6: ADC
ARCHITECTURES
6: ADC
ARCHITECTURES
There are mainly four types of ADC architectures:
Flash-type,
Pipeline,
Successive approximation,
Oversampled ADCs.
There are mainly four types of ADC architectures:
Flash-type,
Pipeline,
Successive approximation,
Oversampled ADCs.
The ADC has a continuous, infinite-valued signal as its
input. The input transitions determine the amount of
INL and DNL associated with the converter.
Flash-type
Figure : Block diagram of a Flash ADC
Flash or parallel converters have the highest speed of any
type of ADC. As shown in Fig., they utilize one comparator
per quantization level (2N - 1) and 2N resistors (a resistor
string DAC). The reference voltage is divided into 2N
values, each of which is fed into a comparator. The input
voltage is compared with each reference value and results
in a thermometer code at the output of the comparators. A
thermometer code will exhibit all zeros for each resistor
level if the value of VIN is less than the value on the
resistor string, and ones if VIN is greater than or equal to
voltage on the resistor string.
Flash or parallel converters have the highest speed of any
type of ADC. As shown in Fig., they utilize one comparator
per quantization level (2N - 1) and 2N resistors (a resistor
string DAC). The reference voltage is divided into 2N
values, each of which is fed into a comparator. The input
voltage is compared with each reference value and results
in a thermometer code at the output of the comparators. A
thermometer code will exhibit all zeros for each resistor
level if the value of VIN is less than the value on the
resistor string, and ones if VIN is greater than or equal to
voltage on the resistor string.
A simple 2N-1, N digital thermometer decoder circuit
converts the compared data into an N-bit digital word.
The obvious advantage of this converter is the speed
with which one conversion can take place. Each clock
pulse generates an output digital word.
The disadvantage of this is doubling of area with each
bit of increased resolution.
For example, an 8-bit converter requires 255
comparators, but a 9-bit ADC requires 511. Flash
converters have traditionally been limited to 8-bit
resolution
A simple 2N-1, N digital thermometer decoder circuit
converts the compared data into an N-bit digital word.
The obvious advantage of this converter is the speed
with which one conversion can take place. Each clock
pulse generates an output digital word.
The disadvantage of this is doubling of area with each
bit of increased resolution.
For example, an 8-bit converter requires 255
comparators, but a 9-bit ADC requires 511. Flash
converters have traditionally been limited to 8-bit
resolution
Another disadvantages of the Flash ADC are the power
requirements of the 2N-1 comparators. The speed is
limited by the switching of the comparators and the
digital logic.
Problem: Design a 3-bit Flash converter, listing the values
of the voltages at each resistor tap, and draw the transfer
curve for VIN = 0 to 5 V. Assume VREF = 5 V. Construct a
table listing the values of the thermometer code and the
output of the decoder for VIN =1.5, 3.0, and 4.5 V.
Problem: Design a 3-bit Flash converter, listing the values
of the voltages at each resistor tap, and draw the transfer
curve for VIN = 0 to 5 V. Assume VREF = 5 V. Construct a
table listing the values of the thermometer code and the
output of the decoder for VIN =1.5, 3.0, and 4.5 V.
Accuracy Issues for the Flash ADC
Accuracy is dependent on the matching of the resistor
string and the input offset voltage of the comparators.
The voltage on the i-th tap of the resistor string was found
to be
where VI ideal is the voltage at the i-th tap if all the resistors
had an ideal value of R. The term, ΔRk is the value of the
resistance error (difference from ideal) due to the
mismatch.
The switching point for the i-th comparator, Vsw,i thenbecomes
where Vos,i, is the input referred offset voltage of the i-th
comparator. The INL for the converter can then be
described as
The worst-case INL will be assumed to be occur at the
middle of the string
where it is assumed that the maximum positive mismatch
occurs in all the resistors in the lower half of the string
and the maximum negative mismatch occurs in the upper
half (or vice versa) and that the comparator at the i-th tap
contains the maximum offset voltage, |Vos,i|max,
where it is assumed that the maximum positive mismatch
occurs in all the resistors in the lower half of the string
and the maximum negative mismatch occurs in the upper
half (or vice versa) and that the comparator at the i-th tap
contains the maximum offset voltage, |Vos,i|max,
Problem: If a 10-bit Flash converter is designed,
determine the maximum offset voltage of the comparators
which will make the INL less than 1/2 LSB. Assume that
the resistor string is perfectly matched and VREF = 5 V.
The offset voltage be equal to 1/2 LSB.(i.e. ½ * Vref/2N) ThereforeThe offset voltage be equal to 1/2 LSB.(i.e. ½ * Vref/2N) Therefore
The DNL calculation for the Flash converter is
calculated Using the definition of DNL,
The maximum DNL will occur assuming ΔRi is at its
maximum, Vos,I is at its maximum positive value, and Vos,i-1is at its maximum negative voltage. Thus,
The maximum DNL will occur assuming ΔRi is at its
maximum, Vos,I is at its maximum positive value, and Vos,i-1is at its maximum negative voltage. Thus,
The Two-Step Flash ADC