Editing Analog-to-digital converter (section)

==Types==
These are several common ways of implementing an electronic ADC.

=== RC charge time ===
[[RC circuit|Resistor-capacitor (RC) circuits]] have a known voltage charging and discharging curve that can be used to solve for an unknown analog value.

==== Wilkinson ====
The '''Wilkinson ADC''' was designed by [[Denys Wilkinson]] in 1950. The Wilkinson ADC is based on the comparison of an input voltage with that produced by a charging capacitor. The capacitor is allowed to charge until a comparator determines it matches the input voltage. Then, the capacitor is discharged linearly by using a constant [[current source]]. The time required to discharge the capacitor is proportional to the amplitude of the input voltage. While the capacitor is discharging, pulses from a high-frequency oscillator clock are counted by a register. The number of clock pulses recorded in the register is also proportional to the input voltage.<ref>{{Harvtxt|Knoll|1989|pp=663–664}}</ref><ref>{{Harvtxt|Nicholson|1974|pp=309–310}}</ref>

==== Measuring analog resistance or capacitance ====
If the analog value to measure is represented by a resistance or capacitance, then by including that element in an [[RC circuit]] (with other resistances or capacitances fixed) and measuring the time to charge the capacitance from a known starting voltage to another known ending voltage through the resistance from a known voltage supply, the value of the unknown resistance or capacitance can be determined using the capacitor charging equation:

<math display="block">V_\text{capacitor}(t) = V_\text{supply}\left(1 - e^{-\frac{t}{RC}}\right)</math>

and solving for the unknown resistance or capacitance using those starting and ending datapoints. This is similar but contrasts to the Wilkinson ADC which measures an unknown voltage with a known resistance and capacitance, by instead measuring an unknown resistance or capacitance with a known voltage.

For example, the positive (and/or negative) pulse width from a [[555 timer IC#Modes|555 Timer IC in monostable or astable mode]] represents the time it takes to charge (and/or discharge) its capacitor from {{Frac|1|3}}&nbsp;''V''<sub>supply</sub> to {{Frac|2|3}}&nbsp;''V''<sub>supply</sub>. By sending this pulse into a microcontroller with an accurate clock, the duration of the pulse can be measured and converted using the capacitor charging equation to produce the value of the unknown resistance or capacitance.

Larger resistances and capacitances will take a longer time to measure than smaller one. And the accuracy is limited by the accuracy of the microcontroller clock and the amount of time available to measure the value, which potentially might even change during measurement or be affected by external [[Parasitic element (electrical networks)|parasitics]].

===Direct-conversion===
{{main|Flash ADC}}
A direct-conversion or flash ADC has a bank of [[comparator]]s sampling the input signal in parallel, each firing for a specific voltage range. The comparator bank feeds a [[digital encoder]] [[logic circuit]] that generates a binary number on the output lines for each voltage range.

ADCs of this type have a large [[Die (integrated circuit)|die]] size and high power dissipation. They are often used for [[video]], [[wideband communications]], or other fast signals in [[optical storage|optical]] and [[magnetic storage]].

The circuit consists of a resistive divider network, a set of op-amp comparators and a priority encoder. A small amount of hysteresis is built into the comparator to resolve any problems at voltage boundaries. At each node of the resistive divider, a comparison voltage is available. The purpose of the circuit is to compare the analog input voltage with each of the node voltages.

The circuit has the advantage of high speed as the conversion takes place simultaneously rather than sequentially. Typical conversion time is 100&nbsp;ns or less. Conversion time is limited only by the speed of the comparator and of the priority encoder. This type of ADC has the disadvantage that the number of comparators required almost doubles for each added bit. Also, the larger the value of n, the more complex is the priority encoder.

===Successive approximation ===
A [[successive-approximation ADC]] uses a comparator and a [[binary search]] to successively narrow a range that contains the input voltage. At each successive step, the converter compares the input voltage to the output of an internal [[digital-to-analog converter]] (DAC) which initially represents the midpoint of the allowed input voltage range. At each step in this process, the approximation is stored in a successive approximation register (SAR) and the output of the digital-to-analog converter is updated for a comparison over a narrower range.

