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A Low Cost DC to 500 MHz Logarithmic Amplifier

A logarithmic amplifier IC that eliminates the need for conversion to baseband, thereby reducing subsystem cost and complexity

April 1, 1998
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A Low Cost DC to 500 MHz Logarithmic Amplifier

Analog Devices Inc.
Wilmington, MA

Logarithmic amplifiers are key components in a wide range of RF system applications. By virtue of its DC to 500 MHz frequency range and 92 dB dynamic range, the model AD8307 logarithmic amplifier can be used to measure signal strength directly at the IF. Housed in a small, eight-pin, micro-SO package, this new amplifier is designed to determine the power level (in volts) of an extremely wide dynamic range of input signals.

The AD8307 amplifier is based on the progressive compression technique. Each of the cascaded gain cells provides a small-signal gain of approximately 14 dB with a 900 MHz bandwidth. Figure 1 shows the amplifier’s simplified block diagram. The resulting combination of stages produces a dynamic range that extends from -75 to +17 dBm (approximately 56 m V to 2.2 V in a 50 W system). Over this range, the logarithmic conformance is typically ±2 dB from DC to 500 MHz and ±1 dB over an 88 dB range. Since no intrinsic minimum frequency limit exists, the amplifier can be used at or below audio frequencies.

The AD8307 amplifier uses a single power supply of 2.7 to 5.5 V DC and draws 8 mA, resulting in very low power consumption of 24 mW at 3 V. A power-down control pin permits further power savings by placing the device into a standby mode with a 750 mA residual current.

The amplifier’s output voltage has a 25 mV/dB slope generated by a nominal 2 m A/dB current through an internal 12.5 kW resistor. This output voltage, shown in Figure 2 , varies from 0.25 V for an input of –74 dBm up to 2.5 V for an input of +16 dBm. The output voltage slope can be trimmed using external adjustments. For example, using a 2.7 V DC supply, the output voltage can be decreased to 15 mV/dB to permit the utilization of the entire dynamic range. The amplifier exhibits good supply voltage insensitivity and temperature stability for the scaling parameters, as shown in Figure 3 . The temperature range of the device is from –40° to +85°C. Applications for the AD8307 amplifier include transmitter antenna power measurement, receiver signal strength indication and low cost radar and sonar signal processing. The device is also suitable for use in network and spectrum analyzers and signal-level determination down to 20 Hz for true decibel AC-mode multimeters.

This new logarithmic amplifier combines low cost, small size and low power consumption along with high accuracy and stability, high dynamic range and wide operating frequency range for use in today’s high performance radar, communications and instrumentation equipment. The amplifier’s ease of use allows system designers to achieve consistent performance while reducing subsystem cost by up to 50 percent. The AD8307 logarithmic amplifier is supplied in SOIC and plastic dual-inline packages and costs approximately $6 in 1000-piece quantities. Additional information can be obtained from the company’s Web site at http://www.analog.com/AD8307.

Analog Devices Inc.,
Wilmington, MA
(781) 937-1428.

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