AD8307
1 MHz. To extend the range below this frequency, an external
capacitor can be added. This permits the high-pass corner to be
lowered to audio frequencies using a capacitor of modest value.
Note that this capacitor has no effect on the minimum signal
frequency for input levels above the offset voltage: this extends
down to dc (for a signal applied directly to the input pins). The
offset voltage varies from part to part; some exhibit essentially
stable offsets of under 100 μV without the benefit of an offset
adjustment.
applied to a tapped attenuator, and progressively smaller signals
are applied to three passive rectifying gm cells whose outputs are
summed with those of the main detectors. With care in design,
the extension to the dynamic range can be seamless over the full
frequency range. For the AD8307, it amounts to a further 27 dB.
Therefore, the total dynamic range is theoretically 113 dB. The
specified range of 90 dB (−74 dBm to +1ꢀ dBm) is for high
accuracy and calibrated operation, and includes the low end
degradation due to thermal noise and the top end reduction due
to voltage limitations. The additional stages are not, however,
redundant, but are needed to maintain accurate logarithmic
conformance over the central region of the dynamic range, and
in extending the usable range considerably beyond the specified
range. In applications where log conformance is less demanding,
the AD8307 can provide over 95 dB of range.
EXTENSION OF RANGE
The theoretical dynamic range for the basic log amp shown in
Figure 2ꢀ is AN. For A = 5.2 (14.3 dB) and N = ꢀ, it is 20,000 or
8ꢀ dB. The actual lower end of the dynamic range is largely
determined by the thermal noise floor, measured at the input of
the chain of amplifiers. The upper end of the range is extended
upward by the addition of top end detectors. The input signal is
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