Feedback Gives Peak Detector More Precision ® Application Note May 8, 2007 AN1309.0 Tamara A. Papalias and Mike Wong The standard method for measuring the peak of a signal involves the use of a diode. If the diode is used alone, the input voltage has to be significantly larger than the turn-on voltage to obtain acceptable accuracy. The turn-on voltage ranges from 200mV in germanium diodes to 700mV in silicon diodes. To achieve 10% error, the simple detector requires an input voltage of 2V to 7V, respectively. 20 3.3k 0 Q1 R1 ERROR (%) 5 -5 -10 0.0 0.5 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 FIGURE 2. PEAK ERROR vs AMPLITUDE Figure 3 plots the error versus frequency for a 1VP-P input signal. Again, errors are small for frequencies below 1MHz. This plot also shows that errors are within 5% up to 3MHz and 10% up to about 15MHz. Recall that a 1VP-P signal into a simple peak detector would exhibit 70% error. 40 EL8203_PEAK DETECTOR CIRCUIT 35 ERROR (%) 30 VS = +5V VIN = 1 VP-P VIN = 1 VP-P 25 20 15 10 5 0 -5 -10 0.01 0.10 1.00 FREQUENCY (MHz) 10.00 100.00 FIGURE 3. PEAK ERROR vs FREQUENCY (USING ISL8116 GENERATOR WITH TRIANGLE SIGNALS) In summary, a high-speed dual op amp and a few components can add resolution and accuracy to a simple peak detector. The given circuit provides 2% error for signals up to 1MHz, making it suitable for audio applications and localized transmission, like infrared. D6 BAS16 R23 100k R2 1.0 VIN_MAX (V) The quality of this peak detector implementation is demonstrated in Figures 2 and 3. Figure 2 plots the error versus input voltage swing for various frequencies. The large bandwidth of the dual op amp package allows for errors within a few percentage points for signals up to 1MHz. The "sweet spot" of operation extends from 700mV to about 4V with a 5V supply. V3 10 0 The resistor network surrounding Q1 provides a clamping circuit. When the input to the circuit is higher than the average, the output of the forward op amp is also higher than average. The diode (D6) conducts as in the simple case, and Q1 is held off. However, when the input drops below the average level, the diode does not conduct and Q1 allows a feedback path to be created around the forward op amp. This switched feedback action forces the negative input of the forward op amp to track the output voltage. This tracking behavior is critical for recovery, especially at high speeds. If this network was removed, the op amp could saturate and would suffer in recovery time. 5V V1 + U52 7 3 V+ 0 6 + 2 V4 EL8202 0 R19 FREQUENCY = 1KHz FREQUENCY = 10KHz FREQUENCY = 100KHz FREQUENCY = 1MHz VS = +5V 15 High-frequency op amps can utilize feedback to compensate for the diode turn-on voltage. The circuit in Figure 1 shows such a configuration. The diode labeled D6 is providing the rectification, just like it would in a simple peak detector. Similarly, R23 and C1 provide the low-pass filtering to average the peak signal. One of the dual op amps buffers this averaged output while the other provides a highimpedance input and feedback node for the circuit. V EL8203_PEAK DETECTOR CIRCUIT 2N2222A/ZTX C1 5V V2 + U51 7 3 V+ 6 + 2 V4 0 EL8202B V 0 10k 1k 22p R2 C3 1k FIGURE 1. EL8202 PEAK DETECTOR 1 CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures. 1-888-INTERSIL or 1-888-468-3774 | Intersil (and design) is a registered trademark of Intersil Americas Inc. Copyright Intersil Americas Inc. 2007. All Rights Reserved All other trademarks mentioned are the property of their respective owners. Application Note 1309 Intersil Corporation reserves the right to make changes in circuit design, software and/or specifications at any time without notice. Accordingly, the reader is cautioned to verify that the Application Note or Technical Brief is current before proceeding. For information regarding Intersil Corporation and its products, see www.intersil.com 2 AN1309.0 May 8, 2007