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LTC1416

APPLICATIONS INFORMATION

0000 0000 00 and 1111 1111 1111 11. For full-scale

adjustment, an input voltage of 2.499544V (FS/2 – 1.5LSB)

is applied to AIN and R2 is adjusted until the output code

flickers between 0111 1111 1111 10 and 0111 1111

1111 11.

applications, however, do not have a –5V supply readily

available and most ADCs have inadequate PSRR to suffi-

ciently attenuate the noise created by a switching or

charge pump supply. The LTC1416’s excellent PSRR

makes it possible to achieve good performance, even at 14

bits, using a switch based regulator for a –5V supply.

Figure 12a shows a circuit using an LT1373 configured as

a Cuk converter creating –5V from a 5V supply. The circuit

shown in Figure 12b uses an LT1054 regulated charge

pump to provide –5V. This circuit has the advantage of

reduced board space and fewer passive components. (For

further details refer to

Linear Technology Magazine, June

1997, Page 29.)

BOARD LAYOUT AND BYPASSING

Wire wrap boards are not recommended for high resolu-

tion or high speed A/D converters. To obtain the best

performance from the LTC1416, a printed circuit board

with ground plane is required. Layout for the printed

circuit board should ensure that digital and analog signal

lines are separated as much as possible. In particular, care

should be taken not to run any digital track alongside an

analog signal track or underneath the ADC. The analog

input should be screened by AGND.

An analog ground plane separate from the logic system

ground should be established under and around the ADC

(see Figure 13). Pin 5 (AGND), Pins 14 and 19 (ADC’s

DGND) and all other analog grounds should be connected

to this single analog ground point. The REFCOMP bypass

capacitor and the DVDD bypass capacitor should also be

connected to this analog ground plane. No other digital

grounds should be connected to this analog ground plane.

Low impedance analog and digital power supply common

returns are essential to low noise operation of the ADC and

the foil width for these tracks should be as wide as

possible. In applications where the ADC data outputs and

control signals are connected to a continuously active

microprocessor bus, it is possible to get errors in the

conversion results. These errors are due to feedthrough

from the microprocessor to the successive approximation

comparator. The problem can be eliminated by forcing the

microprocessor into a Wait state during conversion or by

using three-state buffers to isolate the ADC data bus. The

1416 F11a

011...111

011...110

000...001

000...000

111...111

111...110

100...001

100...000

FS – 1LSB

– (FS – 1LSB)

INPUT VOLTAGE (AIN

+ – A

–)

Figure 11a. LTC1416 Transfer Characteristics

ANALOG

INPUT

1416 F11b

100

Ω

50k

24k

–5V

24k

50k

47k

µF

AIN

–

VREF

REFCOMP

AGND

LTC1416

Figure 11b. Offset and Full-Scale Adjust Circuit

Generating a – 5V Supply

There are several advantages to using

±5V supplies rather

than a single 5V supply. A larger signal magnitude is

possible which increases the dynamic range and

improves the signal-to-noise ratio. Operating on

±5V

supplies also offers increased headroom which eases the

requirements for signal conditioning circuitry, avoids the

limitations of rail-to-rail operation and widens the selec-

tion of high performance operational amplifiers. Some

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