10

LTC1598

MULTICHANNEL A/D USES A SINGLE ANTIALIASING

FILTER

This circuit demonstrates how the LTC1598’s indepen-

dent analog multiplexer can simplify design of a 12-bit

data acquisition system. All eight channels are MUXed into

a single 1kHz, fourth-order Sallen-Key antialiasing filter,

which is designed for single-supply operation. Since the

LTC1598’s data converter accepts inputs from ground to

the positive supply, rail-to-rail op amps were chosen for

the filter to maximize dynamic range. The LT1368 dual rail-

to-rail op amp is designed to operate with 0.1

µF load

capacitors (C1 and C2). These capacitors provide fre-

quency compensation for the amplifiers and help reduce

the amplifier’s output impedance and improve supply

rejection at high frequencies. The filter contributes less

than 1LSB of error due to offsets and bias currents. The

filter’s noise and distortion are less than –72dB for a

The combined MUX and A/D errors result in an integral

nonlinearity error of

±3LSB (maximum) and a differential

nonlinearity error of

±3/4LSB (maximum). The typical

signal-to-noise plus distortion ratio is 71dB, with approxi-

mately –78dB of total harmonic distortion. The LTC1598

is programmed through a 4-wire serial interface that is

compatable with MICROWIRE, SPI and QSPI. Maximum

serial clock speed is 320kHz, which corresponds to a

16.8kHz sampling rate.

The complete circuit consumes approximately 800

µA

from a single 5V supply.

Simple Data Acquisition System Takes Advantage of the LTC1598’s

MUXOUT/ADCIN Pins-to-Filter Analog Signals Prior to A/D Conversion

1

2

3

4

5

6

7

8

9

10

11

12

24

23

22

21

20

19

18

17

16

15

14

13

CH5

CH6

CH7

GND

CLK

CS MUX

COM

GND

CS ADC

NC

CH4

CH3

CH2

CH1

CH0

MUXOUT

ADCIN

CLK

NC

LTC1598

ANALOG INPUTS

0V TO 5V

RANGE

7.5k

7.5k

1

µF

5V

LTC1598 • TA05

DATA IN

CHIP SELECT

CLOCK

DATA OUT

–

+

1

µF

0.015

µF

0.03

µF

C2

0.1

µF

7.5k

7.5k

0.03

µF

0.015

µF

–

+

C1

0.1

µF

5V

1/2

LT1368

1/2

LT1368