ad8475 Analog Devices, Inc., ad8475 Datasheet - Page 20

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

AD8475
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.
INPUT SOURCE
IMPEDANCE
LOW
V
IN
+IN 0.4x
+IN 0.8x
–IN 0.8x
–IN 0.4x
Figure 53. Typical Configuration—16-Lead LFCSP
10µF
1
2
3
4
+
1.25kΩ
1.25kΩ
Rev. A | Page 20 of 24
0.1µF
1.25kΩ
1.25kΩ
–V
+V
S
S
0.1µF
AD8475
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
AD7984
AD7982
AD7690
AD7641
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.
1kΩ
1kΩ
+
10µF
Resolution
12
11
10
9
18 Bits
18 Bits
18 Bits
18 Bits
NC
–OUT
+OUT
VOCM
0.1µF
V
OUT
Sample
Rate
1.33 MSPS
1 MSPS
400 kSPS
2 MSPS
= (V
+OUT
REF
– V
–OUT
)
Description
True differential input,
14 mW, 2.5 V ADC
True differential Input,
7.0 mW, 2.5 V ADC
True differential input,
4.5 mW, 5 V ADC
True differential input,
75 mW, 2.5 V ADC