Fast, Voltage-Out DC–440 MHz,

95 dB Logarithmic Amplifier

AD8310

Rev. E

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FEATURES

Multistage demodulating logarithmic amplifier

Voltage output, rise time <15 ns

95 dB dynamic range: −91 dBV to +4 dBV

Single supply of 2.7 V min at 8 mA typ

DC–440 MHz operation, ±0.4 dB linearity

Slope of +24 mV/dB, intercept of −108 dBV

Highly stable scaling over temperature

Fully differential dc-coupled signal path

100 ns power-up time, 1 mA sleep current

APPLICATIONS

Conversion of signal level to decibel form

Transmitter antenna power measurement

Receiver signal strength indication (RSSI)

Low cost radar and sonar signal processing

Network and spectrum analyzers

Signal-level determination down to 20 Hz

True-decibel ac mode for multimeters

GENERAL DESCRIPTION

The AD8310 is a complete, dc−440 MHz demodulating

logarithmic amplifier (log amp) with a very fast voltage mode

output, capable of driving up to 25 mA into a grounded load in

under 15 ns. It uses the progressive compression (successive

detection) technique to provide a dynamic range of up to 95 dB

to ±3 dB law conformance or 90 dB to a ±1 dB error bound up

to 100 MHz. It is extremely stable and easy to use, requiring no

significant external components. A single-supply voltage of

2.7 V to 5.5 V at 8 mA is needed, corresponding to a power

consumption of only 24 mW at 3 V. A fast-acting CMOS-

compatible enable pin is provided.

Each of the six cascaded amplifier/limiter cells has a small-

signal gain of 14.3 dB, with a −3 dB bandwidth of 900 MHz.

A total of nine detector cells are used to provide a dynamic

range that extends from −91 dBV (where 0 dBV is defined as

the amplitude of a 1 V rms sine wave), an amplitude of about

±40 μV, up to +4 dBV (or ±2.2 V). The demodulated output

is accurately scaled, with a log slope of 24 mV/dB and an

intercept of −108 dBV. The scaling parameters are supply-

and temperature-independent.

FUNCTIONAL BLOCK DIAGRAM

+

–

VPOS

INHI

INLO

COMM

3

8mA

1.0k

Ω

BAND GAP REFERENCE

AND BIASING

SIX 14.3dB 900MHz

AMPLIFIER STAGES

NINE DETECTOR CELLS

SPACED 14.3dB

INPUT-OFFSET

COMPENSATION LOOP

2

2

μA

MIRROR

3k

Ω

3k

Ω

1k

Ω

COMM

COMM

COMM

ENBL

BFIN

VOUT

OFLT

ENABLE

BUFFER

INPUT

OUTPUT

OFFSET

FILTER

AD8310

SUPPLY

+INPUT

–INPUT

COMMON

33pF

8

7

6

5

1

2

3

4

Figure 1.

The fully differential input offers a moderately high impedance

(1 kΩ in parallel with about 1 pF). A simple network can match

the input to 50 Ω and provide a power sensitivity of −78 dBm to

+17 dBm. The logarithmic linearity is typically within ±0.4 dB

up to 100 MHz over the central portion of the range, but it is

somewhat greater at 440 MHz. There is no minimum frequency

limit; the AD8310 can be used down to low audio frequencies.

Special filtering features are provided to support this wide

range.

The output voltage runs from a noise-limited lower boundary of

400 mV to an upper limit within 200 mV of the supply voltage

for light loads. The slope and intercept can be readily altered

using external resistors. The output is tolerant of a wide variety

of load conditions and is stable with capacitive loads of 100 pF.

The AD8310 provides a unique combination of low cost, small

size, low power consumption, high accuracy and stability, high

dynamic range, a frequency range encompassing audio to UHF,

fast response time, and good load-driving capabilities, making

this product useful in numerous applications that require the

reduction of a signal to its decibel equivalent.

The AD8310 is available in the industrial temperature range of

−40°C to +85°C in an 8-lead MSOP package.