# Peak detector
A peak detector is an electronic circuit that captures and stores the highest (peak) voltage reached by an input waveform, typically using a diode that charges a storage capacitor and blocks discharge when the input falls.[1](https://sc-mi.univ-batna2.dz/sites/default/files/mi/files/cours_eln_chap12.pdf?m=1762827680) The same name is used for software functions that find the locations and amplitudes of local maxima and minima in sampled signals.[2](https://www.ni.com/en/support/documentation/supplemental/06/peak-detection-using-labview-and-measurement-studio.html)
| Key fact | Detail |
|---|---|
| Basic circuit | A series diode and capacitor; the capacitor charges to the peak of the input AC signal minus the diode's forward voltage drop[3](https://wiki.analog.com/university/courses/electronics/text/chapter-7) |
| Operation | When the input rises, the diode conducts and charges the capacitor; when the input falls, the diode becomes reverse biased and the capacitor holds the maximum voltage[1](https://sc-mi.univ-batna2.dz/sites/default/files/mi/files/cours_eln_chap12.pdf?m=1762827680) |
| Reset | A switch in parallel with the capacitor is generally needed to periodically reset the output when a new peak detection is desired[3](https://wiki.analog.com/university/courses/electronics/text/chapter-7) |
| Precision version | An op amp placed around the diode reduces the diode-drop error by its forward gain, but the simple form saturates when the diode is reverse biased[3](https://wiki.analog.com/university/courses/electronics/text/chapter-7) |
| Envelope variant | Adding a resistor in parallel with the capacitor lets the output fall slowly, producing an envelope detector; one textbook example uses C = 10 µF, R = 10 kΩ at f = 10 Hz[4](https://left.engr.usu.edu/courses/3410/microelectronics_1_book.pdf) |
| Monolithic example | The Analog Devices PKD01, a monolithic peak detector with reset-and-hold mode, tracked an input until a maximum was reached and retained it on a hold capacitor; the part is now obsolete[5](https://www.analog.com/en/products/pkd01.html) |
| Applications | AM demodulation, signal and voltage-peak measurement, envelope detection in communications, and instrumentation[1](https://sc-mi.univ-batna2.dz/sites/default/files/mi/files/cours_eln_chap12.pdf?m=1762827680) |
## Basic circuit and operation
The simplest peak detector is the series connection of a diode and a capacitor. The diode conducts only when the input exceeds the voltage already stored on the capacitor, so each rising excursion tops the capacitor up to a new maximum; when the input falls below the stored voltage, the reverse-biased diode blocks discharge and the capacitor holds the peak.[3](https://wiki.analog.com/university/courses/electronics/text/chapter-7) • [6](https://www.winlab.rutgers.edu/~crose/322_html/envelope_detector.html) With the diode oriented one way the circuit detects positive peaks; reversing the diode makes it detect negative peaks.[3](https://wiki.analog.com/university/courses/electronics/text/chapter-7)
Two limitations follow directly from this arrangement. First, the output is not the true peak of the input, because the diode's built-in forward voltage drop is subtracted from it. Second, once a peak is held, nothing removes it: a switch in parallel with the capacitor is generally needed to reset the output when a new measurement is wanted.[3](https://wiki.analog.com/university/courses/electronics/text/chapter-7)
## Precision (op amp) peak detectors
Placing an op amp in the circuit reduces the diode-drop error, because the amplifier's forward gain compensates for the diode's built-in voltage. In the basic precision form, however, when the input is more negative than the held capacitor voltage, the diode is reverse biased, which opens the negative-feedback path to the inverting input, and the op amp output saturates at the negative supply rail.[3](https://wiki.analog.com/university/courses/electronics/text/chapter-7) When the input again exceeds the held voltage, the amplifier must first recover from saturation, so the circuit may not respond properly to fast, short-duration positive peaks.[3](https://wiki.analog.com/university/courses/electronics/text/chapter-7)
