AD8310 Analog Devices, AD8310 Datasheet - Page 7

• Manufacturer Part Number: AD8310
• Description: Fast Response, DC - 440 Mhz, Voltage Out, 90 DB Logarithmic Amplifier
• Manufacturer: Analog Devices

GENERAL THEORY
Logarithmic amplifiers perform a more complex operation than
that of classical linear amplifiers, and their circuitry is significantly
different. A good grasp of what log amps do, and how they do
it, will avoid many pitfalls in their application. For a compete
discussion of the theory, refer to the AD8307 data sheet.
The essential purpose of a log amp is not to amplify, though
amplification is needed internally, but to compress a signal of wide
dynamic range to its decibel equivalent. It is thus a measurement
device. A better term might be “logarithmic converter,” since
the function is the conversion of a signal from one domain of
representation to another, via a precise nonlinear transformation:
where V
the logarithm is usually taken to base-ten (in which case V
also the “volts-per-decade”), V
called the “intercept voltage.” Log amps implicitly require two
references, here V
circuit. The accuracy of a log amp cannot be any better than the
accuracy of its scaling references. In the AD8310, these are provided
by a band-gap reference.
While Equation 1, plotted in Figure 19, is fundamentally correct, a
different formula is appropriate for specifying the calibration
attributes or demodulating log amps like the AD8310, operating
in RF applications with a sine wave input:
Here, V
or “RSSI”) output, V
in volts/dB (25 mV/dB for the AD8310), P
expressed in decibels relative to some reference power level and
is P
the same reference level. A widely used reference in RF systems
is decibels above 1 mW in 50 , a level of 0 dBm. Note that the
quantity (P
from the formula because the conversion has already been implic-
itly performed in stating the input in decibels. This is strictly a
concession to popular convention: log amps manifestly do not
respond to power (tacitly “power absorbed at the input”), but,
rather, to input voltage. The input is specified in dBV (decibels
with respect to 1 V rms) throughout this data sheet. This is more
precise, although still incomplete, since the signal waveform is
REV. A
V
Figure 19. General Form of the Logarithmic Function
OUT
0
V
V
the logarithmic intercept, expressed in decibels relative to
V
–2V
V
OUT
OUT
4V
OUT
5V
3V
2V
= 0
IN
V
–40dBc
OUT
OUT
Y
Y
Y
Y
Y
Y
= 10
= V
= V
IN
–2
is the output voltage, V
is the demodulated and filtered baseband (“video”
–P
V
Y
X
SLOPE
0
log (V
) is just dB. The logarithmic function disappears
LOWER INTERCEPT
X
(P
and V
V
SLOPE
IN
0dBc
IN
IN
= V
– P
/V
X
Y
is the logarithmic slope, now expressed
X
, which determine the scaling of the
0
)
)
IN
V
is the input voltage, and V
IN
+40dBc
Y
V
= 10
SHIFT
is called the “slope voltage,”
2
V
X
IN
is the input power,
V
IN
+80dBc
= 10
4
V
X
LOG V
IN
X
(1)
Y
(2)
is
is
–7–
also involved. Since many users specify RF signals in terms of
power—usually in dBm/50
specifying the performance of the AD8310.
Progressive Compression
High-speed high-dynamic range log amps use a cascade of non-
linear amplifier cells to generate the logarithmic function as a
series of contiguous segments, a type of piecewise-linear tech-
nique. The AD8310 employs six cells in its main signal path each
having a small-signal gain of 14.3 dB ( 5.2) and a –3 dB band-
width of about 900 MHz; the overall gain is about 20,000 (86 dB)
and the overall bandwidth of the chain is some 500 MHz, resulting
in a gain-bandwidth product (GBW) of 10,000 GHz, about a
million times that of a typical op amp. This very high GBW is
essential to accurate operation under small-signal conditions
and at high frequencies. The AD8310 exhibits a logarithmic
response down to inputs as small as 40 V at 440 MHz.
Progressive compression log amps either provide a baseband
“video” response or they accept an RF input and demodulate
this signal to develop an output that is essentially the envelope
of the input represented on a logarithmic or decibel scale. The
AD8310 is the latter kind. Demodulation is performed in a
total of nine detector cells, six of which are associated with
the amplifier stages and three are passive detectors that receive a
progressively-attenuated fraction of the full input. The maximum
signal frequency can be 440 MHz but, since all the gain stages
are dc-coupled, operation at very low frequencies is possible.
Slope and Intercept Calibration
All monolithic log amps from Analog Devices use precision
design techniques to control the logarithmic slope and intercept.
The primary source of this calibration is a pair of accurate voltage
references, that provide supply- and temperature-independent
scaling. The slope is set to 24 mV/dB by the bias chosen for the
detector cells and the subsequent gain of the post-detector output
interface. With this slope, the full 95 dB dynamic range can
easily be accommodated within the output swing capacity when
operating from a 2.7 V supply. Intercept positioning at –108 dBV
(–95 dBm re 50 ) has likewise been chosen to provide an output
centered in the available voltage range.
Precise control of the slope and intercept results in a log amp
having stable scaling parameters, making it a true measurement
device as, for example, a calibrated Received Signal Strength
Indicator (RSSI). In this application, the input waveform is
invariably sinusoidal. The input level is correctly specified in
dBV. It may alternatively be stated as an equivalent power, in
dBm, but here we must step carefully, since it is essential to specify
the impedance in which this power is presumed to be measured.
In most RF practice, it is common to assume a reference imped-
ance of 50 , in which 0 dBm (1 mW) corresponds to a sinusoidal
amplitude of 316.2 mV (223.6 mV rms). However, the power
metric is only correct when the input impedance is lowered to
50 , either by a termination resistor added across INHI and
INLO, or by the use of a narrow-band matching network.
It cannot be stated too strongly that log amps do not inherently
respond to power, but to the voltage applied to their input. The
AD8310 presents a nominal input impedance much higher than
50
network can considerably improve the power sensitivity of this
type of log amp. This increases the voltage applied to the input and
(typically 1 k at low frequencies). A simple input matching
—we also use this convention in
AD8310