Analog to Digital Converters Questions and Answers

Q: In successive approximation ADC offset voltage equal to 1/2 LSB is added to the D/A converter's output. This is done to :

The half-LSB offset is added to reduce the maximum quantization error.Without the offset, a successive approximation converter truncates: every input between two code levels is rounded down, so the error ranges from 0 to almost 1 LSB. Shifting the D/A comparison levels by half an LSB makes the converter round to the nearest level instead, so the error lies between -1/2 and +1/2 LSB and its maximum magnitude halves.The offset does not change the number of comparisons, so it does not affect speed; it does not add output bits; and it does not widen the input range, since the full-scale code is unchanged.

Q: An ADC used in \(3\frac{1}{2}\) (Three and half) digital multimeter is generally :

A 3½ digit multimeter generally uses a 12 bit dual slope ADC.A 3½ digit display reads up to ±1999, about 2000 counts, which needs a resolution of at least 11 bits (\(2^{11} = 2048\)); a 12-bit-class converter covers it. Digital multimeters use the dual slope (integrating) method because it is accurate, cheap and rejects mains-frequency noise by integrating over a whole number of cycles; speed does not matter for a meter that updates a few times a second. Single-chip converters such as the ICL7106 are built for exactly this.An 8-bit converter gives only 256 levels, far too few for 2000 counts, whatever its type, so all three 8-bit options are wrong.

Q: Among the following A to D converter, the slowest one is the :

The slowest converter is the Integrating type.An integrating (dual-slope) converter integrates the input for a fixed time and then measures how long a reference voltage takes to bring the integrator back to zero. Each conversion takes up to twice the full count of its counter, typically tens of milliseconds, which is why it is used in digital voltmeters where accuracy and noise rejection matter more than speed.The counting (ramp) type needs up to one full count per conversion, the successive approximation type only one clock period per bit, and the parallel comparator (flash) type converts in a single step, making it the fastest.

Q: Which of the following can be used as a substitute for analog to digital conversion ?

The substitute is a voltage to frequency convertor.A voltage to frequency converter turns an analog voltage into a pulse train whose frequency is proportional to the voltage. Counting those pulses over a fixed time gives a digital number proportional to the input, so a V/F converter with a counter performs analog to digital conversion. It is used in this way in digital voltmeters and in isolated or remote measurement.A frequency to voltage converter works the other way. An astable multivibrator only generates a fixed square wave, and a voltage regulator holds a voltage steady; neither converts an analog value into a digital one.

Q: In terms of increasing speeds the ADCs can be listed as :

In order of increasing speed: dual slope, successive approximation, flash.A dual slope ADC integrates the input for a fixed time and then de-integrates against a reference, taking milliseconds per conversion; it is slow but accurate and rejects noise, which is why it is used in digital multimeters. A successive approximation ADC finds one bit per clock cycle, so an n-bit conversion takes about n clock cycles, typically microseconds. A flash ADC compares the input with all levels at once using 2n − 1 comparators, converting in a single step of nanoseconds.Every other order puts one of these out of place, for example listing flash as the slowest.

Q: Match the List-I with List-II and select the correct answer using the codes given below :List-IList-II(a) Flash ADC(i) Music synthesiser(b) Dual slope ADC(ii) DMM(c) Successive Approximation ADC(iii) Digital Camera(d) DAC(iv) Speech digitisation

The correct code is (a)-(iii), (b)-(ii), (c)-(iv), (d)-(i).A flash ADC is the fastest converter, so it suits high-speed applications such as a digital camera (iii). A dual-slope ADC is slow but very accurate and rejects mains noise, which is why digital multimeters use it (ii). A successive approximation ADC offers moderate speed and resolution, suitable for speech digitisation (iv). A DAC turns digital samples back into an analogue signal, as in a music synthesiser (i).The other codes give the dual-slope converter to speech or a synthesiser, where its slowness would be a handicap, or misplace the DAC.

