Clock line should be short

and cross no other lines.

Single Ground entry for all

Reference Components

All Analog Components

mounted over Analog

area of Ground Plane

Xfmr/Amplifier

Components

mounted over

Digital area of

Digital power line should be routed

away from analog power trace.

COMMON

GROUND

PLANE

Ground entry points

close to ground pins.

Analog power line should be routed

away from Digital power trace.

Driving source

located close

to converter.

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

48

49

50

51

52

53

54

55

56

57

58

59

60

61

62

63

64

V

D

DB5

DB4

DB3

DB2

DB1

DB0/ABb

OEB

DR GND

DA11

DA10

DA9

DA8

DA7

DA6

V

D

AGND

V

RMB

V

RPB

V

RNB

V

REF

AGND

V

A

AGND

MULTIPLEX

V

RNA

V

RPA

V

RMA

ADC12DL040

All Digital

Ground Plane

6 x 100:

6 x 100:

OSC

LATCH

LATCH

ADC12DL040

SNAS250D – FEBRUARY 2005 – REVISED APRIL 2013

www.ti.com

Since digital switching transients are composed largely of high frequency components, total ground plane copper

weight will have little effect upon the logic-generated noise. This is because of the skin effect. Total surface area

is more important than is total ground plane volume.

Generally, analog and digital lines should cross each other at 90° to avoid crosstalk. To maximize accuracy in

high speed, high resolution systems, however, avoid crossing analog and digital lines altogether. It is important to

keep clock lines as short as possible and isolated from ALL other lines, including other digital lines. Even the

generally accepted 90° crossing should be avoided with the clock line as even a little coupling can cause

problems at high frequencies. This is because other lines can introduce jitter into the clock line, which can lead to

degradation of SNR. Also, the high speed clock can introduce noise into the analog chain.

Best performance at high frequencies and at high resolution is obtained with a straight signal path. That is, the

signal path through all components should form a straight line wherever possible.

Figure 40. Example of a Suitable Layout

Be especially careful with the layout of inductors. Mutual inductance can change the characteristics of the circuit

in which they are used. Inductors should not be placed side by side, even with just a small part of their bodies

beside each other.

The analog input should be isolated from noisy signal traces to avoid coupling of spurious signals into the input.

Any external component (e.g., a filter capacitor) connected between the converter's input pins and ground or to

the reference input pin and ground should be connected to a very clean point in the ground plane.

Figure 40 gives an example of a suitable layout. All analog circuitry (input amplifiers, filters, reference

components, etc.) should be placed in the analog area of the board. All digital circuitry and I/O lines should be

placed in the digital area of the board. The ADC12DL040 should be between these two areas. Furthermore, all

components in the reference circuitry and the input signal chain that are connected to ground should be

connected together with short traces and enter the ground plane at a single, quiet point. All ground connections

should have a low inductance path to ground.

28

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