You face a serious problem in using a slow ADC with a fast peak detector. The circuit in Figure 1 allows a slow ADC to measure a fast, sampled signal peak. The 100-MHz peak detector for ultrasonic-pulse sampling uses a fast MAX4231 amplifier from Maxim (www.maxim-ic.com). This amplifier has a shutdown feature that facilitates power savings without losing the sampled information. When the circuit samples a peak with a low-TTL-control input, the output of the peak-detector amplifier shuts off, and the output amplifier switches on to measure the output signal. This technique reduces power consumption by nearly 50%, because only one of the amplifiers is active at any given time. Table 1 shows the circuit functions and the amplifier modes as a function of the control input's status.

The most important advantage of the circuit is that it prevents sampling of another input peak before a measurement takes place. The first peak-detector amplifier—in shutdown mode—does not permit the reading to change. This feature helps a slow ADC to monitor a high-speed sampled peak in a desired sampling interval, an impossible operation with a conventional peak-detecting circuit. You select the R₁ and C₁ values for the desired time constant to hold the peak and then let it decay according to the RC time constant. If peak decay is undesirable, then you can use a transistor switch (not shown) without R₁ to discharge the capacitor to ground before sampling a new input-signal peak.

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