### AN1309: Feedback Gives Peak Detector More Precision

```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.
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