AD8021 datasheet(14/28 Pages) AD

14 / 28 page

AD7685

Rev. C | Page 14 of 28

Transfer Functions

The ideal transfer characteristic for the AD7685 is shown in

Figure 26 and Table 8.

000...000

000...001

000...010

111...101

111...110

111...111

ANALOG INPUT

+FS – 1.5 LSB

+FS – 1 LSB

–FS + 1 LSB

–FS

–FS + 0.5 LSB

Figure 26. ADC Ideal Transfer Function

TYPICAL CONNECTION DIAGRAM

Figure 27 shows an example of the recommended connection

diagram for the AD7685 when multiple supplies are available.

AD7685

REF

GND

VDD

IN–

IN+

VIO

SDI

SCK

SDO

CNV

3- OR 4-WIRE INTERFACE5

100nF

10µF2

≥7V

≤–2V

1.8V TO VDD

REF1

0 TO VREF

33Ω

2.7nF

NOTES

1. SEE VOLTAGE REFERENCE INPUT SECTION FOR REFERENCE SELECTION.

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

3. SEE DRIVER AMPLIFIER CHOICE SECTION.

4. OPTIONAL FILTER. SEE ANALOG INPUT SECTION.

5. SEE DIGITAL INTERFACE FOR MOST CONVENIENT INTERFACE MODE.

Figure 27. Typical Application Diagram with Multiple Supplies

Table 8. 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).

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