AD7475ARM datasheet(14/24 Pages) AD

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AD7475/AD7495

Rev. B | Page 14 of 24

TYPICAL CONNECTION DIAGRAM

Figure 13 and Figure 15 show typical connection diagrams for

the AD7475 and AD7495, respectively. In both setups the GND

pin is connected to the analog ground plane of the system. In

Figure 13, REF IN is connected to a decoupled 2.5 V supply

from a reference source, the AD780, to provide an analog input

range of 0 V to 2.5 V. Although the AD7475 is connected to a

VDD of 5 V, the serial interface is connected to a 3 V micro-

processor. The VDRIVE pin of the AD7475 is connected to the

same 3 V supply of the microprocessor to allow a 3 V logic

interface (see the Digital Inputs section.) In Figure 15, the REF

OUT pin of the AD7495 is connected to a buffer and then

applied to a level-shifting circuit used on the analog input to

allow a bipolar signal to be applied to the AD7495. A minimum

100 nF capacitance is required on the REF OUT pin to GND.

The conversion result from both ADCs is output in a 16-bit

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

result. For applications where power consumption is of concern,

the power-down modes should be used between conversions or

bursts of several conversions to improve power performance.

See the Operating Modes section for more information.

VDD

VIN

GND

SUPPLY

2.5V

AD780

SUPPLY

AD7475

0V TO

2.5V

INPUT

SDATA

μC/μP

SCLK

SERIAL

INTERFACE

0.1

μF

(MIN)

VDRIVE

REF IN

0.1

μF

0.1

μF

Figure 13. AD7475 Typical Connection Diagram

Analog Input

Figure 14 shows an equivalent circuit of the analog input

structure of the AD7475/AD7495. The D1 and D2 diodes

provide ESD protection for the analog inputs. Care must be

taken to ensure that the analog input signal never exceeds the

supply rails by more than 200 mV. This causes these diodes to

become forward-biased and start conducting current into the

substrate. The maximum current these diodes can conduct

without causing irreversible damage to the part is 20 mA.

The capacitor C1 in Figure 14 is typically about 4 pF and can

primarily be attributed to pin capacitance. The resistor R1 is a

lumped component made up of the on resistance of a switch.

This resistor is typically about 100 Ω. The capacitor C2 is

the ADC sampling capacitor and has a capacitance of 16 pF,

typically. For ac applications, it is recommended to remove

high frequency components from the analog input signal

using an RC low-pass filter on the relevant analog input pin. In

applications where harmonic distortion and signal-to-noise

ratio are critical, the analog input should be driven from a low

impedance source. Large source impedances significantly affect

the ac performance of the ADC. This may necessitate the use of

an input buffer amplifier. The choice of the op amp is a function

of the particular application.

VIN

16pF

4pF

VDD

CONVERSION PHASE: SWITCH OPEN

TRACK PHASE: SWITCH CLOSED

Figure 14. Equivalent Analog Input Circuit

VDD

VIN

GND

SUPPLY

AD7495

0V TO

2.5V

INPUT

SDATA

μC/μP

SCLK

SERIAL

INTERFACE

0.1

μF

(MIN)

VDRIVE

REF OUT

0.1

μF

0.1

μF

Figure 15. AD7495 Typical Connection Diagram

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