**Holding the value.** After a peak is captured, the stored charge must be protected. A high-performance design uses an op amp with very low input bias current to buffer the capacitor voltage; the Texas Instruments OPA111, for example, has a maximum input bias current of 1 picoamp, so it does not drain the charge on a low-leakage capacitor while presenting the stored voltage to the rest of the circuit.[7](https://www.digikey.com/en/articles/hardware-vs-software-tradeoffs-part-2-peak-detectors) Leakage currents of the peak-detecting diode and the reset switch are also a design concern; one [National Semiconductor](https://www.edgepedia.org/national-semiconductor) (now TI) application circuit for extended hold used isolation so that these leakage currents are absorbed before reaching the storage capacitor, at the cost of slower acquisition.[8](https://www.ti.com/lit/an/snoa818/snoa818.pdf)
A complete peak-detection function needs more than the hold circuit: a comparator compares the held peak value with the present, real-time input, and its output indicates that a peak has occurred when the current value falls below the held value.[7](https://www.digikey.com/en/articles/hardware-vs-software-tradeoffs-part-2-peak-detectors)
**Speed and capacitance tradeoffs.** Any peak detector trades precision against speed of detection. If the threshold difference between the held peak and the present value is set very low, such as a few millivolts or a few percent of full scale, peaks are identified quickly, but signal noise can produce false peaks. An all-analog peak detector can operate from near DC to megahertz and even into the gigahertz range given suitable amplifiers, and the choice of capacitor value is left to the designer.[7](https://www.digikey.com/en/articles/hardware-vs-software-tradeoffs-part-2-peak-detectors)
## Envelope detector
Adding a resistor in parallel with the capacitor turns the peak detector into an envelope detector: the diode rapidly charges the capacitor, and the resistor slowly discharges it, so the output falls when the input falls and tracks the envelope of the waveform rather than holding a single maximum.[4](https://left.engr.usu.edu/courses/3410/microelectronics_1_book.pdf) This is the classic AM demodulation circuit, sometimes called a leaky peak detector, where "leaky" refers to the discharge path provided by the resistor.[9](https://www.allaboutcircuits.com/TEXTBOOK/radio-frequency-analysis-design/radio-frequency-demodulation/how-to-demodulate-an-am-waveform/)
For AM demodulation the resistance is chosen so that the discharge is slow enough to smooth out the carrier-frequency variations but fast enough not to smooth out the lower-frequency envelope variations; the detector should retain the peak relative to the carrier but not relative to the envelope.[9](https://www.allaboutcircuits.com/TEXTBOOK/radio-frequency-analysis-design/radio-frequency-demodulation/how-to-demodulate-an-am-waveform/) Most practical envelope detectors use half-wave or full-wave rectification followed by low-pass filtering to recover the modulating signal. The filtering is rarely perfect, so some ripple remains on the output, particularly for low-frequency inputs such as bass guitar notes; more filtering gives a smoother result but reduces the high-frequency response to the original modulating signal.[3](https://wiki.analog.com/university/courses/electronics/text/chapter-7)
The simple diode demodulator is cheap, needing only a diode, a capacitor, and a resistor, but real diodes are nonlinear, so the demodulated output is distorted in a way that depends on the diode's I/V characteristic; most AM transistor radios produce roughly 5 to 10 percent distortion, acceptable for casual listening but not for accurate reproduction.[6](https://www.winlab.rutgers.edu/~crose/322_html/envelope_detector.html)
## Monolithic and integrated implementations
Dedicated integrated peak detectors have existed since at least 1981, when [Analog Devices](https://www.edgepedia.org/analog-devices) released the PKD01, a monolithic peak detector with reset-and-hold mode. The PKD01 tracked an analog input until a maximum amplitude was reached and retained that value on a hold capacitor; its monolithic construction provided matching characteristics that reduce charge injection and droop-rate errors compared with hybrid modules and discrete designs, and it included an uncommitted comparator for application flexibility. The part is now listed as obsolete.[5](https://www.analog.com/en/products/pkd01.html) Peak-detection functions also appear inside larger integrated circuits; the SMSC EMC1702 current-sense monitor, for example, includes peak current detection circuitry that works independently of its current-measurement integration and asserts a THERM output and PEAK status bit when a programmed peak condition is met.[10](https://ww1.microchip.com/downloads/jp/DeviceDoc/1702.pdf)