===Ramp-compare===
A ramp-compare ADC produces a [[Sawtooth wave|saw-tooth signal]] that ramps up or down then quickly returns to zero.<ref>Couch - 2001 - Digital and analog communication systems - Prentice Hall - New Jersey, USA</ref>
When the ramp starts, a timer starts counting. When the ramp voltage matches the input, a comparator fires, and the timer's value is recorded. Timed ramp converters can be implemented economically,{{efn|A very simple (nonlinear) ramp converter can be implemented with a microcontroller and one resistor and capacitor.<ref>{{cite web |url=http://www.atmel.com/dyn/resources/prod_documents/doc0942.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.atmel.com/dyn/resources/prod_documents/doc0942.pdf |archive-date=2022-10-09 |url-status=live |title=Atmel Application Note AVR400: Low Cost A/D Converter |website=atmel.com}}</ref>}}  however, the ramp time may be sensitive to temperature because the circuit generating the ramp is often a simple analog [[integrator]]. A more accurate converter uses a clocked counter driving a DAC. A special advantage of the ramp-compare system is that converting a second signal just requires another comparator and another register to store the timer value. To reduce sensitivity to input changes during conversion, a [[sample and hold]] can charge a capacitor with the instantaneous input voltage and the converter can time the time required to discharge with a [[constant current]].

===Integrating ===
An '''[[integrating ADC]]''' (also '''dual-slope''' or '''multi-slope''' ADC) applies the unknown input voltage to the input of an [[operational amplifier applications#Inverting integrator|integrator]] and allows the voltage to ramp for a fixed time period (the run-up period). Then a known reference voltage of opposite polarity is applied to the integrator and is allowed to ramp until the integrator output returns to zero (the run-down period). The input voltage is computed as a function of the reference voltage, the constant run-up time period, and the measured run-down time period. The run-down time measurement is usually made in units of the converter's clock, so longer integration times allow for higher resolutions. Likewise, the speed of the converter can be improved by sacrificing resolution. Converters of this type (or variations on the concept) are used in most [[digital voltmeter]]s for their linearity and flexibility.
; Charge balancing ADC: The principle of charge balancing ADC is to first convert the input signal to a frequency using a [[voltage-to-frequency converter]]. This frequency is then measured by a counter and converted to an output code proportional to the analog input. The main advantage of these converters is that it is possible to transmit frequency even in a noisy environment or in isolated form. However, the limitation of this circuit is that the output of the voltage-to-frequency converter depends upon an RC product whose value cannot be accurately maintained over temperature and time.
; Dual-slope ADC: The analog part of the circuit consists of a high input impedance buffer, precision integrator and a voltage comparator. The converter first integrates the analog input signal for a fixed duration and then it integrates an internal reference voltage of opposite polarity until the integrator output is zero. The main disadvantage of this circuit is the long duration time. They are particularly suitable for accurate measurement of slowly varying signals such as [[thermocouple]]s and [[weighing scale]]s.

===Delta-encoded===
A ''delta-encoded'' or ''counter-ramp'' ADC has an up-down [[Counter (digital)|counter]] that feeds a DAC. The input signal and the DAC both go to a comparator. The comparator controls the counter. The circuit uses negative [[feedback]] from the comparator to adjust the counter until the DAC's output matches the input signal and number is read from the counter. Delta converters have very wide ranges and high resolution, but the conversion time is dependent on the input signal behavior, though it will always have a guaranteed worst-case. Delta converters are often very good choices to read real-world signals as most signals from physical systems do not change abruptly. Some converters combine the delta and successive approximation approaches; this works especially well when high frequency components of the input signal are known to be small in magnitude.

===Pipelined===
A ''pipelined ADC'' (also called ''subranging quantizer'') uses two or more conversion steps. First, a coarse conversion is done. In a second step, the difference to the input signal is determined with a DAC. This difference is then converted more precisely, and the results are combined in the last step. This can be considered a refinement of the successive-approximation ADC wherein the feedback reference signal consists of the interim conversion of a whole range of bits (for example, four bits) rather than just the next-most-significant bit. By combining the merits of the successive approximation and flash ADCs this type is fast, has a high resolution, and can be implemented efficiently.

