AD1671

REV. B

–8–

85

40

0

50

45

–45

–50

55

60

65

70

75

80

–5

–10

–15

–20

–25

–30

–35

–40

ANALOG INPUT – dB

Figure 7. Spurious Free Dynamic Range vs. Input

APPLYING THE AD1671

GROUNDING AND DECOUPLING RULES

Proper grounding and decoupling should be a primary design

objective in any high speed, high resolution system. The

AD1671 separates analog and digital grounds to optimize the

management of analog and digital ground currents in a system.

The AD1671 is designed to minimize the current flowing from

REF COM (Pin 20) by directing the majority of the current

log ground currents hence reduces the potential for large ground

voltage drops. This can be especially true in systems that do not

utilize ground planes or wide ground runs. REF COM is also

configured to be code independent, therefore reducing input de-

pendent analog ground voltage drops and errors. Code depen-

dent ground current is diverted to ACOM (Pin 27). Also critical

in any high speed digital design is the use of proper digital

grounding techniques to avoid potential CMOS “ground

bounce.” Figure 3 is provided to assist in the proper layout,

grounding and decoupling techniques.

AIN1

REF IN

BPO/UPO

ACOM

BIT 1

BIT 12

DCOM

AD1671

ENCODE

DAV

OTR

MSB

AGP*

DGP*

+5V

–5V

*GROUND PLANE RECOMMENDED

AIN2

REF OUT

SHA OUT

REF COM

1

µF

V

(

±5V)

IN

1

18

28

+5V

0.1

µF

10

µF

23

22

20

27

19

25

26

24

21

13

2

17

16

15

14

0.1

µF10µF

0.1

µF10µF

Figure 8. AD1671 Grounding and Decoupling

Figure 4 plots both S/(N+D) and Effective Number of Bits

(ENOB) for a 100 kHz input signal sampled from 666 kHz to

1.25 MHz.

SAMPLING FREQUENCY – kHz

72.5

68

1250

68.5

70.5

714

666

69

69.5

70

71

71.5

72

1111

1000

909

833

769

11.75

11.50

11.25

11.00

Figure 5 is a THD plot for a full-scale 100 kHz input signal with

the sample frequency swept from 666 kHz to 1.25 MHz.

–68

–86

1250

–84

–76

714

666

–82

–80

–78

–74

–72

–70

1111

1000

909

833

769

SAMPLING FREQUENCY – kHz

The AD1671’s SFDR performance is ideal for use in communi-

cation systems such as high speed modems and digital radios.

The SFDR is better than 84 dB with sample rates up to 1.11 MHz

and increases as the input signal amplitude is attenuated by ap-

proximately 3 dB. Note also the SFDR is typically better than

80 dB with input signals attenuated by up to –7 dB.

1250

714

666

1111

1000

909

833

769

SAMPLING FREQUENCY – kHz

–86

–68

–84

–76

–82

–80

–78

–74

–72

–70

–88

–90

Figure 6. Spurious Free Dynamic Range vs. Sampling