a FEATURES High Accuracy 0.02% Max Nonlinearity, 0 V to 2 V RMS Input 0.10% Additional Error to Crest Factor of 3 Wide Bandwidth 8 MHz at 2 V RMS Input 600 kHz at 100 mV RMS Computes: True RMS Square Mean Square Absolute Value dB Output (60 dB Range) Chip Select-Power Down Feature Allows: Analog “3-State” Operation Quiescent Current Reduction from 2.2 mA to 350 A Side-Brazed DIP, Low Cost Cerdip and SOIC PRODUCT DESCRIPTION
The AD637 is a complete high accuracy monolithic rms-to-dc converter that computes the true rms value of any complex waveform. It offers performance that is unprecedented in integrated circuit rms-to-dc converters and comparable to discrete and modular techniques in accuracy, bandwidth and dynamic range. A crest factor compensation scheme in the AD637 permits measurements of signals with crest factors of up to 10 with less than 1% additional error. The circuit’s wide bandwidth permits the measurement of signals up to 600 kHz with inputs of 200 mV rms and up to 8 MHz when the input levels are above 1 V rms. As with previous monolithic rms converters from Analog Devices, the AD637 has an auxiliary dB output available to the user. The logarithm of the rms output signal is brought out to a separate pin allowing direct dB measurement with a useful range of 60 dB. An externally programmed reference current allows the user to select the 0 dB reference voltage to correspond to any level between 0.1 V and 2.0 V rms. A chip select connection on the AD637 permits the user to decrease the supply current from 2.2 mA to 350 µA during periods when the rms function is not in use. This feature facilitates the addition of precision rms measurement to remote or hand-held applications where minimum power consumption is critical. In addition when the AD637 is powered down the output goes to a high impedance state. This allows several AD637s to be tied together to form a wide-band true rms multiplexer. The input circuitry of the AD637 is protected from overload voltages that are in excess of the supply levels. The inputs will not be damaged by input signals if the supply voltages are lost.
High Precision, Wide-Band RMS-to-DC Converter AD637 FUNCTIONAL BLOCK DIAGRAMS
The AD637 is available in two accuracy grades (J, K) for commercial (0°C to +70°C) temperature range applications; two accuracy grades (A, B) for industrial (–40°C to +85°C) applications; and one (S) rated over the –55°C to + 125°C temperature range. All versions are available in hermetically-sealed, 14-pin side-brazed ceramic DIPs as well as low cost cerdip packages. A 16-pin SOIC package is also available. PRODUCT HIGHLIGHTS
1. The AD637 computes the true root-mean-square, mean square, or absolute value of any complex ac (or ac plus dc) input waveform and gives an equivalent dc output voltage. The true rms value of a waveform is more useful than an average rectified signal since it relates directly to the power of the signal. The rms value of a statistical signal is also related to the standard deviation of the signal. 2. The AD637 is laser wafer trimmed to achieve rated performance without external trimming. The only external component required is a capacitor which sets the averaging time period. The value of this capacitor also determines low frequency accuracy, ripple level and settling time. 3. The chip select feature of the AD637 permits the user to power down the device down during periods of nonuse, thereby, decreasing battery drain in remote or hand-held applications. 4. The on-chip buffer amplifier can be used as either an input buffer or in an active filter configuration. The filter can be used to reduce the amount of ac ripple, thereby, increasing the accuracy of the measurement.
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