DN138 - Micropower ADC and DAC in SO-8 Give PCs a 12-Bit Analog Interface

Micropower ADC and DAC in SO-8 Give PCs
a 12-Bit Analog Interface – Design Note 138
Kevin R. Hoskins
Introduction
Adding two channels of analog input/output to a PC
computer is simple, inexpensive, low powered and
compact when using the LTC ®1298 ADC and LTC1446
DAC. The LTC1298 and the LTC1446 are the first SO-8
packaged 2-channel devices of their kind. While the
application shown is for PC data acquisition, the two
converters provide the smallest, lowest power solutions
for many other analog I/O applications.
PC 2-Channel Analog I/O Interface
The circuit shown in Figure 1 connects to a PC’s serial
interface using four interface lines: DTR, RTS, CTS and
TX. DTR is used to transmit the serial clock signal, RTS
is used to transfer data to the DAC and ADC, CTS is
used to receive conversion results from the LTC1298,
and the signal on TX selects either the LTC1446 or
the LTC1298 to receive input data. The LTC1298’s and
LTC1446’s low power dissipation allow the circuit to
be powered from the serial port. The TX and RTS lines
charge capacitor C4 through diodes D5 and D6. An
LT®1021-5 regulates the voltage to 5V. Returning the
TX and RTS lines to a logic high after sending data to
the DAC or completion of an ADC conversion provides
constant power to the LT1021-5.
Small, Micropower ADC and DAC
The LTC1298 features a 12-bit ADC, 2-channel multiplexer, and an input S/H. Operating on a 5V supply at
maximum conversion rate, the LTC1298 draws just
340μA. A built-in auto shutdown further reduces power
dissipation at reduced sampling rates (to 30μA at 1ksps).
The serial interface allows remote operation.
Using a 486-33 PC, the throughput was 3.3ksps for the
LTC1298 and 2.2ksps for the LTC1446. Your mileage
may vary.
The LTC1446 is the first 12-bit dual micropower SO-8
packaged DAC. It features a 1mV/LSB (0V to 4.095V)
rail-to-rail output, internal 2.048V reference and a
simple, cost-effective, 3-wire serial interface. A DOUT pin
allows cascading multiple LTC1446s without increasing
the number of serial interconnections. Operating on a
supply of 5V, the LTC1446 draws just 1mA (typ).
Listing 1 is C code that prompts the user to either read
a conversion result from the ADC’s CH0 or write a data
word to both DAC channels.
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
U3 1/2 74HC74
U1 LTC1298
D1
510Ω
INPUT 1
D2
1
510Ω 2
3
INPUT 2
510Ω
D3
510Ω 4
D4
CS
VCC
CH0
CLK
CH1
DOUT
GND
DIN
8
7
2
C1
0.1μF
6
4
3
D
PR
Q
CLR
CK
Q
6
5
1
5V
+
U2 LTC1446
2
3
4
AOUT1
AOUT2
CLK
VOUTB
DIN
VCC
CS/LD
GND
DOUT
VOUTA
C4
150μF
D6
5
4
1
+
4
C5
47μF
6
D5
2
U4
LT1021-5
U3 1/2 74HC74
8
12
7
6
10
C2
0.1μF
11
D
PR
Q
CLR
CK
Q
8
9
2
1 51k SELECT
6
5 51k DIN
12
13 51k SCLK
11
10
9
13
5V
8
5
5V
3
DOUT
C3
0.1μF
D1 TO D6: 1N914
Figure 1. Communicating Over the Serial Port, the LTC1298 and LTC1446 in SO-8 Create a
Simple, Low Power 2-Channel Analog Interface for PCs
09/96/138_conv
TX
RTS
DTR
CTS
DN138 F01
Conclusion
These SO-8 packaged data converters offer unprecedented space, power and economy to data acquisition
system designers. They form a very nice 12-bit analog
interface to a wide variety of portable, battery-powered
and size constrained products. They are extremely easy
to apply and require a minimum of passive support
components and interconnections.
