LTC1273ACSW datasheet(20/24 Pages) LINER

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LTC1273

LTC1275/LTC1276

127356fa

APPLICATI

I FOR ATIO

This is a two byte read instruction which loads the ADC

data (address B000) into the HL register pair. During the

first read operation, BUSY forces the microprocessor to

WAIT for the LTC1273 conversion. No WAIT states are

inserted during the second read operation when the mi-

croprocessor is reading the high data byte.

TMS32010 Microcomputer

Figure 20 shows an LTC1273/TMS32010 interface. The

LTC1273 is operating in the ROM Mode.

The LTC1273 is mapped at a port address. The following

I/O instruction starts a conversion and reads the previous

conversion result into data memory.

IN A,PA

(PA = PORT ADDRESS)

When conversion is complete, a second I/O instruction

reads the up-to-date data into memory and starts another

conversion. A delay at least as long as the ADC conversion

time must be allowed between I/O instructions.

MUXing with CD4051

The high input impedance of the LTC1273/LTC1275/

LTC1276 provides an easy, cheap, fast, and accurate way

to multiplex many channels of data through one con-

verter. Figure 21 shows a low cost CD4051 connected to

the LTC1275. The LTC1275’s input draws no DC input

Figure 20. TMS32010 Interface

DATA BUS

LTC1273/75/76 • F20

PORT ADDRESS BUS

D11

DEN

PA0

PA2

TMS32010

ADDRESS

DECODE

D0/8

D11

HBEN

LTC1273/75/76

LINEAR CIRCUITRY OMITTED FOR CLARITY

current so it can be accurately driven by the unbuffered

MUX. The CD4520 counter increments the MUX channel

after each sample is taken. Figure 22 shows the acquisi-

tion time of LTC1275 vs the source resistance. For a

500

Ω maximum “on” resistance of the CD4051, the

acquisition time of the ADC is not greatly affected. For

larger source resistances, modest increases in acquisi-

tion time must be allowed.

VDD

VSS

VEE AB C

–5V

8 INPUT

CHANNELS

±2.8V

INPUT

VARIES

BUFFER

REQUIRED

D11

•

BUSY

LTC1275

AIN

µP

DSP

ENABLE

RESET

CD4520

COUNTER

LTC1273/75/76 • F21

CD4051

Figure 21. MUXing the LTC1275 with CD4051

SOURCE RESISTANCE (

Ω)

100

10k

LTC1273/75/76 • F22

LTC1275

AIN

RSOURCE

VIN

500

Ω

Figure 22. Acqusition Time of LTC1275 vs Source Resistance

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