AN-277| Application Note
The New MICRO-DAC Product Line for Microprocessor Systems
The New MICRO-DAC TM Product Line for Microprocessor Systems
A second generation of the popular MDAC (or multiplying DAC) is now available which has been designed to provide an easy interface to microprocessor systems These new MICRO-DAC products are low power drain CMOS converters which typically require only 0 5 mA supply current (2 mA max) and draw only approximately 600 mA from a 10 VDC reference supply The basic problems which are inherent in bipolar designs are not present in this CMOS product CMOS devices have nearly infinite current gain therefore there are no b or a errors in the design Also there is no analogous term to offset voltage in these products rather an ON CMOS switch is nothing more than a small resistor which can be controlled by device geometry To avoid the temperature coefficient and piezoresistive problems of diffused resistors silicon chromium thin-film resistors are used These resistors track within 1 ppm C which insures excellent temperature tracking characteristics Also the feedback resistor which is needed with an external op amp is provid
ed on the chip which insures a low temperature coefficient of the gain or full-scale reading of the DAC Bipolar designs in the 10-bit region can have a power dissipation of 300 mW Unless extreme care is taken to insure an almost perfect thermal die layout it is very possible to have a 1 C temperature gradient on the die If a diffused resistor ladder were to be used in the presence of this gradiMSB 1 2b1 1 2
National Semiconductor Application Note 277 James B Cecil July 1981
ent it will cause a 0 15% error This means that all of the allowable error in a 10-bit DAC will be used up due to this thermal gradient From this it is obvious that the CMOS DAC with its combination of a low temperature coefficient thin-film resistor ladder and an on-chip power dissipation of 30 mW max will overcome one of the major problems in bipolar designs DATA FORMATS AND DATA BUFFERS From the digital viewpoint a DAC seems little more than a write only memory where the information in the memory is made available as the analog output voltage Problems arise concerning data formatting Is the data to be left-justified (fractional binary) or right-justified (positionally weighted binary) Also updating a 10-bit DAC from an 8-bit bus can cause transient output voltage errors until the complete new word has been transferred The data format options are shown in Figure 1 Early converter manufacturers favored fractional binary and this has caused the MSB to be labeled as ``Bit 1'' on DAC products As may be expecte
d this convention has been changed in the new converter products to match the notation of the bits on the data bus of mPs People supplying converter products still favor the fractional binary format but it appears that the user groups are approximately split on the question of which to use
0 2b2 1 4
1 2b3 1 8
0 2b4 1 16
1 2b5 1 32
0 2b6 1 64
1 2b7 1 128
0 2b8 1 256
2 2b9 1 512
LSB 0 2b10 1 1024
VOUT e (fractional binary number) c VREF
a) Left-Justified Data
MSB 1 29 512
0 28 256
1 27 128
0 26 64
1 25 32
0 24 16
1 23 8
0 22 4
1 21 2
LSB 0 20 1
VOUT e (positionally weighted binary number) c VLSB where VLSB e VREF 1024
b) Right-Justified Data FIGURE 1 Data Formats for a 10-Bit Converter
AN-277
MICRO-DACTM and BI-FETTM are trademarks of National Semiconductor Corp C1995 National Semiconductor Corporation
TL H 8717
RRD-B30M115 Printed in U S A
AN-277 Application Note national Download PDF
Add this permalink to your bookmarks for future download of AN-277 ApplicationNote
Permalink: http://application.emcelettronica.com/national/AN-277
