Datasheet

AD8307
Rev. D | Page 19 of 24
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 k on-chip load resistor (see Figure 31) with 50 k, 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, −60 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
M
M
dB
−
+
=Δ
1
1
log20
10
(7)
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.
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 R
S
to zero.
VR2 is adjusted while applying an accurately known CW signal
near the lower end of the dynamic range to minimize the effect
of any residual uncertainty in the slope. For example, to position
the intercept to −80 dBm, a test level of −65 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.
01082-037
C2 = C
C
C1 = C
C
20mV/dB
±10%
AD8307
INPUT
–75dBm TO
+16dBm
INP VPS ENB INT
INM COM OFS OUT
NC
NC = NO CONNECT
8765
234
1
4.7Ω
VR2
50kΩ
R
S
0.1µ
F
V
P
, 2.7V TO 5.5V
AT ~8mA
±3dB
VR1
50kΩ
32.4kΩ
FOR V
P
= 3V, R
S
= 20kΩ
V
P
= 5V, R
S
= 51kΩ
Figure 37. Slope and Intercept Adjustments