AD7679

Rev. A | Page 16 of 28

Transfer Functions

Except in 18-bit interface mode, the AD7679 offers straight

binary and twos complement output coding when using OB/2C.

See

and

for the ideal transfer characteristic.

Figure 24

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

03085-0-026

Figure 24. ADC Ideal Transfer Function

Table 8. Output Codes and Ideal Input Voltages

Description

Analog Input

Straight

Binary

(Hex)

Twos

Complement

(Hex)

FSR –1 LSB

4.095962 V

3FFFF1

1FFFF1

FSR – 2 LSB

4.095924 V

3FFFE

1FFFE

Midscale +

1 LSB

31.25 μV

20001

00001

Midscale

0 V

20000

00000

Midscale –

1 LSB

–31.25 μV

1FFFF

3FFFF

–FSR + 1 LSB

-4.095962 V

00001

20001

–FSR

-4.096 V

000002

200002

AVDD

AGND

DGND

DVDD

OVDD

OGND

CNVST

BUSY

SDOUT

SCLK

RD

CS

RESET

PD

2.5V REF

NOTE 1

REFBUFIN

20

Ω

D

CLOCK

AD7679

μC/μP/DSP

SERIAL PORT

DIGITAL SUPPLY

(3.3V OR 5V)

ANALOG

SUPPLY

(5V)

DVDD

OB/2C

NOTE 6

PDBUF

DVDD

50k

Ω

100nF

1M

Ω

IN+

ANALOG INPUT+

2.7nF

U1

NOTE 3

NOTE 4

AD8021

–

+

30

Ω

NOTE 2

NOTE 5

ADR421

10

μF

100nF

+

10

μ F

100nF

+

100nF

+

10

μ F

IN–

ANALOG INPUT–

2.7nF

U2

NOTE 3

NOTE 4

AD8021

–

+

30

Ω

50

Ω

100nF

NOTES

1. SEE VOLTAGE REFERENCE INPUT SECTION.

2. OPTIONAL CIRCUITRY FOR HARDWARE GAIN CALIBRATION.

3.THE AD8021 IS RECOMMENDED. SEE DRIVER AMPLIFIER CHOICE SECTION.

4. SEE ANALOG INPUTS SECTION.

5. OPTION, SEE POWER SUPPLY SECTION.

6. OPTIONAL LOW JITTER CNVST, SEE CONVERSION CONTROL SECTION.

47

μF

MODE1

MODE0

NOTE 1

REF

REFGND

03085-0-027

Figure 25. Typical Connection Diagram (Internal Reference Buffer, Serial Interface)