AD7677AST datasheet(12/20 Pages) AD

12 / 20 page

REV. 0

AD7677

–12–

Transfer Functions

Using the OB/

2C digital input, the AD7677 offers two output

codings: straight binary and two’s complement. The ideal trans-

fer characteristic for the AD7677 is shown in Figure 4.

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 4. ADC Ideal Transfer Function

TYPICAL CONNECTION DIAGRAM

Figure 5 shows a typical connection diagram for the AD7677.

Different circuitry shown on this diagram are optional and are

discussed below.

Analog Inputs

Figure 6 shows a simplified analog input section of AD7677.

IN+

IN–

AGND

AVDD

R+ = 168

R– = 168

Figure 6. Simplified Analog Input

The diodes shown in Figure 6 provide ESD protection for the

inputs. Care must be taken to ensure that the analog input sig-

nal never exceeds the absolute ratings on these inputs. This will

cause these diodes to become forward-biased and start conduct-

ing current. These diodes can handle a forward-biased current

of 120 mA maximum. This condition could eventually occur

when the input buffer’s (U1) or (U2) supplies are different from

AVDD. In such case, an input buffer with a short-circuit current

limitation can be used to protect the part.

This analog input structure is a true differential structure. By

using these differential inputs, signals common to both inputs

are rejected as shown in Figure 7, which represents the typical

CMRR over frequency.

AVDD

AGND

DGND

DVDD

OVDD

OGND

SER/

PAR

CNVST

BUSY

SDOUT

SCLK

RESET

REFGND

CREF

2.5V REF

NOTE 1

REF

100

CLOCK

AD7677

C/ P/DSP

SERIAL PORT

DIGITAL SUPPLY

(3.3V OR 5V)

ANALOG

SUPPLY

(5V)

DVDD

OB/

NOTE 7

BYTESWAP

DVDD

50k

100nF

IN+

ANALOG INPUT+

2.7nF

NOTE 4

NOTE 5

AD8021

–

NOTE 2

NOTE 3

NOTE 5

ADR421

10 F

100nF

10 F

100nF

10 F

IN–

ANALOG INPUT–

2.7nF

NOTE 4

NOTE 5

AD8021

–

1 F

NOTES

1. SEE VOLTAGE REFERENCE INPUT SECTION.

2. WITH THE RECOMMENDED VOLTAGE REFERENCES, CREF IS 47 F. SEE CHAPTER VOLTAGE REFERENCE INPUT SECTION.

3. OPTIONAL CIRCUITRY FOR HARDWARE GAIN CALIBRATION.

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

5. SEE ANALOG INPUT SECTION.

6. OPTION, SEE POWER SUPPLY SECTION.

7. OPTIONAL LOW JITTER

CNVST, SEE CONVERSION CONTROL SECTION.

Figure 5. Typical Connection Diagram

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