A precision full wave rectifier (often called an absolute value circuit) uses operational amplifiers to eliminate the forward voltage drop ($V_f$) of standard diodes, allowing accurate rectification of millivolt-level AC signals. In power supply design, this topology is critical for true RMS ripple measurement, AC current sensing, and precision overcurrent protection loops. Where a standard silicon diode (0.6V drop) or even a Schottky diode (0.2V drop) would completely blind a feedback controller to small AC perturbations on a DC rail, a precision active rectifier preserves signal integrity down to the microvolt level.
Topology Comparison: Rectification for PSU Feedback
When designing the sensing front-end for a power supply, you must choose how to convert the AC ripple or AC current sense signal into a DC voltage that an MCU ADC or analog error amplifier can read. The table below compares the four primary rectification topologies used in PSU feedback and monitoring circuits.
| Topology | Threshold / $V_f$ | Efficiency (at 50mV sense) | Heat Dissipation | Noise Floor | Relative Cost |
|---|---|---|---|---|---|
| Standard Silicon Bridge | ~1.2V (2x $V_f$) | 0% (Signal lost) | High ($I imes 1.2V$) | Low (Thermal only) | $ |
| Schottky Bridge | ~0.4V (2x $V_f$) | ~10% (Severe clipping) | Moderate ($I imes 0.4V$) | Low (Thermal only) | $$ |
| Precision Active (Op-Amp) | < 10µV ($V_{os}$ limited) | ~99% (Signal preserved) | Low (Quiescent only) | Medium (Op-amp $e_n$) | $$$ |
| Synchronous MOSFET | $I imes R_{DS(on)}$ (~5mV) | ~98% (Power delivery) | Very Low ($I^2R$) | High (Switching noise) | $$$$ |
Linear vs. Switching Loads: Ripple and Input Range
The requirements for your precision full wave rectifier change drastically depending on whether you are monitoring a linear or switching power supply. The input range, op-amp selection, and ripple expectations are fundamentally different.
Linear Power Supplies (100Hz/120Hz Ripple)
Linear regulators (like the LM317 or discrete pass-transistor designs) produce very low-frequency ripple (twice the mains frequency) with exceptionally low noise floors, often in the microvolt range.
Ripple Expectation: 1mV to 10mV RMS at 120Hz.
Op-Amp Requirement: You need a zero-drift, chopper-stabilized op-amp with ultra-low offset voltage ($V_{os}$) and low 1/f noise. The OPA2188 ($V_{os}$ max 25µV) is ideal here. Slew rate is irrelevant at 120Hz, but DC precision is paramount.
Switching Power Supplies (100kHz to 2MHz Ripple)
Switching converters (Buck, Boost, LLC) generate high-frequency triangular or sawtooth ripple with fast edges and high $dv/dt$.
Ripple Expectation: 10mV to 100mV peak-to-peak at 500kHz+, often riding on top of high-frequency switching spikes.
Op-Amp Requirement: DC precision matters less than AC performance. You need a high Gain-Bandwidth Product (GBW > 100MHz) and a fast Slew Rate (SR > 50V/µs) to prevent slew-induced crossover distortion when the signal crosses zero. The ADA4817 (400MHz GBW, 240V/µs SR) or OPA350 are standard choices.
Input Range Constraints: The op-amp must handle the common-mode voltage of the sense node. If you are measuring ripple directly on a 12V DC rail, the op-amp inputs will see 12V DC. Most high-speed op-amps cannot tolerate this. You must use a high-impedance resistive divider to attenuate the DC common-mode voltage to near 0V (or a virtual ground), while passing the AC ripple via a coupling capacitor, or use a dedicated high-side current sense amplifier topology.
Design Example: 500kHz Precision Rectifier for Ripple Sensing
Let’s design a precision full wave rectifier to measure the 500kHz AC ripple on a 5V switching buck converter output. The goal is to output a 0–500mV DC signal proportional to the AC ripple amplitude, which will be read by a 3.3V MCU ADC.
Input/Output Specifications
- Input Signal: 10mV to 500mV AC (peak), 500kHz, superimposed on 5V DC (AC-coupled via 100nF C0G capacitor).
- Output Signal: 0V to 500mV DC (full-wave rectified and low-pass filtered).
- Bandwidth: DC to 1MHz (-3dB).