Listing 1. Configure Analog Interface with this C Code
#define port 0x3FC/* Control register, RS232 */
#define inprt 0x3FE/* Status reg. RS232 */
#define LCR 0x3FB/* Line Control Register */
#define high 1
#define low 0
#define Clock 0x01/* pin 4, DTR */
#define Din 0x02/* pin 7, RTS */
#define Dout 0x10/* pin 8, CTS input */
#include<stdio.h>
#include<dos.h>
#include<conio.h>
/* Function module sets bit to high or low */
void set_control(int Port,char bitnum,int flag)
{
char temp;
temp = inportb(Port);
if (flag==high)
temp |= bitnum; /* set output bit to high */
else
temp &= ~bitnum; /* set output bit to low */
outportb(Port,temp);
}
/* This function brings CS high or low (consult the
schematic) */
void CS_Control(direction)
{
if (direction)
{
set_control(port,Clock,low);/* set clock high for Din to
be read */
set_control(port,Din,low); /* set Din low */
set_control(port,Din,low);/* set Din high to make CS goes
high */
}
else {
outportb(port, 0x01);/* set Din & clock low */
Delay(10);
outportb(port, 0x03);/* Din goes high to make CS
goes low */
}
}
/* This function outputs a 24 bit(2x12) digital code to
LTC1446L */
void Din_(long code,int clock)
{
int x;
for(x = 0; x<clock; ++x)
{
code <<= 1;/* align the Din bit */
if (code & 0x1000000)
{
set_control(port,Clock,high);\/* set Clock low */
set_control(port,Din,high);/* set Din bit high */
}
else {
set_control(port,Clock,high); /* set Clock low */
set_control(port,Din,low);/* set Din low */
}
set_control(port,Clock,low);/* set Clock high for
DAC to latch */
}
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}
/* Read bit from ADC to PC */
Dout_()
{
int temp, x, volt =0;
for(x = 0; x<13; ++x)
{
set_control(port,Clock,high);
set_control(port,Clock,low);
temp = inportb(inprt);/* read status reg. */
volt <<= 1;/* shift left one bit for serial
transmission */
if(temp & Dout)
volt += 1;
/* add 1 if input bit is high */
}
return(volt & 0xfff);
}
/* menu for the mode selection */
char menu()
{
printf(“Please select one of the following:\na: ADC\nd:
DAC\nq: quit\n\n”);
return (getchar());
}
void main()
{
long code;
char mode_select;
int temp,volt=0;
/* Chip select for DAC & ADC is controlled by RS232 pin
3 TX line. When LCR’s bit 6 is set, the DAC is selected
and the reverse is true for the ADC. */
outportb(LCR,0x0);/* initialize DAC */
outportb(LCR,0x64);/* initialize ADC */
while((mode_select = menu()) != ‘q’)
{
switch(mode_select)
{
case ‘a’:
{
outportb(LCR,0x0);/* selecting ADC */
CS_Control(low);/* enabling the ADC CS */
Din_(0x680000, 0x5);/* channel selection */
volt = Dout_();
outportb(LCR,0x64);/* bring CS high */
set_control(port,Din,high);/* bring Din signal
high */
printf(“\ncode: %d\n”,volt);
}
break;
case ‘d’:
{
printf(“Enter DAC input code (0 - 4095):\n”);
scanf(“%d”, &temp);
code = temp;
code += (long)temp << 12;/* converting 12 bit to 24 bit
word */
outportb(LCR,0x64);/* selecting DAC */
CS_Control(low);/* CS enable */
Din_(code,24);/* loading digital data to DAC */
outportb(LCR,0x0);/* bring CS high */
outportb(LCR,0x64);/* disabling ADC */
set_control(port,Din,high);/* bring Din signal high */
}
break;
}
}
}
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