AD8307
The intercept can be raised, for example, to 100 μV, with the
rationale that the dc precision does not warrant operation in
the first decade (from 10 μV to 100 μV). Likewise, the slope can
be raised to 50 mV/dB, using R7 = 3 kΩ, R8 = 2 kΩ , or to
100 mV/dB, to simplify decibel measurements on a DVM,
using R7 = 8 kΩ, R8 = 2 kΩ, which raises the maximum output
11 V, thus requiring a 15 V supply for the AD830. The output
can be made to swing in a negative direction by simply
reversing Pins 1 and 2. Low-pass filtering capacitor, C3, sets the
output rise time to about 1 ms.
Next, it is necessary to set the intercept. This is the purpose of
VR2, which should be adjusted after VR1. The simplest method
is to short the input and adjust VR2 for an output of 0.3 V,
corresponding to the noise floor. For more exacting
applications, a temporary sinusoidal test voltage of 1 mV in
amplitude, at about 1 MHz, should be applied, which can
require the use of a temporary on-board input attenuator. For
20 mV/dB scaling, a 10 μV dc intercept (which is ꢀ dB below
the ac intercept) requires adjusting the output to 0.ꢀ8 V; for
100 mV/dB scaling, this becomes 3.4 V. If a 100 μV intercept is
preferred (usefully lowering the maximum output voltage),
these become 0.28 V and 1.4 V, respectively.
6.0
5.5
5.0
Finally, the slope must be adjusted. This can be performed by
applying a low frequency square wave to the main input, having
precisely determined upper and lower voltage levels, provided
by a programmable waveform generator. A suitable choice is a
100 Hz square wave with levels of 10 mV and 1 V. The output is
a low-pass filtered square wave, and its amplitude should be
0.8 V for 20 mV/dB scaling, or 4 V for 100 mV/dB scaling.
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
1.0
0.5
0
–0.5
–1.0
OPERATION ABOVE 500 MHZ
The AD8307 is not intended for use above 500 MHz. However,
it does provide useful performance at higher frequencies.
Figure 47 shows a plot of the logarithmic output of the AD8307
for an input frequency of 900 MHz. The device shows good
logarithmic conformance from −50 dBm to −10 dBm. There is a
bump in the transfer function at −5 dBm, but if this is
acceptable, the device is usable over a ꢀ0 dB dynamic range
(−50 dBm to +10 dBm).
10µ
100µ
1m
10m
100m
1
10
V
IN
Figure 46. Ideal Output and Law-Conformance Error for the DC-Coupled
AD8307 at 50 mV/dB
Figure 4ꢀ shows the output and the law-conformance error, in
the absence of noise and input offset, for the 50 mV/dB option.
Note that the error ripple for dc excitation is about twice that
for the more usual sinusoidal excitation. In practice, both the
noise and the internal offset voltage degrade the accuracy in the
first decade of the dynamic range. The latter is now manually
nulled by VR1, using a simple method that ensures very low
residual offsets.
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
A temporary ac signal, typically a sine wave of 100 mV in
amplitude at a frequency of about 100 Hz, is applied via the
capacitor at node TEMP; this has the effect of disturbing the
offset nulling voltage. The output voltage is then viewed on an
oscilloscope and VR1 is adjusted until the peaks of the
(frequency-doubled) waveform are exactly equal in amplitude.
This procedure can provide an input null down to about 10 μV.
The temperature drift is very low, though not specified since the
AD8307 is not principally designed to operate as a baseband log
amp; in ac modes, this offset is nulled continuously and
automatically.
0.4
0.2
0
–60
–50
–40
–30
–20
–10
0
10
P
(dBm)
IN
Figure 47. Output vs. Input Level for a 900 MHz Input Signal
Rev. C | Page 23 of 24
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