Data Sheet

AD8318

1 MHz to 8 GHz, 70 dB Logarithmic Detector/Controller

Rev. E

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FEATURES

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Wide bandwidth: 1 MHz to 8 GHz

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High accuracy: ±1.0 dB over 55 dB range (f < 5.8 GHz)

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Stability over temperature: ±0.5 dB

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Low noise measurement/controller output (VOUT)

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Pulse response time: 10 ns/12 ns (fall/rise)

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Integrated temperature sensor

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Small footprint LFCSP

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Power-down feature: <1.5 mW at 5 V

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Single-supply operation: 5 V at 68 mA

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Fabricated using high speed SiGe process

APPLICATIONS

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RF transmitter PA setpoint control and level monitoring

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RSSI measurement in base stations, WLAN, WiMAX, and radars

FUNCTIONAL BLOCK DIAGRAM

Figure 1.

GENERAL DESCRIPTION

The AD8318 is a demodulating logarithmic amplifier, capable of

accurately converting an RF input signal to a corresponding deci-

bel-scaled output voltage. It employs the progressive compression

technique over a cascaded amplifier chain, each stage of which is

equipped with a detector cell. The device is used in measurement

or controller mode. The AD8318 maintains accurate log conform-

ance for signals of 1 MHz to 6 GHz and provides useful operation

to 8 GHz. The input range is typically 60 dB (referenced to 50 Ω)

with error less than ±1 dB. The AD8318 has a 10 ns response

time that enables RF burst detection to beyond 45 MHz. The

device provides unprecedented logarithmic intercept stability vs.

ambient temperature conditions. A 2 mV/°C slope temperature

sensor output is also provided for additional system monitoring. A

single supply of 5 V is required. Current consumption is typically 68

mA. Power consumption decreases to <1.5 mW when the device is

disabled.

The AD8318 can be configured to provide a control voltage to a

VGA, such as a power amplifier or a measurement output, from Pin

VOUT. Because the output can be used for controller applications,

wideband noise is minimal.

In this mode, the setpoint control voltage is applied to VSET.

The feedback loop through an RF amplifier is closed via VOUT,

the output of which regulates the amplifier output to a magnitude

corresponding to VSET. The AD8318 provides 0 V to 4.9 V output

capability at the VOUT pin, suitable for controller applications. As a

measurement device, Pin VOUT is externally connected to VSET to

produce an output voltage, VOUT, which is a decreasing linear-in-dB

function of the RF input signal amplitude.

The logarithmic slope is nominally -25 mV/dB but can be adjusted

by scaling the feedback voltage from VOUT to the VSET interface.

The intercept is 20 dBm (referenced to 50 Ω, CW input) using the

INHI input. These parameters are very stable against supply and

temperature variations.

The AD8318 is fabricated on a SiGe bipolar IC process and is

available in a 4 mm × 4 mm, 16-lead LFCSP for the operating

temperature range of –40oC to +85oC.

Figure 2. Typical Logarithmic Response and Error vs. Input Amplitude at 5.8

GHz