AD637 DATASHEET PDF

All versions are available in hermetically-sealed, lead side-brazed ceramic DIPs as well as low cost cerdip packages. A lead SOIC package is also available. The AD 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. The AD is laser wafer trimmed to achieve rated perfor- mance without external trimming.

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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. As with previous monolithic rms converters from Analog Devices, the AD 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.

A chip select connection on the AD permits the user to decrease the supply current from 2. A 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 AD is powered down the output goes to a high impedance state. This allows several ADs to be tied together to form a wide-band true rms multiplexer. The input circuitry of the AD 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. All versions are available in hermetically-sealed, lead side-brazed ceramic DIPs as well as low cost cerdip packages.

A lead SOIC package is also available. The AD 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. The AD 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. The chip select feature of the AD permits the user to power down the device down during periods of nonuse, thereby, decreasing battery drain in remote or hand-held applications. 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.

E Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other rights of third parties which may result from its use.

No license is granted by implication or otherwise under any patent or patent rights of Analog Devices. One Technology Way, P. Box , Norwood, MA , U. Analog Devices, Inc.

C, and? A —2— REV. Specifications subject to change without notice. Specifications shown in boldface are tested on all production units at final electrical test. Results from those tests are used to calculate outgoing quality levels. All min and max specifications are guaranteed, although only those shown in boldface are tested on all production units. Indefinite Storage Temperature Range. I3 BIAS 24k? A1 6k? Electrostatic charges as high as V readily accumulate on the human body and test equipment and can discharge without detection.

Although the AD features proprietary ESD protection circuitry, permanent damage may occur on devices subjected to high energy electrostatic discharges. Therefore, proper ESD precautions are recommended to avoid performance degradation or loss of functionality. The actual computation performed by the AD follows the equation?

V rms? If the RC time constant of the filter is much greater than the longest period of the input signal than A4s output will be proportional to the average of I4. The output of this filter amplifier is used by A3 to provide the denominator current I3 which equals Avg. A nominal 5 pF capacitor should be used to insure stability. Standard RMS Connection The performance of the AD is tolerant of minor variations in the power supply voltages, however, if the supplies being used exhibit a considerable amount of high frequency ripple it is advisable to bypass both supplies to ground through a 0.

F ceramic disc capacitor placed as close to the device as possible. The output signal range of the AD is a function of the supply voltages, as shown in Figure 3. The output signal can be used buffered or nonbuffered depending on the characteristics of the load.

If no buffer is needed, tie buffer input Pin 1 to common. The output of the AD is capable of driving 5 mA into a 2 k?

In the standard rms connection shown in Figure 2, only a single external capacitor is required to set the averaging time constant. In this configuration, the AD will compute the true rms of any input signal. An averaging error, the magnitude of which will be dependent on the value of the averaging capacitor, will be present at low frequencies. For example, if the filter capacitor CAV, is 4?

F this error will be 0. If it is desired to measure only ac signals, The AD includes a chip select feature which allows the user to decrease the quiescent current of the device from 2.

This is done by driving the CS, Pin 5, to below 0. Under these conditions, the output will go into a high impedance state. In addition to lowering power consumption, this feature permits bussing the outputs of a number of ADs to form a wide bandwidth rms multiplexer. If the chip select is not being used, Pin 5 should be tied high. These trims will result in significant reduction in the maximum total error as shown in Figure 4.

This remaining error is due to a nontrimmable input offset in the absolute value circuit and the irreducible nonlinearity of the device. The trimming procedure on the AD is as follows: l.

Alternatively R1 can be adjusted to give the correct output with the lowest expected value of VIN. Connect the desired full scale input to VIN, using either a dc or a calibrated ac signal, trim R3 to give the correct output at Pin 9, i. Of course, a 2 V peak-to-peak sine wave should give 0. Remaining errors are due to the nonlinearity. F of averaging capacitance. As shown in Figure 6, the averaging error is defined as the peak value of the ac component, ripple, plus the value of the dc error.

The uncertainty can be significantly reduced through the use of a post filtering network or by increasing the value of the averaging capacitor. Max Total Error vs. F CAV Figure 5. The deviation from the ideal rms value is due to an averaging error. The averaging error is comprised of an ac and dc component. Both components are REV. E —5— The ac ripple component of averaging error can be greatly reduced by increasing the value of the averaging capacitor.

A preferable method of reducing the ripple is through the use of the post filter network, shown in Figure 8. This network can be used in either a one or two pole configuration. For most applications the single pole filter will give the best overall compromise between ripple and settling time. Two Pole Sallen-Key Filter Figure 9a shows values of CAV and the corresponding averaging error as a function of sine-wave frequency for the standard rms connection.

Figure 9b shows the relationship between averaging error, signal frequency settling time and averaging capacitor value. This graph is drawn for filter capacitor values of 3. This ratio sets the magnitude of the ac and dc errors equal at 50 Hz. As an example, by using a 1? F averaging capacitor and a 3. F filter capacitor, the ripple for a 60 Hz input signal will be reduced from 5. This gives a factor of thirty reduction in ripple and yet the settling time would only increase by a factor of three.

The values of CAV and C2, the filter capacitor, can be calculated for the desired value of averaging error and settling time by using Figure 9b. The symmetry of the input signal also has an effect on the magnitude of the averaging error.

Table I gives practical component values for various types of 60 Hz input signals. These capacitor values can be directly scaled for frequencies other than 60 Hz, i. For applications that are extremely sensitive to ripple, the two pole configuration is suggested. This configuration will minimize capacitor values and settling time while maximizing performance. Figure 9c can be used to determine the required value of CAV, C2 and C3 for the desired level of ripple and settling time.

CAV 10 10 1. F ms Crest factor is often overlooked in determining the accuracy of an ac measurement. Crest factor is defined as the ratio of the peak signal amplitude to the rms value of the signal C. Waveforms which resemble low duty cycle pulse trains, such as those occurring in switching power supplies and SCR circuits, have high crest factors.

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