===Delta-sigma===
{{main|Delta-sigma modulation }}
A '''delta-sigma ADC''' (also known as a '''sigma-delta ADC''') is based on a [[negative feedback]] loop with an analog filter and low resolution (often 1 bit) but high [[sampling rate]] ADC and DAC. The feedback loop continuously corrects accumulated quantization errors and performs [[noise shaping]]: quantization noise is reduced in the low frequencies of interest, but is increased in higher frequencies. Those higher frequencies may then be removed by a [[downsampling]] [[digital filter]], which also converts the data stream from that high sampling rate with low [[Audio bit depth|bit depth]] to a lower rate with higher bit depth.

===Time-interleaved===
A [[time-interleaved ADC]] uses M parallel ADCs where each ADC samples data every M:th cycle of the effective sample clock. The result is that the sample rate is increased M times compared to what each individual ADC can manage. In practice, the individual differences between the M ADCs degrade the overall performance reducing the [[spurious-free dynamic range]] (SFDR).<ref>{{cite journal|last=Vogel|first=Christian|title=The Impact of Combined Channel Mismatch Effects in Time-interleaved ADCs|journal=IEEE Transactions on Instrumentation and Measurement|year=2005|volume=55|issue=1|pages=415–427|doi=10.1109/TIM.2004.834046|bibcode=2005ITIM...54..415V |citeseerx=10.1.1.212.7539|s2cid=15038020}}</ref> However, techniques exist to correct for these time-interleaving mismatch errors.<ref>{{citation |url=https://www.analog.com/en/analog-dialogue/articles/interleaving-adcs.html |author1=Gabriele Manganaro |author2=David H. Robertson |title=Interleaving ADCs: Unraveling the Mysteries |date=July 2015 |publisher=[[Analog Devices]] |access-date=2021-10-07}}</ref>

===Intermediate FM stage===
An ADC with an intermediate FM stage first uses a [[voltage-to-frequency converter]] to produce an oscillating signal with a frequency proportional to the voltage of the input signal, and then uses a [[frequency counter]] to convert that frequency into a digital count proportional to the desired signal voltage. Longer integration times allow for higher resolutions. Likewise, the speed of the converter can be improved by sacrificing resolution. The two parts of the ADC may be widely separated, with the frequency signal passed through an [[opto-isolator]] or transmitted wirelessly. Some such ADCs use sine wave or square wave [[frequency modulation]]; others use [[pulse-frequency modulation]]. Such ADCs were once the most popular way to show a digital display of the status of a remote analog sensor.<ref>
[http://www.analog.com/static/imported-files/tutorials/MT-028.pdf Analog Devices MT-028 Tutorial: "Voltage-to-Frequency Converters"] by Walt Kester and James Bryant 2009,
apparently adapted from Kester, Walter Allan (2005) [https://books.google.com/books?id=0aeBS6SgtR4C&pg=RA2-PA274 ''Data conversion handbook''], Newnes, p. 274, {{ISBN|0750678410}}.</ref><ref>
[http://ww1.microchip.com/downloads/en/AppNotes/00795a.pdf Microchip AN795 "Voltage to Frequency / Frequency to Voltage Converter"] p. 4: "13-bit A/D converter"
</ref><ref>Carr, Joseph J. (1996) [https://books.google.com/books?id=1yBTAAAAMAAJ ''Elements of electronic instrumentation and measurement''], Prentice Hall, p. 402, {{ISBN|0133416860}}.</ref><ref>[http://www.globalspec.com/reference/3127/Voltage-to-Frequency-Analog-to-Digital-Converters "Voltage-to-Frequency Analog-to-Digital Converters"]. globalspec.com</ref><ref>Pease, Robert A. (1991) [https://books.google.com/books?id=3kY4-HYLqh0C&pg=PA130 ''Troubleshooting Analog Circuits''], Newnes, p. 130, {{ISBN|0750694998}}.</ref>

