REV. B
AD8309
–16–
Slope and Intercept Adjustment
The AD8309 provides limited opportunities for adjustment of
its basic scaling parameters, which are controlled to within tight
limits through robust design. In applications involving the ob-
servation of measured signal levels on a DVM a slope of 10 mV
per decade is convenient: the reading is then directly in deci-
bels, needing only the positioning of the decimal point. This
may be simply achieved and at the same time trimmed to this
exact value using the scheme shown in Figure 35. A large filter
capacitor C
FILT
may be added as shown when the voltage is to
be measured on a DVM; this lowers the fluctuation in the lower-
order display digits.
A precision attenuator or signal generator is required to provide
several test levels at 10 dB intervals. The adjustment may also
made using an AM modulated signal, at about the center of the
dynamic range. 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
∆dB = 20 log
10
(1+M)/(1–M) (12)
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 output would
thus be adjusted to have a peak-to-peak amplitude of 150 mV.
1
2
3
4
5
6
7
8
VLOG
VPS2
PADL
LMHI
LMLO
PADL
FLTR
LMDR
COM2
VPS1
PADL
INHI
INLO
PADL
COM1
ENBL
AD8309
9
10
11
14
15
16
10V
RSSI OUTPUT
10mV/dB 6 10%
10V
12
13
V
S
0.1mF 0.1mF
C
FILT
10mF
DVM
VR1
2kV
SLOPE
LIMITER
MAY BE
DISABLED
FOR RSSI
ONLY MODE
8.87kV
8.87kV
INPUT
COUPLING
Figure 35. Trimming Slope to 10 mV/dB
±
10%
The intercept can be adjusted by the use of the auxiliary circuit
shown in Figure 36, without changing the slope, which remains
20 mV/dB. This circuit provides a range of about ±4 dB on a
nominal intercept of –113 dBV (–100 dBm), with a fairly low
residual temperature sensitivity (+0.008 dB/°C). This is suffi-
cient to absorb the worst-case intercept error in the AD8309
plus system-level gain errors. 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 uncer-
tainty in the slope. For example, to position the intercept to
exactly –100 dBm, a test level of –60 dBm may be applied and
VR2 adjusted to produce a dc output of 40 dB above the inter-
cept, which is +0.8 V. This trim can optionally be combined
with the slope trim described above.
1
2
3
4
5
6
7
8
VLOG
VPS2
PADL
LMHI
LMLO
PADL
FLTR
LMDR
COM2
VPS1
PADL
INHI
INLO
PADL
COM1
ENBL
AD8309
9
10
11
14
15
16
0.1mF
10V
R
LIM
RSSI
0.1mF
10V
12
13
V
S
47kV
9.6kV
VR2
2kV
INTERCEPT
IN914
OR SIMILAR
Figure 36. Trimming Intercept to –113 dBV
±
4 dB
APPLICATIONS
The AD8309 is a versatile and easily applied log-limiting amp.
Being complete, it can be used with very few external compo-
nents, and most applications can be accommodated using the
simple connections shown in the preceding section. A few ex-
amples of more specialized applications are provided here.
Log Amp with High Slope Voltage
Where a higher RSSI slope voltage is required, and/or complete
calibration with good temperature stability and minimal interac-
tion between trims, the interface shown in Figure 37 may be
used. Note that at 50 mV/dB, the full 100 dB dynamic range of
the AD8309 requires a 5 V swing. This can be provided by a
single supply operational amplifier having a rail-to-rail output
stage and operating from a 6 V supply. Where a lower range is
sufficient, or when using the 40 mV/dB option, a 5 V supply will
be adequate.
In this application, the supply current into the VPS2 pin is only
slightly dependent on the current delivered to the load resis-
tance, R
L
, so a voltage dropping resistor, R
D
, may be added to
lower the supply to the AD8309, which can meet all of its speci-
fications with a 2.7 V supply. The lower chip dissipation and
the resulting reduction in operating temperature will minimize
degradation of noise figure at high ambient temperatures. R
D
is
calculated as follows:
R
V
mA
R
D
S
LIM
=
−
≥
3
25
100 Ω
(13)
which allows for operation at ambient temperatures up to +85°C.
Table II may be used to select the component values for various
different operating conditions. The slope adjustment range is
±10% and the intercept adjustment range is ±3 dB. Since the
intercept offset bias is derived from the supply, there is a sensi-
tivity to this voltage. Where supply stability is poor, a regulator
may be needed to bias VR2 and R4.
