The Type AD8307 monolithic logarithmic amplifier is intended for a number of applications, among which:
• Conversion of signal level to decibel form
• Transmitter antenna power measurement
• Receiver signal strength indication (RSSI)
• Network and spectrum analysers (up to 120 dB)
• Signal level determination down to 20 Hz
• True decibel AC mode for multimeters
Its operation is based on the progressive compression (successive detection) technique, providing a dynamic range of 92 dB to ±3 dB law-conformance and 88 dB to a tight ±1 dB error bound at all frequencies up to 100 MHz.
The device is very stable and easy to use. It needs a supply voltage of 2.7–5.5 V at 7.5 mA, corresponding to a low power consumption of 22.5 mW at 3 V. A fast-acting CMOS-compatible control pin can disable the AD8307 to a standby current of not more than 150 μA.
Each of the cascaded amplifier/ limiter cells has a small signal gain of 14.3 dB with a –3 dB bandwidth of 900 MHz. The input is fully differential at a moderately high impedance (1.1 kΩ in parallel with 1.4 pF). The device provides a basic dynamic range extending from about –75 dBm (decibel referred to 1 mW) to around +17 dBm. A simple input-matching network can lower this range to –88 dBm to +3 dBm. The logarithmic linearity is typically within 0.3 dB up to 100 MHz over the central portion of this range, and is degraded only slightly at 500 MHz. There is no minimum frequency limit: the AD8307 may be used at audio frequencies down to DC.
The output is a voltage-scaled 25 mV dB–1, generated by a nominal current of 2 μA dB–1 through an internal 12.5 kΩ resistor. This voltage varies from 0.25 V at an input of –74 dBm (that is, the a.c. intercept is at –84 dBm, a 20 μV r.m.s. sinusoidal input), up to 2.5 V from an input of +16 dBm. This slope and intercept can be trimmed with external adjustments. For instance, with a 2.7 V supply, the output scaling may be lowered to 15 mV dB–1 to permit utilization of the full dynamic range.
The AD8307 has good supply insensitivity and temperature stability of the scaling parameters. The combination of low cost, small size, low power consumption, high accuracy and stability, large dynamic range, and a frequency range from DC to UHF make it useful in numerous applications requiring the conversion of a signal to its decibel equivalent.
Further information on the Internet: www.analog.com/AD8307