DN112 - A Versatile 8-Channel Multiplexer

A Versatile 8-Channel Multiplexer – Design Note 112
Kevin R. Hoskins
Introduction
Available in either 16-pin DIP or narrow body SOIC
packages, the CMOS LTC ®1390 is a high performance
8-to-1 analog multiplexer. It is addressed through a
3-wire digital interface featuring bidirectional serial data.
The LTC1390 features a typical on-resistance of 45Ω,
typical switch leakage of 50pA and guaranteed breakbefore-make switch operation. Charge injection is ±10pC
(max). All digital inputs remain logic compatible whether
operating on single or dual supplies. The inputs are
robust, easily withstanding 100mA fault currents. The
LTC1390 operates over a wide power supply voltage
range of 3V to ±15V.
Low Power, Daisy-Chain Serial Interface,
8-Channel A/D System
Figure 1 shows the LTC1390 connected to the LTC1286
12-bit micropower A/D converter. The Clock (CLK)
and Chip Select (CS) signals are connected in parallel
to the LTC1390 and LTC1286. Both the LTC1390 and
the LTC1286 are designed for serial data transfer. The
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their
respective owners.
5V – 5V
0.1μF
1
2
3
4
ANALOG
INPUTS
5
6
7
8
V+
S0
S1
LTC1390
S2
5V
0.1μF
OPTIONAL A/D
INPUT FILTER
16
1
15
2
14
V–
3
D
S3
DATA 2
S4
DATA 1
S5
CS
S6
CLK
S7
GND
VREF
VCC
8
0.1μF
7
CLK
LTC1286
6
–IN
DOUT
4
5
GND CS/SHDN
13
12
+IN
11
10
9
DATA
CLK
CS
DN112 • F01
3-WIRE
SERIAL
INTERFACE
TO MUX AND ADC
Figure 1. The LTC1390 Expands the 12-Bit Micropower LTC1286 ADC Input Capacity to Eight Channels
CLK
CS
EN = LOW
EN = HIGH
DATA 1
B2
B1
B2
B0
B1
B0
ANY
ANALOG
INPUTS
D
DN112 • F02
tON
tOFF
Figure 2. The LTC1390 Clocks in Data While CS is High and Latches the Data,
Enabling the Selected Channel, When CS Goes Low
09/95/112_conv
rail-to-rail precision LT®1366 operational amplifier. The
filter’s cutoff frequency is set to 1kHz.
LTC1390 also includes provisions for daisy-chain operation. This allows full bidirectional communication over
a single serial data line. While CS is high the LTC1286
is inactive. The serial data used to select a multiplexer
channel is applied to the LTC1390 DATA 1 input. Four bits
are clocked into the LTC1390. The LTC1286 DOUT is in a
high impedance state, ignoring these bits. The LTC1390
latches the four data bits when the CS signal goes low,
selecting a channel and initiating an A/D conversion
from the LTC1286. Subsequent clock cycles shift out
LTC1286 conversion data through the LTC1390 (from
DATA 1 to DATA 2) to a host microprocessor. Figure 2
shows the LTC1390 timing diagram.
Conclusion
The LTC1390 provides designers of a serially interfaced
data acquisition system with a flexible, low power
and cost-effective way to expand the number of A/D
channels.
VCC VEE
0.1μF
0.1μF
1
2
3
It is very easy to expand the width of the multiplexer and
take advantage of the LTC1390 daisy-chain capability.
Figure 3 shows the simple connections needed to add
multiplexer channels. The Data 2 on each additional
LTC1390 is connected to the Data 1 on the preceding
LTC1390. When operating with multiple LTC1390s, the
channel selecting data is sent in groups of four bits.
4
ANALOG
INPUTS 5
6
7
8
1
2
3
Another useful feature of the LTC1390 is the ability to add
analog signal processing between the LTC1390 output
and an A/D input. This helps save overall system cost
because only one signal processing circuit is needed
instead of one circuit per multiplexer input channel.
4
ANALOG
INPUTS 5
6
7
8
16
1
15
S1 LTC1390 D
14
–
A
S2
V
13
S3
DATA 2
12
S4
DATA 1
11
S5
CS
10
S6
CLK
9
S7
GND
2
V+
S0
V+
S0
VCC
VREF
8
7
CLK
LTC1286
6
–IN
DOUT
4
5
CS
GND
+IN
3
16
15
S1 LTC1390 D
14
B
S2
V–
13
S3
DATA 2
12
S4
DATA 1
11
S5
CS
10
S6
CLK
9
S7
GND
DATA
CS
CLK
DN112 • F03
Figure 4 shows an active 2nd order lowpass filter connected between the LTC1390 output and the LTC1286
input. The heart of the lowpass filter is the single supply
5V
VCC
0.1μF
Figure 3. Additional Multiplexer Channels Are Easy
to Add Without Adding Additional Serial Lines
5V 0.1μF
3000pF
5V
0.1μF
–
1
2
3
4
5
6
7
8
V+
S0
S1
S2
LTC1390
D
16
15
14
V–
S3
DATA 2
S4
DATA 1
S5
CS
S6
CLK
S7
GND
75k
75k
1500pF
13
LT1366
+
1
2
VREF
VCC
8
0.1μF
7
CLK
LTC1286
3
6
–IN
DOUT
4
5
CS
GND
+IN
12
11
10
9
DN112 • F04
CS
CLK
DATA
Figure 4. The Connection Between the LTC1390 Output and the LTC1286 Input
Is the Perfect Place for Signal Conditioning Circuits Such as the LT1366-Based
2nd Order Active Lowpass Filter
Data Sheet Download
www.linear.com
Linear Technology Corporation
For applications help,
call (408) 432-1900
dn112f_conv LT/GP 0995 160K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 1995
Similar pages