AD7475ARMZ datasheet(14/24 Pages) AD

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AD7475/AD7495

Data Sheet

Rev. D | 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 connects to a VDD

of 5 V, the serial interface connects to a 3 V microprocessor. The

VDRIVE pin of the AD7475 connects 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, use the power-down

modes 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. 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 device is 20 mA. The capacitor C1 in Figure 14 is

typically about 4 pF and is 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, drive the analog input 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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