REV. B

AD7663

–13–

100nF

10 F

100nF

10 F

AVDD

10 F

100nF

AGND

DGND

DVDD

OVDD

OGND

SER/

PAR

CNVST

BUSY

SDOUT

SCLK

RD

CS

RESET

PD

REFGND

2.5V REF

REF

100

D

CLOCK

AD7663

C/ P/DSP

SERIAL

PORT

DIGITAL SUPPLY

(3.3V OR 5V)

ANALOG

SUPPLY

(5V)

DVDD

OB/

2C

NOTE 8

BYTESWAP

DVDD

50k

100nF

1M

INA

100nF

U2

IND

INGND

ANALOG

INPUT

( 10V)

2.7nF

U1

15

10 F

NOTE 2

NOTE 1

NOTE 3

NOTE 7

NOTE 4

50

INC

INB

NOTE

6

NOTES

1. SEE VOLTAGE REFERENCE INPUT SECTION.

3. OPTIONAL CIRCUITRY FOR HARDWARE GAIN CALIBRATION.

4. FOR BIPOLAR RANGE ONLY. SEE SCALER REFERENCE INPUT SECTION.

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

6. WITH 0V TO 2.5V RANGE ONLY. SEE ANALOG INPUTS SECTION.

7. OPTION. SEE POWER SUPPLY SECTION.

8. OPTIONAL LOW JITTER

CNVST. SEE CONVERSION CONTROL SECTION.

+

+

+

++

+

+

AD8031

AD8021

50

ADR421

NOTE 5

Figure 5. Typical Connection Diagram (±10 V Range Shown)

Figure 6 shows a simplified analog input section of the AD7663.

INC

INB

INA

4R

2R

R

IND

4R

AGND

AVDD

R1

R = 1.28k

Figure 6. Simplified Analog Input

The four resistors connected to the four analog inputs form a resis-

tive scaler that scales down and shifts the analog input range to a

common input range of 0 V to 2.5 V at the input of the switched

capacitive ADC.

By connecting the four inputs INA, INB, INC, and IND to the

input signal itself, the ground, or a 2.5 V reference, other analog

input ranges can be obtained.

The diodes shown in Figure 6 provide ESD protection for the four

analog inputs. The inputs INB, INC, and IND have a high voltage

protection (–11 V to +30 V) to allow wide input voltage range.

Care must be taken to ensure that the analog input signal never

exceeds the absolute ratings on these inputs, including INA

(0 V to 5 V). This will cause these diodes to become forward-

biased and start conducting current. These diodes can handle a

forward-biased current of 120 mA maximum. For instance, when

using the 0 V to 2.5 V input range, these conditions could even-

tually occur on the input INA when the input buffer’s (U1) supplies

are different from AVDD. In such cases, an input buffer with a

short-circuit current limitation can be used to protect the part.

This analog input structure allows the sampling of the differential

signal between the output of the resistive scaler and INGND. Unlike

other converters, the INGND input is sampled at the same time as

the inputs. By using this differential input, small signals common

to both inputs are rejected as shown in Figure 7, which represents

the typical CMRR over frequency. For instance, by using INGND

to sense a remote signal ground, the difference of ground potentials

between the sensor and the local ADC ground is eliminated.