Datasheet

21
LTC1594L/LTC1598L
15948lfb
TYPICAL APPLICATIONS N
U
MULTICHANNEL A/D USES A SINGLE
ANTIALIASING FILTER
This circuit demonstrates how the LTC1598L’s indepen-
dent analog multiplexer can simplify design of a 12-bit
data acquisition system. All eight channels are MUXed into
a single 1kHz, 4th order Sallen-Key antialiasing filter,
which is designed for single supply operation. Since the
LTC1598L’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 0.1µF 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
100Hz, 2V
P-P
offset sine input.
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 68dB, with approxi-
mately –78dB of total harmonic distortion. The LTC1598L
is programmed through a 4-wire serial interface that is
compatible with MICROWIRE, SPI and QSPI. Maximum
serial clock speed is 200kHz, which corresponds to a
10.5kHz sampling rate.
The complete circuit consumes approximately 600µA
from a single 3V supply.
Simple Data Acquisition System Takes Advantage of the LTC1598L’s
MUXOUT/ADCIN Pins to Filter Analog Signals Prior to A/D Conversion
1594L/98L TA05
CH0
CH1
CH2
CH3
CH4
CH5
CH6
CH7
20
21
22
23
24
1
2
3
+
–
8-CHANNEL
MUX
8 COM
GND
4, 9
10
6
5, 14
7
11
CSADC
CSMUX
CLK
D
IN
D
OUT
12
13
NC
NC
12-BIT
SAMPLING
ADC
SERIAL DATA LINK
MICROWIRE AND SPI
COMPATIBLE
ADCINMUXOUT
18
3
8
1
2
17 16
3.3V
3.3V
15, 19
C7
1µF
V
REF
V
CC
LTC1598L
C6
0.1µF
–
+
C2
0.015µF
C8
0.01µF
R2
7.5k
R3
7.5k
R4
7.5k
R1
7.5k
C4
0.03µF
C3
0.1µF
C5
0.015µF
C1
0.03µF
1/2 LT1368
5
7
6
4
–
+
1/2 LT1368