1、1 FEATURES Computes True rms value Average rectified value Absolute value Provides 200 mV full-scale input range (larger inputs with input attenuator) High input impedance: 1012 Low input bias current: 25 pA maximum High accuracy: 0.3 mV 0.3% of reading RMS conversion with signal crest factors up to
2、 5 Wide power supply range: +2.8 V, 3.2 V to 16.5V Low power: 200 mA maximum supply current Buffered voltage output No external trims needed for specified accuracy AD737an unbuffered voltage output version with chip power-down also available GENERAL DESCRIPTION The AD736 is a low power, precision, m
3、onolithic true rms-to-dc converter. It is laser trimmed to provide a maximum error of 0.3 mV 0.3% of reading with sine wave inputs. Furthermore, it maintains high accuracy while measuring a wide range of input waveforms, including variable duty-cycle pulses and triac (phase)-controlled sine waves. T
4、he low cost and small size of this converter make it suitable for upgrading the performance of non-rms precision rectifiers in many applications. Compared to these circuits, the AD736 offers higher accuracy at an equal or lower cost. The AD736 can compute the rms value of both ac and dc input voltag
5、es. It can also be operated as an ac-coupled device by adding one external capacitor. In this mode, the AD736 can resolve input signal levels of 100 Vrms or less, despite variations in temperature or supply voltage. High accuracy is also maintained for input waveforms with crest factors of 1 to 3. I
6、n addition, crest factors as high as 5 can be measured (introducing only 2.5% additional error) at the 200 mV full-scale input level. The AD736 has its own output buffer amplifier, thereby pro-viding a great deal of design flexibility. Requiring only 200 A of power supply current, the AD736 is optim
7、ized for use in portable multimeters and other battery-powered applications. 2 The AD736 allows the choice of two signal input terminals: a high impedance FET input (1012 ) that directly interfaces with High-Z input attenuators and a low impedance input (8 k) that allows the measurement of 300 mV in
8、put levels while operating from the minimum power supply voltage of +2.8 V, 3.2 V. The two inputs can be used either single ended or differentially. The AD736 has a 1% reading error bandwidth that exceeds 10 kHz for the input amplitudes from 20 mV rms to 200 mV rms while consuming only 1 mW. The AD7
9、36 is available in four performance grades. The AD736J and AD736K grades are rated over the 0C to +70C and 20C to +85C commercial temperature ranges. The AD736A and AD736B grades are rated over the 40C to +85C industrial temperature range. The AD736 is available in three low cost, 8-lead packages: P
10、DIP, SOIC, and CERDIP. PRODUCT HIGHLIGHTS 1. The AD736 is capable of computing the average rectified value, absolute value, or true rms value of various input signals. 2. Only one external component, an averaging capacitor, is required for the AD736 to perform true rms measurement. 3. The low power
11、consumption of 1 mW makes the AD736 suitable for many battery-powered applications. 4. A high input impedance of 1012 eliminates the need for an external buffer when interfacing with input attenuators. 5. A low impedance input is available for those applications that require an input signal up to 30
12、0 mV rms operating from low power supply voltages. SPECIFICATIONS At 25C 5 V supplies, ac-coupled with 1 kHz sine wave input applied, unless otherwise noted. Specifications in bold are tested on all production units at final electrical test. Results from those tests are used to calculate outgoing qu
13、ality levels. Stresses above those listed under Absolute Maximum Ratings may cause permanent damage to the device. This is a stress rating only； functional operation of the device at these or any other conditions above those indicated in the operational section of this specification is not implied.
14、Exposure to absolute maximum rating conditions for extended periods may affect device reliability. 3 THEORY OF OPERATION As shown by Figure 18, the AD736 has five functional subsections: the input amplifier, full-wave rectifier (FWR), rms core, output amplifier, and bias section. The FET input ampli
15、fier allows both a high impedance, buffered input (Pin 2) and a low impedance, wide dynamic range input (Pin 1). The high impedance input, with its low input bias current, is well suited for use with high impedance input attenuators. The output of the input amplifier drives a full-wave precision rec
16、tifier that, in turn, drives the rms core. The essential rms operations of squaring, averaging, and square rooting are performed in the core using an external averaging capacitor, CAV. Without CAV, the rectified input signal travels through the core unprocessed, as is done with the average respondin
17、g connection (see Figure 19). A final subsection, an output amplifier, buffers the output from the core and allows optional low-pass filtering to be performed via the external capacitor, CF, which is connected across the feedback path of the amplifier. In the average responding connection, this is w
18、here all of the averaging is carried out. In the rms circuit, this additional filtering stage helps reduce any output ripple that was not removed by the averaging capacitor, CAV. TYPES OF AC MEASUREMENT The AD736 is capable of measuring ac signals by operating as either an average responding converter or a true rms-to-dc converter. As its name implies, an average