Q: An 8-bit successive approximation analog to digital converter has full scale reading of 2.55 V and its conversion time for an analog input of 1 V is 20 µS. The conversion time for a 2 V input will be :

The conversion time is still 20 µS.A successive approximation ADC tests one bit per clock cycle, starting from the most significant bit, so an n-bit conversion always takes n clock cycles, whatever the input voltage. An 8-bit converter that takes 20 µs for a 1 V input therefore also takes 20 µs for a 2 V input.The options that halve or double the time assume the conversion time depends on the input level. That is true of a counting (ramp) ADC, whose counter must count up to the input, but not of a successive approximation ADC.

Q: A digital multimeter normally uses :

A digital multimeter normally uses a dual slope ADC (spelt 'Duel' in the paper).The dual slope converter integrates the input for a fixed time, then measures how long a reference voltage takes to integrate back to zero. Because the result depends on a ratio of times, it is insensitive to drift in the integrator components, and integrating over a whole number of mains cycles rejects 50 Hz hum. It is slow, but a multimeter needs only a few readings a second, so accuracy and noise rejection matter more.Flash ADCs are very fast but costly and of low resolution, used for video and oscilloscopes. Counter and successive approximation ADCs are faster than dual slope but do not average out noise in the same way.

In successive approximation ADC offset voltage equal to 1/2 LSB is added to the D/A converter's output. This is done to :

A.Improve the speed of operation
B.Reduce maximum quantization error
C.Increase the number of bits at the output
D.Increase the range of input voltage that can be converted

Exam Year(s)

MH SET - 2018, SET Exam - 2018,

Answer: B

The half-LSB offset is added to reduce the maximum quantization error.

Without the offset, a successive approximation converter truncates: every input between two code levels is rounded down, so the error ranges from 0 to almost 1 LSB. Shifting the D/A comparison levels by half an LSB makes the converter round to the nearest level instead, so the error lies between -1/2 and +1/2 LSB and its maximum magnitude halves.

The offset does not change the number of comparisons, so it does not affect speed; it does not add output bits; and it does not widen the input range, since the full-scale code is unchanged.

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An ADC used in \(3\frac{1}{2}\) (Three and half) digital multimeter is generally :

A.12 bit dual slope
B.8-bit successive approximation
C.8-bit flash
D.8-bit dual slope

Exam Year(s)

MH SET - 2019, SET Exam - 2019,

Answer: A

A 3½ digit multimeter generally uses a 12 bit dual slope ADC.

A 3½ digit display reads up to ±1999, about 2000 counts, which needs a resolution of at least 11 bits (\(2^{11} = 2048\)); a 12-bit-class converter covers it. Digital multimeters use the dual slope (integrating) method because it is accurate, cheap and rejects mains-frequency noise by integrating over a whole number of cycles; speed does not matter for a meter that updates a few times a second. Single-chip converters such as the ICL7106 are built for exactly this.

An 8-bit converter gives only 256 levels, far too few for 2000 counts, whatever its type, so all three 8-bit options are wrong.

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Among the following A to D converter, the slowest one is the :

A.Parallel comparator type
B.Successive approximation type
C.Integrating type
D.Counting type

Exam Year(s)

MH SET - 2019, SET Exam - 2019,

Answer: C

The slowest converter is the Integrating type.

An integrating (dual-slope) converter integrates the input for a fixed time and then measures how long a reference voltage takes to bring the integrator back to zero. Each conversion takes up to twice the full count of its counter, typically tens of milliseconds, which is why it is used in digital voltmeters where accuracy and noise rejection matter more than speed.

The counting (ramp) type needs up to one full count per conversion, the successive approximation type only one clock period per bit, and the parallel comparator (flash) type converts in a single step, making it the fastest.

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Which of the following can be used as a substitute for analog to digital conversion ?

A.Frequency to voltage convertor
B.Astable multivibrator
C.Voltage to frequency convertor
D.Voltage regulator

Exam Year(s)

MH SET - 2020, SET Exam - 2020,

Answer: C

The substitute is a voltage to frequency convertor.

A voltage to frequency converter turns an analog voltage into a pulse train whose frequency is proportional to the voltage. Counting those pulses over a fixed time gives a digital number proportional to the input, so a V/F converter with a counter performs analog to digital conversion. It is used in this way in digital voltmeters and in isolated or remote measurement.

