Fast, Voltage-Out, DC to 440 MHz,
95 dB Logarithmic Amplifier
AD8310
Rev. F
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FEATURES
Multistage demodulating logarithmic amplifier
Voltage output, rise time <15 ns
High current capacity: 25 mA into grounded R
L
95 dB dynamic range: −91 dBV to +4 dBV
Single supply of 2.7 V min at 8 mA typ
DC to 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
FUNCTIONAL BLOCK DIAGRAM
+
–
VPOS
INHI
INLO
COMM
3
8mA
1.0kΩ
BAND GAP REFERENCE
AND BIASING
SIX 14.3dB 900MHz
AMPLIFIER STAGES
NINE DETECTOR CELLS
SPACED 14.3dB
INPUT-OFFSET
COMPENSATION LOOP
2
2μA
/
dB
MIRROR
3kΩ
3kΩ
1kΩ
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
01084-001
Figure 1.
GENERAL DESCRIPTION
The AD8310 is a complete, dc to 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.
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.