Component Selection and Headroom Math
We will use the standard dual-op-amp absolute value topology. U1 acts as an inverting half-wave rectifier, and U2 acts as a summing amplifier to combine the original and inverted signals.
| Ref Des | Part Number | Function / Value | Critical Parameter |
|---|---|---|---|
| U1, U2 | ADA4817-2 (Dual) | Precision High-Speed Op-Amp | 400MHz GBW, 240V/µs Slew Rate |
| D1, D2 | BAS316 | Fast Switching Diodes | 4ns $t_{rr}$, 1.5pF Capacitance |
| R1, R2, R3, R4 | 10kΩ 0.1% Thin Film | Gain and Summing Resistors | Matched to 0.1% for CMRR |
| U3 | TPS7A4901 | Positive LDO (+5V Rail) | 310mV Dropout, 20µV RMS Noise |
| U4 | TPS7A3001 | Negative LDO (-5V Rail) | 216mV Dropout, High PSRR |
Regulator Dropout and Headroom Math:
To prevent single-supply crossover distortion near 0V, the ADA4817 requires dual supplies. We generate $\pm 5V$ rails using the TPS7A4901 (positive) and TPS7A3001 (negative) LDOs, fed from an isolated $\pm 7V$ auxiliary winding on the main transformer.
The TPS7A4901 has a maximum dropout voltage of 310 mV at 500 mA. If our unregulated positive bus sags to 5.5 V under heavy load, the available headroom is $5.5 V - 5.0 V = 500 mV$. This safely exceeds the 310 mV dropout requirement, ensuring the op-amp rail remains regulated.
The ADA4817 output swing requires at least 1.2V of headroom from the supply rails. With $\pm 5V$ supplies, the maximum linear output swing is $\pm 3.8V$. Since our maximum rectified output is only 500mV, we have 3.3V of unused headroom, guaranteeing zero clipping even during transient over-shoots.
Thermal Derating and Protection Strategies
High-speed op-amps in precision rectifier circuits are susceptible to thermal drift and input overvoltage faults, especially when probing live power supply nodes.
Thermal Derating Calculations
The ADA4817-2 draws a quiescent current ($I_q$) of 4.5mA per amplifier, totaling 9mA for the dual package. Operating on $\pm 5V$ rails (10V total span), the quiescent power dissipation is:
$$P_{D(quiescent)} = 10V \times 9mA = 90mW$$
The ADA4817 in an 8-lead SOIC package has a junction-to-ambient thermal resistance ($\theta_{JA}$) of approximately 120°C/W. The temperature rise above ambient is:
$$\Delta T = 90mW \times 120°C/W = 10.8°C$$
In a 40°C enclosure, the junction temperature will sit at ~51°C, well below the 125°C maximum rating. However, if the rectifier is required to drive a low-impedance load (e.g., a 50Ω coaxial cable to an oscilloscope), the output stage will dissipate significant additional heat. In our design, the output feeds a high-impedance MCU ADC ( >1MΩ ), so load-induced thermal derating is negligible.
Input Protection and Fault Survival
When measuring a switching power supply, a fault condition (like a shorted output capacitor or a control loop oscillation) can cause the AC ripple to spike from 50mV to several volts. The ADA4817 inputs are protected by internal ESD diodes, but these are only rated for a few milliamps of continuous current.
To protect the precision full wave rectifier without degrading its 1MHz bandwidth, implement the following front-end protection:
- Series Current Limiting: Place a 100Ω thin-film resistor in series with the non-inverting input. This limits fault current to safe levels.
- Transient Voltage Suppression (TVS): Place a low-capacitance TVS diode (e.g., PRTR5V0U2X or a 3.3V SMAJ3.3A) from the input node to ground. This clamps high-energy transients before they reach the op-amp.
- Phase Reversal Prevention: Some high-speed op-amps exhibit phase reversal when the input common-mode range is exceeded, causing the output to slam to the positive rail and potentially damaging downstream ADCs. The ADA4817 is specifically designed with anti-phase-reversal circuitry, but adding a 1kΩ resistor in series with the output and a 3.3V Schottky clamp to the ADC VREF pin provides a secondary hardware fail-safe.
By matching the rectifier topology to the specific noise profile of your power supply—chopper-stabilized for linear, high-GBW for switching—and strictly managing LDO headroom and thermal limits, a precision full wave rectifier becomes an indispensable tool for closing the loop on modern, ultra-low-noise power conversion systems.
References:
1. Texas Instruments, "Absolute Value Circuits" (sboa068), Application Report.
2. Analog Devices, "Ask The Applications Engineer - 33: Precision Rectifiers", Analog Dialogue.
3. Analog Devices, "MT-068: Tutorial - Op Amp Input/Output Voltage Ranges".
