Free Active Peak Detector & Hold Capacitor Tool

1. Active Peak Detector Operation & Diode Isolation

Passive diode-capacitor peak detectors suffer from a major flaw: the forward diode drop ($V_f approx 0.6 ext{V}$ for silicon, $0.3 ext{V}$ for Schottky) creates massive non-linear deadbands and temperature drift.

An active peak detector places the rectifying diode inside the negative feedback loop of an operational amplifier. The op-amp drives the diode with whatever voltage is necessary to equalize the voltage on the holding capacitor ($C_{hold}$) with the input signal ($V_{in}$), effectively dividing $V_f$ by the open-loop gain ($A_{OL} > 100,000$) down to mere microvolts.

2. Sizing the Holding Capacitor ($C_{hold}$)

Selecting $C_{hold}$ requires satisfying two opposing constraints:

• Acquisition Speed ($C_{hold} le C_{max}$): To capture a narrow transient pulse of duration $t_{pulse}$, the op-amp output current $I_{out}$ must charge $C_{hold}$ up to $V_{peak}$: $$C_{max} = rac{I_{out} cdot t_{pulse}}{V_{peak}}$$
• Voltage Hold Droop ($C_{hold} ge C_{min}$): During the hold phase, total leakage current $I_{total} = I_{diode_leak} + I_{buffer_bias}$ slowly drains charge off the capacitor: $$rac{dV}{dt} = rac{I_{total}}{C_{hold}} implies C_{min} = rac{I_{total} cdot t_{hold}}{Delta V_{droop_allowable}}$$

3. Dielectric Absorption (Soakage) Trap

Standard X7R/Y5V ceramic capacitors and aluminum electrolytics exhibit dielectric absorption (memory effect). Even if discharged to zero, trapped molecular dipoles slowly release residual charge, creating phantom voltages ($0.5% - 2%$ error). Precision sample-and-hold circuits strictly require low-absorption dielectrics: Polypropylene (PP), Polystyrene, Teflon (PTFE), or C0G/NP0 ceramic (dielectric absorption $< 0.02%$).