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