1 MHz to 8 GHz, 70 dB

Logarithmic Detector/Controller

Data Sheet

AD8318

Rev. D

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FEATURES

Wide bandwidth: 1 MHz to 8 GHz

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

Stability over temperature: ±0.5 dB

Low noise measurement/controller output (VOUT)

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

Integrated temperature sensor

Small footprint LFCSP

Power-down feature: <1.5 mW at 5 V

Single-supply operation: 5 V at 68 mA

Fabricated using high speed SiGe process

APPLICATIONS

RF transmitter PA setpoint control and level monitoring

RSSI measurement in base stations, WLAN, WiMAX, and

radars

FUNCTIONAL BLOCK DIAGRAM

TEMP

SENSOR

GAIN

BIAS

SLOPE

DET

DET

DET

DET

INHI

INLO

I

V

VOUT

I

V

VSET

CLPF

TEMP

VPSI

ENBL

TADJ

VPSO

CMOP

CMIP

Figure 1.

GENERAL DESCRIPTION

The AD8318 is a demodulating logarithmic amplifier, capable

of accurately converting an RF input signal to a corresponding

decibel-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 conformance 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

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

2.4

0

–65

10

2.2

2.0

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

6

–6

5

4

3

2

1

0

–1

–2

–3

–4

–5

–60 –55 –50 –45 –40 –35 –30 –25 –20 –15 –10 –5

0

5

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