Lead LFCSP
HIGH PERFORMANCE ADC DRIVING
The AD8475 is ideally suited for broadband dc-coupled and
industrial applications. The circuit in Figure 55 shows an
industrial front-end connection for an AD8475 driving an
AD7982, a 18-bit, 1 MSPS ADC, with dc coupling on the
AD8475 input and output. (The AD7982 achieves its optimum
performance when driven differentially.) The AD8475 performs
the attenuation of a 20 V p-p input signal, level shifts it, and
converts it to a differential signal without the need for any
external components. The AD8475 eliminates the need for dual
supplies at the front end to accept large bipolar signals. It also
eliminates the need for a precision resistor network for attenua-
tion, and a transformer to drive the ADC and perform the single-
ended-to-differential conversion.
The ac and dc performance of the AD8475 are compatible with
the 18-bit, 1 MSPS AD7982 PulSAR® ADC and other 16-bit and
18-bit members of the family, which have sampling rates up to
4 MSPS. Some suitable high performance differential ADCs are
listed in Table 6.
Table 6. High Performance SAR ADCs
Part Resolution
Sample
Rate Description
AD7984 18 Bits 1.33 MSPS True differential input,
14 mW, 2.5 V ADC
AD7982 18 Bits 1 MSPS True differential Input,
7.0 mW, 2.5 V ADC
AD7690 18 Bits 400 kSPS True differential input,
4.5 mW, 5 V ADC
AD7641 18 Bits 2 MSPS True differential input,
75 mW, 2.5 V ADC
In this example, the AD8475 is powered with a single 5 V
supply and used in a gain of 0.4, with a single-ended input
converted to a differential output. The input is a 20 V p-p
symmetric, ground-referenced bipolar signal. With an output
common-mode voltage of 2.5 V, each AD8475 output swings
between 0.5 V and 4.5 V, opposite in phase, providing an 8 V p-p
differential signal to the ADC input.