AD7643

Rev. 0 | Page 16 of 28

TRANSFER FUNCTIONS

Using the OB/2C digital input, except in 18-bit interface mode,

the AD7643 offers two output codings: straight binary and twos

262,144, which is 15.623 μV. Refer to Figure 22 and Table 8 for

the ideal transfer characteristic.

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 22. ADC Ideal Transfer Function

Table 8. Output Codes and Ideal Input Voltages

Digital Output Code (Hex)

Description

Analog Input

Straight

Binary

Twos

Complement

FSR −1 LSB

+2.0479844 V

0x3FFFF1

0x1FFFF1

FSR − 2 LSB

+2.0479688 V

0x3FFFE

0x1FFFE

Midscale + 1 LSB

+15.625 μV

0x20001

0x00001

Midscale

0 V

0x20000

0x00000

Midscale − 1 LSB

−15.625 μV

0x1FFFF

0x3FFFF

−FSR + 1 LSB

−2.0479844 V

0x00001

0x20001

−FSR

−2.048 V

0x000002

0x200002

RD

CS

100nF

100nF

AVDD

10µF

100nF

AGND

DGND

DVDD

OVDD

OGND

CNVST

BUSY

SDOUT

SCLK

RESET

PD

REFBUFIN

10Ω

D

CLOCK

AD7643

MICROCONVERTER/

MICROPROCESSOR/

DSP

SERIAL

PORT

DIGITAL

INTERFACE

SUPPLY

(2.5V OR 3.3V)

ANALOG

SUPPLY (2.5V)

OVDD

DIGITAL

SUPPLY (2.5V)

IN+

IN–

U2

15Ω

NOTE 5

50Ω

50pF

NOTE 1

ANALOG

INPUT +

2.7nF

2.7nF

U1

15Ω

NOTE 1

MODE0

MODE1

D0/OB/2C

REFGND

REF

PDBUF

PDREF

100nF

ANALOG

INPUT –

NOTE 2

NOTE 2

NOTE 3

NOTE 4

NOTE 3

NOTE 7

NOTE 6

10µF

10µF

10µF

10kΩ

50pF

1. SEE ANALOG INPUTS SECTION.

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

3. THE CONFIGURATION SHOWN IS USING THE INTERNAL REFERENCE. SEE VOLTAGE REFERENCE INPUT SECTION.

4. A 10µF CERAMIC CAPACITOR (X5R, 1206 SIZE) IS RECOMMENDED (FOR EXAMPLE, PANASONIC ECJ3YB0J106M).

SEE VOLTAGE REFERENCE INPUT SECTION.

5. OPTION, SEE POWER SUPPLY SECTION.

6. OPTION, SEE POWER-UP SECTION.

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

Figure 23. Typical Connection Diagram