A frequency to voltage converter works the other way. An astable multivibrator only generates a fixed square wave, and a voltage regulator holds a voltage steady; neither converts an analog value into a digital one.

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In terms of increasing speeds the ADCs can be listed as :

A.Successive approximation, dual slope, flash
B.Dual slope, successive approximation, flash
C.Dual slope, flash, successive approximation
D.Flash, dual slope, successive approximation

Exam Year(s)

MH SET - 2020, SET Exam - 2020,

Answer: B

In order of increasing speed: dual slope, successive approximation, flash.

A dual slope ADC integrates the input for a fixed time and then de-integrates against a reference, taking milliseconds per conversion; it is slow but accurate and rejects noise, which is why it is used in digital multimeters. A successive approximation ADC finds one bit per clock cycle, so an n-bit conversion takes about n clock cycles, typically microseconds. A flash ADC compares the input with all levels at once using 2ⁿ − 1 comparators, converting in a single step of nanoseconds.

Every other order puts one of these out of place, for example listing flash as the slowest.

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Match the List-I with List-II and select the correct answer using the codes given below :

List-I	List-II
(a) Flash ADC	(i) Music synthesiser
(b) Dual slope ADC	(ii) DMM
(c) Successive Approximation ADC	(iii) Digital Camera
(d) DAC	(iv) Speech digitisation

A.(a)-(iii), (b)-(iv), (c)-(i), (d)-(ii)
B.(a)-(i), (b)-(ii), (c)-(iii), (d)-(iv)
C.(a)-(iii), (b)-(ii), (c)-(iv), (d)-(i)
D.(a)-(ii), (b)-(i), (c)-(iv), (d)-(iii)

Exam Year(s)

MH SET - 2021, SET Exam - 2021,

Answer: C

The correct code is (a)-(iii), (b)-(ii), (c)-(iv), (d)-(i).

A flash ADC is the fastest converter, so it suits high-speed applications such as a digital camera (iii). A dual-slope ADC is slow but very accurate and rejects mains noise, which is why digital multimeters use it (ii). A successive approximation ADC offers moderate speed and resolution, suitable for speech digitisation (iv). A DAC turns digital samples back into an analogue signal, as in a music synthesiser (i).

The other codes give the dual-slope converter to speech or a synthesiser, where its slowness would be a handicap, or misplace the DAC.

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An 8-bit successive approximation analog to digital converter has full scale reading of 2.55 V and its conversion time for an analog input of 1 V is 20 µS. The conversion time for a 2 V input will be :

A.10 µS
B.20 µS
C.40 µS
D.50 µS

Exam Year(s)

MH SET - 2023, SET Exam - 2023,

Answer: B

The conversion time is still 20 µS.

A successive approximation ADC tests one bit per clock cycle, starting from the most significant bit, so an n-bit conversion always takes n clock cycles, whatever the input voltage. An 8-bit converter that takes 20 µs for a 1 V input therefore also takes 20 µs for a 2 V input.

The options that halve or double the time assume the conversion time depends on the input level. That is true of a counting (ramp) ADC, whose counter must count up to the input, but not of a successive approximation ADC.

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A digital multimeter normally uses :

A.Flash ADC
B.counter ADC
C.Successive approximation ADC
D.Duel slope ADC

Exam Year(s)

MH SET - 2023, SET Exam - 2023,

Answer: D

A digital multimeter normally uses a dual slope ADC (spelt 'Duel' in the paper).

The dual slope converter integrates the input for a fixed time, then measures how long a reference voltage takes to integrate back to zero. Because the result depends on a ratio of times, it is insensitive to drift in the integrator components, and integrating over a whole number of mains cycles rejects 50 Hz hum. It is slow, but a multimeter needs only a few readings a second, so accuracy and noise rejection matter more.

Flash ADCs are very fast but costly and of low resolution, used for video and oscilloscopes. Counter and successive approximation ADCs are faster than dual slope but do not average out noise in the same way.

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