Data Sheet

AD8307

Rev. F | Page 13 of 24

In the AD8307, this is achieved by an on-chip filter, providing

sufficient suppression of HF feedback to allow operation above

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.

EXTENSION OF RANGE

The theoretical dynamic range for the basic log amp shown in

Figure 26 is AN. For A = 5.2 (14.3 dB) and N = 6, it is 20,000 or

86 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

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 +16 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 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.