AD8307
SLOPE AND INTERCEPT ADJUSTMENTS
Where higher calibration accuracy is needed, the adjustments
shown in Figure 37 can be used, either singly or in combination.
The log slope is lowered to 20 mV/dB by shunting the nominally
12.5 kΩ on-chip load resistor (see Figure 31) with 50 kΩ, adjusted
by VR1. The calibration range is 10% (18 mV/dB to 22 mV/dB),
including full allowance for the variability in the value of the
internal load. The adjustment can be made by alternately
applying two input levels, provided by an accurate signal
generator, spaced over the central portion of the log amp’s
dynamic range, for example −ꢀ0 dBm and 0 dBm. An AM
modulated signal, at the center of the dynamic range, can also
be used. For a modulation depth M, expressed as a fraction, the
decibel range between the peaks and troughs over one cycle of
the modulation period is given by
The log intercept is adjustable over a 3 dB range, which is
sufficient to absorb the worst-case intercept error in the
AD8307, plus some system level errors. For greater range, set RS
to zero. VR2 is adjusted while applying an accurately known
CW signal near the lower end of the dynamic range in order to
minimize the effect of any residual uncertainty in the slope. For
example, to position the intercept to −80 dBm, a test level of
−ꢀ5 dBm can be applied and VR2 adjusted to produce a dc
output of 15 dB above zero at 25 mV/dB, which is +0.3 V.
0.1µF
4.7Ω
V , 2.7V TO 5.5V
P
AT ~8mA
VR2
50kΩ
R
S
C1 = C
C
±3dB
8
7
6
5
INPUT
–75dBm TO
+16dBm
INP VPS ENB INT
FOR V = 3V, R = 20kΩ
P
P
S
1 + M
AD8307
V
= 5V, R = 51kΩ
S
ΔdB = 20log
(7)
10 1 − M
INM COM OFS OUT
1
2
3
4
NC
20mV/dB
±10%
For example, using an rms signal level of −40 dBm with a 70%
modulation depth (M = 0.7), the decibel range is 15 dB, as the
signal varies from −47.5 dBm to −32.5 dBm.
C2 = C
C
32.4kΩ
VR1
50kΩ
NC = NO CONNECT
Figure 37. Slope and Intercept Adjustments
Rev. C | Page ±9 of 24
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