AD7441/AD7451

Rev. D | Page 14 of 24

TYPICAL CONNECTION DIAGRAM

Figure 22 shows a typical connection diagram for the device.

In this setup, the GND pin is connected to the analog ground

a 2.5 V decoupled reference source. The signal source is connected

tantalum capacitor in parallel with a 0.1 μF ceramic capacitor.

The reference pin is decoupled to AGND with a capacitor of at

least 0.1 μF. The conversion result is output in a 16-bit word

with four leading zeros followed by the MSB of the 12-bit or

10-bit result. The 10-bit result of the AD7441 is followed by two

trailing zeros.

AD7441/

AD7451

0.1µF

0.1µF

10µF

DC INPUT

VOLTAGE

SCLK

2.7V TO 5.25V

SUPPLY

SERIAL

INTERFACE

µC/µP

SDATA

CS

GND

2.5V

AD780

p-p

Figure 22. Typical Connection Diagram

ANALOG INPUT

The AD7441/AD7451 have a pseudo differential analog input.

this input provides an offset from ground or a pseudo ground

input signal ground from the ADC ground, allowing dc common-

mode voltages to be cancelled.

Because the ADC operates from a single supply, it is necessary

to level shift ground-based bipolar signals to comply with the

input requirements. An op amp (for example, the AD8021) can

be configured to rescale and level shift a ground-based (bipolar)

signal so that it is compatible with the input range of the AD7441/

AD7451 (see Figure 23).

When a conversion takes place, the pseudo ground corresponds

to 0, and the maximum analog input corresponds to 4096 for

the AD7451 and 1024 for the AD7441.

R

2.5V

1.25V

0V

+1.25V

0V

–1.25V

R

3R

0.1µF

R

AD7441/

AD7451

EXTERNAL

Figure 23. Op Amp Configuration to Level Shift a Bipolar Input Signal

ANALOG INPUT STRUCTURE

Figure 24 shows the equivalent circuit of the analog input

structure of the AD7441/AD7451. The four diodes provide

ESD protection for the analog inputs. Care must be taken to

ensure that the analog input signals never exceed the supply

rails by more than 300 mV. This causes these diodes to become

forward-biased and start conducting into the substrate. These

diodes can conduct up to 10 mA without causing irreversible

damage to the part. The C1 capacitors (see Figure 24) are

typically 4 pF and can be attributed primarily to pin capaci-

tance. The resistors are lumped components made up of

the on resistance of the switches. The value of these resistors

is typically about 100 Ω. The C2 capacitors are the ADC

sampling capacitors and have a capacitance of 16 pF typically.

For ac applications, removing high frequency components from

the analog input signal through the use of an RC low-pass filter

on the relevant analog input pins is recommended. In applica-

tions where harmonic distortion and the signal-to-noise ratio

are critical, it is recommended that the analog input be driven

from a low impedance source. Large source impedances

significantly affect the ac performance of the ADC, which can

necessitate the use of an input buffer amplifier. The choice of

the amplifier is a function of the particular application.

C1

C2

R1

D

D

C1

C2

R1

D

D

Figure 24. Equivalent Analog Input Circuit;

Conversion Phase—Switches Open;

Track Phase—Switches Closed