LTC2445 datasheet(6/28 Pages) LINER

6 / 28 page

LTC2444/LTC2445/

LTC2448/LTC2449

sn2444589 2444589fs

default mode of operation is CH0-CH1, OSR of 256, and 1X

mode. The serial data input contains an enable bit which

determines if a new channel/speed is selected. If this bit is

low the following conversion remains at the same speed

and selected channel. The serial data input is applied to the

device under control of the serial clock (SCK) during the

data output cycle. The first conversion following a new

channel/speed is valid.

FO (Pin 35): Frequency Control Pin. Digital input that

controls the internal conversion clock. When FO is con-

nected to VCC or GND, the converter uses its internal

oscillator running at 9MHz. The conversion rate is deter-

mined by the selected OSR such that tCONV (ms) = 40 •

OSR + 170/fOSC (kHz). The first digital filter null is located

at 8/tCONV, 7kHz at OSR = 256 and 55Hz (Simultaneous 50/

60Hz) at OSR = 32768. This pin may be driven with a

maximum external clock of 10.24MHz resulting in a maxi-

mum 8kHz output rate (OSR = 64, 2X Mode).

CS (Pin 36): Active Low Chip Select. A LOW on this pin

enables the SDO ditital output and wakes up the ADC.

Following each conversion the ADC automatically enters

the sleep mode and remains in this low power state as long

as CS is HIGH. A LOW-to-HIGH transition on CS during the

Data Output aborts the data transfer and starts a new

conversion.

SDO (Pin 37): Three-State Digital Output. During the data

output period, this pin is used as serial data output. When

the chip select CS is HIGH (CS = VCC) the SDO pin is in a

high impedance state. During the conversion and sleep

periods, this pin is used as the conversion status output.

The conversion status can be observed by pulling CS

LOW. This signal is HIGH while the conversion is in

progress and goes LOW once the conversion is complete.

SCK (Pin 38): Bidirectional Digital Clock Pin. In internal

serial clock operation mode, SCK is used as a digital output

for the internal serial interface clock during the data output

period. In the external serial clock operation mode, SCK is

used as the digital input for the external serial interface

clock during the data output period. The serial clock

operation mode is determined by the logic level applied to

the EXT pin.

GND – 0.3V to VCC + 0.3V. Within these limits, the two

selected inputs (IN+ and IN–) provide a bipolar input range

(VIN = IN+ – IN–) from –0.5 • VREF to 0.5 • VREF. Outside this

input range, the converter produces unique over-range

and under-range output codes.

CH0 to CH15 (Pins 8-23): LTC2448/LTC2449 Analog

Inputs. May be programmed for single-ended or differen-

tial mode.

CH0 to CH7 (Pins 9, 10, 13, 14, 17, 18, 21, 22): LTC2444/

LTC2445 Analog Inputs. May be programmed for single-

ended or differential mode.

NC (Pins 8, 11, 12, 15, 16, 19, 20, 23): LTC2444/

LTC2445 No Connect/Channel Isolation Shield. May be

left floating or tied to any voltage 0 to VCC in order to

provide isolation for pairs of differential input channels.

NC (Pins 24, 25, 26, 27): LTC2444/LTC2448 No Connect.

These pins can either be tied to ground or left floating.

MUXOUTP (Pin 24): LTC2445/LTC2449 Positive Multi-

plexer Output. Used to drive the input to an external buffer/

amplifier.

ADCINP (Pin 25): LTC2445/LTC2449 Positive ADC Input.

Tie to output of buffer/amplifier driven by MUXOUTP.

ADCINN (Pin 26): LTC2445/LTC2449 Negative ADC Input.

Tie to output of buffer/amplifier driven by MUXOUTN.

MUXOUTN (Pin 27): LTC2445/LTC2449 Negative Multi-

plexer Output. Used to drive the input to an external buffer/

amplifier.

VCC (Pin 28): Positive Supply Voltage. Bypass to GND with

a 10µF tantalum capacitor in parallel with a 0.1µF ceramic

capacitor as close to the part as possible.

REF+ (Pin 29), REF– (Pin 30): Differential Reference

Input. The voltage on these pins can have any value

between GND and VCC as long as the reference positive

input, REF+, is maintained more positive than the negative

reference input, REF+, by at least 0.1V.

SDI (Pin 34): Serial Data Input. This pin is used to select

the speed, 1X or 2X mode, resolution, and input channel,

for the next conversion cycle. At initial power up, the

PI FU CTIO S

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