250 MHz Demodulating Logarithmic Amplifier

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Product Details

The AD641 is a 250 MHz, demodulating logarithmic amplifier with an accuracy of ±2.0 dB and 44 dB dynamic range. The AD641 uses a successive detection architecture to provide an output current that is logarithmically proportional to its input voltage. The output current can be converted to a voltage using one of several on-chip resistors to select the slope. A single AD641 provides up to 44 dB of dynamic range at speeds up to 250 MHz, and two cascaded AD641s together can provide 58 dB of dynamic range at speeds up to 250 MHz. The AD641 is fully stable and well characterized over either the industrial or military temperature ranges.

The AD641 is not a logarithmic building block, but rather a complete logarithmic solution for compressing and measuring wide dynamic range signals. The AD641 is comprised of five stages and each stage has a full wave rectifier, whose current depends on the absolute value of its input voltage. The output of these stages are summed together to provide the demodulated output current scaled at 1 mA per decade (50 µA/dB).

Without utilizing the 10× input attenuator, log conformance of 2.0 dB is maintained over the input range –44 dBm to 0 dBm. The attenuator offers the most flexibility without significantly impacting performance.

The 250 MHz bandwidth and temperature stability make this product ideal for high speed signal power measurement in RF/ IF systems. ECM/Radar and Communication applications are routinely in the 100 MHz–180 MHz range for power measurement. The bandwidth and accuracy, as well as dynamic range, make this part ideal for high speed, wide dynamic range signals. The AD641 is offered in industrial (–40°C to +85°C) and military (–55°C to +125°C) package temperature ranges. Industrial versions are available in plastic DIP and PLCC; MIL versions are packaged in cerdip.


  • IF/RF signal processing
  • Received Signal Strength Indicator (RSSI)
  • High speed signal compression
  • High speed spectrum analyzer
  • ECM/radar

Product Lifecycle

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At least one model within this product family is in production and available for purchase. The product is appropriate for new designs but newer alternatives may exist.

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ADI has always placed the highest emphasis on delivering products that meet the maximum levels of quality and reliability. We achieve this by incorporating quality and reliability checks in every scope of product and process design, and in the manufacturing process as well.  "Zero defects" for shipped products is always our goal.

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The USA list pricing shown is for BUDGETARY USE ONLY, shown in United States dollars (FOB USA per unit for the stated volume), and is subject to change. International prices may differ due to local duties, taxes, fees and exchange rates. For volume-specific price or delivery quotes, please contact your local Analog Devices, Inc. sales office or authorized distributor. Pricing displayed for Evaluation Boards and Kits is based on 1-piece pricing.

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