## Applications
Peak detectors serve in instrumentation and communications wherever a maximum value must be captured or an envelope followed: AM radio demodulation, signal measurement of voltage peaks, and envelope detection in communication systems.[1](https://sc-mi.univ-batna2.dz/sites/default/files/mi/files/cours_eln_chap12.pdf?m=1762827680) In particle-detection instrumentation, the CITIROC read-out ASIC used for the focal-plane detectors of the ASTRI-Horn Cherenkov telescope implements a peak detector reading mode in which each channel's shaped signal maximum is captured during an adjustable time interval; the mode allows calibration of SiPM sensor gain without external light sources, with two parallel preamplifier chains reading charge from 160 fC to 320 pC, corresponding to 1 to 2000 photo-electrons at a SiPM gain of 106.[11](https://arxiv.org/abs/2001.01339) Fast peak detection also appears in power-electronics protection, where current-shunt monitoring circuits must react to peak over-current conditions in SiC or GaN switch systems, in many cases shutting down in less than 1 µs to prevent damage.[12](https://www.ti.com/document-viewer/lit/html/sboa593)
## Software peak detection
The term also covers digital signal processing. When applied in software, peak detection means determining where a signal's local maxima and minima occur and how large they are, subject to conditions that may be as basic as a threshold or as strict as a demand that the peak's shape match a prototype. It is used in fields such as chemistry, biology, and music, including spectroscopy, chromatography, and tone detection. LabVIEW and Measurement Studio provide peak detector functions that can process a signal broken into multiple data blocks, retaining internal state and a history buffer between calls so that peaks near block boundaries are located correctly.[2](https://www.ni.com/en/support/documentation/supplemental/06/peak-detection-using-labview-and-measurement-studio.html)
## References
1. [Chapter 12: Diodes and Semiconductors, Fundamental Electronics, University of Batna 2](https://sc-mi.univ-batna2.dz/sites/default/files/mi/files/cours_eln_chap12.pdf?m=1762827680)
2. [Peak Detection Using LabVIEW and Measurement Studio, National Instruments](https://www.ni.com/en/support/documentation/supplemental/06/peak-detection-using-labview-and-measurement-studio.html)
3. [Chapter 7: Diode Application Topics, Analog Devices Wiki](https://wiki.analog.com/university/courses/electronics/text/chapter-7)
4. [Microelectronics I, Utah State University course text](https://left.engr.usu.edu/courses/3410/microelectronics_1_book.pdf)
5. [PKD01: Monolithic Peak Detector with Reset-and-Hold Mode, Analog Devices](https://www.analog.com/en/products/pkd01.html)
6. [The Envelope Detector, WINLAB, Rutgers University](https://www.winlab.rutgers.edu/~crose/322_html/envelope_detector.html)
7. [Hardware vs. Software Tradeoffs Part 2: Peak Detectors, DigiKey](https://www.digikey.com/en/articles/hardware-vs-software-tradeoffs-part-2-peak-detectors)
8. [Applying a New Precision Op Amp (AN-242), Texas Instruments](https://www.ti.com/lit/an/snoa818/snoa818.pdf)
9. [How to Demodulate an AM Waveform, All About Circuits](https://www.allaboutcircuits.com/TEXTBOOK/radio-frequency-analysis-design/radio-frequency-demodulation/how-to-demodulate-an-am-waveform/)
10. [EMC1702 High-Side Current-Sense and Dual 1°C Temperature Monitor Datasheet, SMSC/Microchip](https://ww1.microchip.com/downloads/jp/DeviceDoc/1702.pdf)
11. [Use of the Peak-Detector Mode for Gain Calibration of SiPM Sensors with ASIC CITIROC Read-Out, arXiv:2001.01339](https://arxiv.org/abs/2001.01339)
12. [Quickly Detecting Overcurrent Faults With Low-Side Current Shunt Measurement (SBOA593), Texas Instruments](https://www.ti.com/document-viewer/lit/html/sboa593)
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