AD7997 datasheet(16/32 Pages) AD

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AD7997/AD7998

Rev. 0 | Page 16 of 32

TYPICAL CONNECTION DIAGRAM

The typical connection diagram for the AD7997/AD7998 is

shown in Figure 22. In this figure, the address select pin (AS)

is tied to VDD; however, AS can also be tied to AGND or left

floating, allowing the user to select up to five AD7997/AD7998

devices on the same serial bus. An external reference must be

applied to the AD7997/AD7998. This reference can be in the

range of 1.2 V to VDD. A precision reference like the REF 19x

family, AD780, ADR03, or ADR381 can be used to supply the

reference voltage to the ADC.

SDA and SCL form the 2-wire I2C-/SMBus-compatible

interface. External pull-up resisters are required for both SDA

and SCL lines.

The AD7998-0/AD7997-0 support standard and fast I2C

interface modes. The AD7998-1/AD7997-1 support standard,

fast, and high speed I2C interface modes. Therefore if operating

in either standard or fast mode, up to five AD7997/AD7998

devices can be connected to the bus, as noted:

3 × AD7997-0/AD7998-0 and 2 × AD7997-1/ AD7998-1

3 × AD7997-1/AD7998-1 and 2 × AD7997-0/AD7998-0

In high speed mode, up to three AD7997-1/AD7998-1 devices

can be connected to the bus.

Wake-up from shutdown and acquisition prior to a conversion

is approximately 1 µs, and conversion time is approximately

2 µs. The AD7997/AD7998 enters shutdown mode again after

each conversion, which is useful in applications where power

consumption is a concern.

ANALOG INPUT

Figure 21 shows an equivalent circuit of the AD7997/AD7998

analog input structure. The two diodes, D1 and D2, provide

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

ensure that the analog input signal does not exceed the supply

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

forward biased and start conducting current into the substrate.

These diodes can conduct a maximum current of 10 mA

without causing irreversible damage to the part.

VIN

VDD

30pF

4pF

CONVERSION PHASE—SWITCH OPEN

TRACK PHASE—SWITCH CLOSED

Figure 21. Equivalent Analog Input Circuit

Capacitor C1 in Figure 21 is typically about 4 pF and can

primarily be attributed to pin capacitance. Resistor R1 is a

lumped component made up of the on resistance (RON) of

a track-and-hold switch, and also includes the RON of the

input multiplexer. The total resistance is typically about 400 Ω.

C2, the ADC sampling capacitor, has a typical capacitance of

30 pF.

VDD

VIN1

AGND

5V SUPPLY

REF 19x

µF

0.1

µF

AD7997/

AD7998

0V to REFIN

INPUT

SDA

µC/µP

SCL

2-WIRE SERIAL

INTERFACE

CONVST

ALERT

REFIN

VDD

VIN8

0.1

µF

Figure 22. AD7997/AD7998 Typical Connection Diagram

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