AD8310
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 postdetector output interface. With this slope, the full
95 dB dynamic range can be easily accommodated within the
output swing capacity, when operating from a 2.7 V supply.
Intercept positioning at−108 dBV (−95 dBm re 50
Ω
) 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
with 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 can alternatively be stated as an equivalent power, in
dBm, but in this case, it is necessary to specify the impedance in
which this power is presumed to be measured. In RF practice, it
is common to assume a reference impedance of 50 Ω, in which
0 dBm (1 mW) corresponds to a sinusoidal amplitude of
316.2 mV (223.6 mV rms). However, the power metric is correct
only when the input impedance is lowered to 50 Ω, either by a
termination resistor added across INHI and INLO, or by the use
of a narrow-band matching network.
Note 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 Ω (typically
1 kΩ at low frequencies). A simple input matching network can
considerably improve the power sensitivity of this type of log
amp. This increases the voltage applied to the input and,
therefore, alters the intercept. For a 50 Ω reactive match, the
voltage gain is about 4.8, and the whole dynamic range moves
down by 13.6 dB. The effective intercept is a function of
waveform. For example, a square-wave input reads 6 dB higher
than a sine wave of the same amplitude, and a Gaussian noise
input reads 0.5 dB higher than a sine wave of the same rms
value.
OFFSET CONTROL
In a monolithic log amp, direct coupling is used between the
stages for several reasons. First, it avoids the need for coupling
capacitors, which typically have a chip area at least as large as
that of a basic gain cell, considerably increasing die size. Second,
the capacitor values predetermine the lowest frequency at which
the log amp can operate. For moderate values, this can be as
high as 30 MHz, limiting the application range. Third, the
parasitic back-plate capacitance lowers the bandwidth of the
cell, further limiting the scope of applications.
However, the very high dc gain of a direct-coupled amplifier
raises a practical issue. An offset voltage in the early stages of
the chain is indistinguishable from a real signal. If it were as
high as 400 µV, it would be 18 dB larger than the smallest ac
signal (50 µV), potentially reducing the dynamic range by this
amount. This problem can be averted by using a global feedback
path from the last stage to the first, which corrects this offset in
a similar fashion to the dc negative feedback applied around an
op amp. The high frequency components of the feedback signal
must, of course, be removed to prevent a reduction of the HF
gain in the forward path.
An on-chip filter capacitor of 33 pF provides sufficient suppres-
sion of HF feedback to allow operation above 1 MHz. The
−
3 dB point in the high-pass response is at 2 MHz, but the
usable range extends well below this frequency. To further lower
the frequency range, an external capacitor can be added at
OFLT (Pin 3). For example, 300 pF lowers it by a factor of 10.
Operation at low audio frequencies requires a capacitor of about
1 µF. Note that this filter has no effect for input levels well above
the offset voltage, where the frequency range would extend
down to dc (for a signal applied directly to the input pins). The
dc offset can optionally be nulled by adjusting the voltage on
the OFLT pin (see the Applications section).
Rev. D | Page 10 of 24
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