LTC1277CS datasheet(10/20 Pages) LINER

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LTC1274/LTC1277

TI I G DIAGRA S

CS to RD Setup Timing

CS to CONVST Setup Timing

LTC1274/77 • TD01

CONVST

LTC1274/77 • TD02

NAP to CONVST Wake-Up Timing (LTC1277)

SLEEP to REFRDY Wake-Up Timing

NAP

CONVST

LTC1274/77 • TD03

t14

SLEEP

REFRDY

LTC1274/77 • TD04

APPLICATIONS INFORMATION

CONVERSION DETAILS

The LTC1274/LTC1277 use a successive approximation

algorithm and an internal sample-and-hold circuit to con-

vert an analog signal to a 12-bit parallel output. The ADCs

are complete with a precision reference and an internal

clock. The control logic provides easy interface to micro-

processors and DSPs. (Please refer to the Digital Interface

section for the data format.)

Conversion start is controlled by the CS and CONVST

inputs. At the start of conversion the successive approxi-

mation register (SAR) is reset. Once a conversion cycle

has begun it cannot be restarted.

During conversion, the internal 12-bit capacitive DAC out-

put is sequenced by the SAR from the most significant bit

(MSB) to the least significant bit (LSB). Referring to

Figure 1, the AIN (LTC1274) or AIN

+ (LTC1277) input con-

nects to the sample-and-hold capacitor during the acquire

phase, and the comparator offset is nulled by the feedback

switch. In this acquire phase, a minimum delay of 2

µs will

provide enough time for the sample-and-hold capacitor to

acquire the analog signal. During the convert phase, the

comparator feedback switch opens, putting the comparator

into the compare mode. The input switch connects CSAMPLE

to ground (LTC1274) or AIN

– (LTC1277), injecting the

analog input charge onto the summing junction. This input

charge is successively compared with the binary-weighted

charges supplied by the capacitive DAC. Bit decisions are

made by the high speed comparator. At the end of a

conversion, the DAC output balances the AIN (LTC1274) or

AIN+ – AIN– (LTC1277) input charge. The SAR contents (a 12-

bit data word) which represent the AIN (LTC1274) or

AIN

+ –AIN–(LTC1277)areloadedintothe12-bitoutputlatches.

VDAC

1274 • F01

–

CDAC

DAC

SAMPLE

HOLD

CSAMPLE

12-BIT

LATCH

COMPAR-

ATOR

SAMPLE

AIN

Figure 1. LTC1274 AIN Input

DYNAMIC PERFORMANCE

The LTC1274/LTC1277 have excellent high speed sam-

pling capability. FFT (Fast Fourier Transform) test tech-

niques are used to test the ADCs’ frequency response,

distortion and noise at the rated throughput. By applying

a low distortion sine wave and analyzing the digital output

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