===Time-stretch===
A [[time-stretch analog-to-digital converter]] (TS-ADC) digitizes a very wide bandwidth analog signal, that cannot be digitized by a conventional electronic ADC, by time-stretching the signal prior to digitization. It commonly uses a [[photonic]] [[preprocessor]] to time-stretch the signal, which effectively slows the signal down in time and compresses its bandwidth. As a result, an electronic ADC, that would have been too slow to capture the original signal, can now capture this slowed-down signal. For continuous capture of the signal, the front end also divides the signal into multiple segments in addition to time-stretching. Each segment is individually digitized by a separate electronic ADC. Finally, a [[digital signal processor]] rearranges the samples and removes any distortions added by the preprocessor to yield the binary data that is the digital representation of the original analog signal.

=== Measuring physical values other than voltage ===
{{Unreferenced section|date=October 2023}}
Although the term ADC is usually associated with measurement of an analog voltage, some partially-electronic devices that convert some measurable physical analog quantity into a digital number can also be considered ADCs, for instance:

* [[Rotary encoder]]s convert from an analog physical quantity that mechanically produces an amount of [[Rotation around a fixed axis|rotation]] into a stream of digital [[Gray code]] that a [[microcontroller]] can digitally interpret to derive the direction of rotation, angular position, and rotational speed.<ref>{{Cite web |date=2019-10-01 |title=How to Use Rotary Encoders to Quickly Convert Mechanical Rotation into Digital Signals |url=https://www.techbriefs.com/component/content/article/tb/supplements/md/features/articles/35291 |access-date=2023-10-09 |website=Techbriefs}}</ref>
* [[Capacitive sensing]] converts from the analog physical quantity of a [[capacitance]]. That capacitance could be a [[Proxy (statistics)|proxy]] for some other physical quantity, such as the distance some metal object is from a metal sensing plate, or the amount of water in a tank, or the [[permittivity]] of a [[dielectric]] material.
** Capacitive-to-digital (CDC) converters determine capacitance by applying a known excitation to a plate of a [[capacitor]] and measuring its charge.<ref>{{Cite web |last=Jia |first=Ning |date=2012-05-01 |title=ADI Capacitance-to-Digital Converter Technology in Healthcare Applications |url=https://www.analog.com/en/analog-dialogue/articles/capacitance-to-digital-converter-technology-healthcare.html |url-status=live |archive-url=https://web.archive.org/web/20230707111410/https://www.analog.com/en/analog-dialogue/articles/capacitance-to-digital-converter-technology-healthcare.html |archive-date=2023-07-07 |access-date=2023-10-09 |website=[[Analog Dialogue]]}}</ref>
* [[Digital calipers]] convert from the analog physical quantity of an amount of displacement between two sliding rulers.
* Inductive-to-digital converters measure a change of [[inductance]] by a conductive target moving in an [[inductor]]'s [[Alternating current|AC]] [[magnetic field]].<ref>{{Cite web |last=Kasemsadeh |first=Ben |date=2015-07-31 |title=How To Sense Lateral Movement Using An Inductance-to-Digital Converter |url=https://www.fierceelectronics.com/components/how-to-sense-lateral-movement-using-inductance-to-digital-converter |url-status=live |archive-url=https://web.archive.org/web/20231009202429/https://www.fierceelectronics.com/components/how-to-sense-lateral-movement-using-inductance-to-digital-converter |archive-date=2023-10-09 |access-date=2023-10-09 |website=Fierce Electronics}}</ref>
* [[Time-to-digital converter|Time-to-digital converters]] recognize events and provide a digital representation of the analog [[time]] they occurred.
** [[Time of flight]] measurements for instance can convert from some analog quantity that affects a [[propagation delay]] for an event.
* [[Sensor]]s in general that don't directly produce a voltage may indirectly produce a voltage or through other ways be converted into a digital value.
** Resistive output (e.g. from a [[potentiometer]] or a [[force-sensing resistor]]) can be made into a voltage by sending a known current through it, or can be made into a [[RC time constant|RC charging time]] measurement, to produce a digital result.