LTC1865LCMS datasheet(13/24 Pages) LINER

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LTC1864/LTC1865

18645fb

LTC1864 OPERATION

Operating Sequence

The LTC1864 conversion cycle begins with the rising edge

of CONV. After a period equal to tCONV, the conversion is

ﬁnished. If CONV is left high after this time, the LTC1864

goes into sleep mode drawing only leakage current. On the

falling edge of CONV, the LTC1864 goes into sample mode

and SDO is enabled. SCK synchronizes the data transfer

with each bit being transmitted from SDO on the falling

SCK edge. The receiving system should capture the data

from SDO on the rising edge of SCK. After completing the

data transfer, if further SCK clocks are applied with CONV

low, SDO will output zeros indeﬁnitely. See Figure 1.

Analog Inputs

The LTC1864 has a unipolar differential analog input. The

converter will measure the voltage between the “IN+”

and “IN–” inputs. A zero code will occur when IN+ minus

IN– equals zero. Full scale occurs when IN+ minus IN–

equals VREF minus 1LSB. See Figure 2. Both the “IN+” and

“IN–” inputs are sampled at the same time, so common

mode noise on the inputs is rejected by the ADC. If “IN–”

is grounded and VREF is tied to VCC, a rail-to-rail input

span will result on “IN+” as shown in Figure 3.

Reference Input

The voltage on the reference input of the LTC1864 deﬁnes

the full-scale range of the A/D converter. The LTC1864 can

operate with reference voltages from VCC to 1V.

APPLICATIONS INFORMATION

Figure 1. LTC1864 Operating Sequence

Figure 2. LTC1864 Transfer Curve

Figure 3. LTC1864 with Rail-to-Rail Input Span

CONV

tCONV

SCK

SDO

B15 B14

B12

B10

B0*

Hi-Z

18645 F01

Hi-Z

B13

B11

SLEEP MODE

tSMPL

*AFTER COMPLETING THE DATA TRANSFER, IF FURTHER SCK CLOCKS ARE

APPLIED WITH CONV LOW, THE ADC WILL OUTPUT ZEROS INDEFINITELY

tsuCONV

VIN*

*VIN = IN

+ – IN–

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0

•

18645 F02

VREF

IN+

IN–

GND

VCC

SCK

SDO

CONV

LTC1864

18645 F03

VIN = 0V TO VCC

VCC

1μF

SERIAL DATA LINK TO

ASIC, PLD, MPU, DSP

OR SHIFT REGISTERS

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