Data Conversion Circuits – D/A and A/D Converters
513
Solution
• In the case of a successive approximation type A/D converter, the final analogue output of its D/A
converter portion always settles at a value below the analogue input voltage to be digitized within
the resolution of the converter.
• The analogue input voltage = 4.365 V.
• The resolution = 10 mV.
• The number of steps = 4.365/(10 × 10
−3 = 436.5.
• Step number 436 will produce a D/A converter output of 436 × 10 = 4360mV = 436V, and step
number 437 will produce a D/Aconverter of 4.37 V.
• The A/D converter will settle at step 436.
• The digital output will be the binary equivalent of(436) 10 which is 0110110100.
Note. When this converter actually performs the conversion, in the tenth clock cycle, the LSB will be
set to ‘1’ initially. This would produce a D/A converter output of 4.37 V which exceeds the analogue
input voltage of 4.365 V. The comparator changes state, which in turn resets the LSB to ‘0’, bringing
the D/A converter output to 4.36 V. This is how a converter of this type settles where a D/A converter
output settles at a value that is one step below the value that makes it exceed the analogue input to be
digitized.
Example 12.10
Compare the average conversion time of an eight-bit counter-type A/D converter with that of an eight-bit
successive approximation type A/D converter if both are working at a 10 MHz clock frequency.
Solution
• The clock time period = 0.1 s.
• The average conversion time in the case of a counter-type A/D converter is given by
[(2
8
− 1/2 × 01 = 1275s.
• The conversion time in the case of a successive approximation type A/D converter is given by
8 × 0.1 = 0.8 s.
12.12 Integrated Circuit A/D Converters
This section presents application-relevant information of some of the popular A/D converter IC type
numbers, as it is not possible to give a detailed description of each one of them. The type numbers
included for this purpose are ADC 0800, ADC 0808, ADC 80, ADC 84, ICL 7106/ICL 7107 and AD
7820.
12.12.1 ADC-0800
ADC-0800 is a successive approximation type eight-bit A/D converter. The internal architecture of
ADC-0800 is shown in Fig. 12.41. The digital output is in complementary form and is also tristate to
permit bussing on common data lines. Its salient features include ratiometric conversion, no missing
codes, tristate outputs and a conversion time of 50 s (typical), ±1-LSB linearity and a clock frequency
range of 50–800 kHz.
513
Solution
• In the case of a successive approximation type A/D converter, the final analogue output of its D/A
converter portion always settles at a value below the analogue input voltage to be digitized within
the resolution of the converter.
• The analogue input voltage = 4.365 V.
• The resolution = 10 mV.
• The number of steps = 4.365/(10 × 10
−3 = 436.5.
• Step number 436 will produce a D/A converter output of 436 × 10 = 4360mV = 436V, and step
number 437 will produce a D/Aconverter of 4.37 V.
• The A/D converter will settle at step 436.
• The digital output will be the binary equivalent of(436) 10 which is 0110110100.
Note. When this converter actually performs the conversion, in the tenth clock cycle, the LSB will be
set to ‘1’ initially. This would produce a D/A converter output of 4.37 V which exceeds the analogue
input voltage of 4.365 V. The comparator changes state, which in turn resets the LSB to ‘0’, bringing
the D/A converter output to 4.36 V. This is how a converter of this type settles where a D/A converter
output settles at a value that is one step below the value that makes it exceed the analogue input to be
digitized.
Example 12.10
Compare the average conversion time of an eight-bit counter-type A/D converter with that of an eight-bit
successive approximation type A/D converter if both are working at a 10 MHz clock frequency.
Solution
• The clock time period = 0.1 s.
• The average conversion time in the case of a counter-type A/D converter is given by
[(2
8
− 1/2 × 01 = 1275s.
• The conversion time in the case of a successive approximation type A/D converter is given by
8 × 0.1 = 0.8 s.
12.12 Integrated Circuit A/D Converters
This section presents application-relevant information of some of the popular A/D converter IC type
numbers, as it is not possible to give a detailed description of each one of them. The type numbers
included for this purpose are ADC 0800, ADC 0808, ADC 80, ADC 84, ICL 7106/ICL 7107 and AD
7820.
12.12.1 ADC-0800
ADC-0800 is a successive approximation type eight-bit A/D converter. The internal architecture of
ADC-0800 is shown in Fig. 12.41. The digital output is in complementary form and is also tristate to
permit bussing on common data lines. Its salient features include ratiometric conversion, no missing
codes, tristate outputs and a conversion time of 50 s (typical), ±1-LSB linearity and a clock frequency
range of 50–800 kHz.