AD636 Low Level, True RMS-to-DC Converter

The AD636 is a low power monolithic IC that performs true rms-to-dc conversion on low level signals. It offers performance that is comparable or superior to that of hybrid and modular converters costing much more. The AD636 is specified for a signal range of 0 mV to 200 mV rms. Crest factors up to 6 can be accommodated with less than 0.5% additional error, allowing accurate measurement of complex input waveforms.

The low power supply current requirement of the AD636, typically 800 μA, is ideal for battery-powered portable instruments. It operates from a wide range of dual and single power supplies, from ±2.5 V to ±16.5 V or from +5 V to +24 V. The input and output terminals are fully protected; the input signal can exceed the power supply with no damage to the device (allowing the presence of input signals in the absence of supply voltage), and the output buffer amplifier is short-circuit protected.

The AD636 includes an auxiliary dB output derived from an internal circuit point that represents the logarithm of the rms output. The 0 dB reference level is set by an externally supplied current and can be selected to correspond to any input level from 0 dBm (774.6 mV) to −20 dBm (77.46 mV). Frequency response ranges from 1.2 MHz at 0 dBm to greater than 10 kHz at −50 dBm.

The AD636 is easy to use. The device is factory-trimmed at the wafer level for input and output offset, positive and negative waveform symmetry (dc reversal error), and full-scale accuracy at 200 mV rms. Therefore, no external trims are required to achieve full-rated accuracy.

The AD636 is available in two accuracy grades. The total error of the J-version is typically less than ±0.5 mV ± 1.0% of reading, while the total error of the AD636K is less than ±0.2 mV to ±0.5% of reading. Both versions are temperature rated for operation between 0°C and 70°C and available in 14-lead SBDIP and 10-lead TO-100 metal can.

The AD636 computes the true root-mean-square of a complex ac (or ac plus dc) input signal and gives an equivalent dc output level. The true rms value of a waveform is a more useful quantity than the average rectified value because it is a measure of the power in the signal. The rms value of an ac-coupled signal is also its standard deviation.

The 200 mV full-scale range of the AD636 is compatible with many popular display-oriented ADCs. The low power supply current requirement permits use in battery-powered hand-held instruments. An averaging capacitor is the only external component required to perform measurements to the fully specified accuracy is. Its value optimizes the trade-off between low frequency accuracy, ripple, and settling time.

An optional on-chip amplifier acts as a buffer for the input or the output signals. Used in the input, it provides accurate performance from standard 10 MΩ input attenuators. As an output buffer, it sources up to 5 mA.

Features and Benefits
  • Laser-trimmed to high accuracy
    0.5% maximum error (AD636K)
    1.0% maximum error (AD636J) 
  • Wide response capability
    Computes rms of ac and dc signals
    1 MHz, −3 dB bandwidth: V rms > 100 mV
    Signal crest factor of 6 for 0.5% error 
  • True rms-to-dc conversion
  • 200 mV full scale
  • dB output with 50 dB range
  • Low power: 800 μA quiescent current
  • Single or dual supply operation
  • Monolithic integrated circuit
  • Low cost
Linear Products
SPICE Models
Data Sheets
Documentnote
AD636: Low Level, True RMS-to-DC Converter Data Sheet (Rev. E)PDF 463 kB
Application Notes
Documentnote
AN-653: Improving Temperature, Stability, and Linearity of High Dynamic Range RMS RF Power Detectors (Rev. A)PDF 415 kB
AN-268: RMS-to-DC Converters Ease Measurement TasksPDF 638 kB
Technical Books
Documentnote
Order Information
Part NumberPackagePacking QtyTemp RangePrice 100-499Price 1000+RoHS
AD636JDZ Production14 ld Side-Brazed CerDIPOTH 250 to 70C22.6220.37Y
AD636JHZ ProductionROUND HEADER/METAL CANOTH 1000 to 70C13.2311.82Y
AD636KDZ Production14 ld Side-Brazed CerDIPOTH 250 to 70C40.1840.18Y
AD636KHZ ProductionROUND HEADER/METAL CANOTH 1000 to 70C18.9218.92Y
AD636: Low Level, True RMS-to-DC Converter Data Sheet (Rev. E) ad636
AD636P SPICE Macro Model ad636
AN-653: Improving Temperature, Stability, and Linearity of High Dynamic Range RMS RF Power Detectors (Rev. A) ad8314
AN-653: 改善高动态范围均方根射频功率检波器的温度稳定性和线性度 (Rev. A) ad8314
AN-268: 均方根- 直流转换器使测量工作轻松自如 ad536a
AN-268: RMS-to-DC Converters Ease Measurement Tasks ad536a