AD7980 datasheet(11/22 Pages) AD

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Preliminary Technical Data

AD7980

Rev Pr C | Page 11 of 22

Transfer Functions

The ideal transfer characteristic for the AD7980 is shown in

Figure 12 and Table 7.

000...000

000...001

000...010

111...101

111...110

111...111

ANALOG INPUT

+FSR – 1.5 LSB

FSR – 1 LSB

–FSR + 1 LSB

–FSR

–FSR + 0.5 LSB

Figure 12. ADC Ideal Transfer Function

Table 7. Output Codes and Ideal Input Voltages

Description

Analog Input

VREF = 5 V

Digital Output Code Hexa

FSR – 1 LSB

4.999924 V

FFFF1

Midscale + 1 LSB

2.500076 V

8001

Midscale

2.5 V

8000

Midscale – 1 LSB

2.499924 V

7FFF

–FSR + 1 LSB

76.3 μV

0001

–FSR

0 V

00002

1. This is also the code for an overranged analog input (VIN+ − VIN− above VREF −

VGND).

2. This is also the code for an underranged analog input (VIN+ − VIN− below VGND).

TYPICAL CONNECTION DIAGRAM

Figure 13 shows an example of the recommended connection

diagram for the AD7980 when multiple supplies are available.

AD7980

REF

GND

VDD

IN–

IN+

VIO

SDI

SCK

SDO

CNV

3- OR 4-WIRE INTERFACE5

100nF

2.5V

μF2

V+

V-

1.8V TO 5V

REF1

0 TO VREF

2.7nF

1SEE REFERENCE SECTION FOR REFERENCE SELECTION.

2CREF IS USUALLY A 10μF CERAMIC CAPACITOR (X5R).

3SEE DRIVER AMPLIFIER CHOICE SECTION.

4OPTIONAL FILTER. SEE ANALOG INPUT SECTION.

5SEE DIGITAL INTERFACE FOR MOST CONVENIENT INTERFACE MODE.

Figure 13. Typical Application Diagram with Multiple Supplies

ANALOG INPUT

Figure 14 shows an equivalent circuit of the input structure of

the AD7980.

The two diodes, D1 and D2, provide ESD protection for the

analog inputs IN+ and IN−. Care must be taken to ensure that

the analog input signal never exceeds the supply rails by more

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