AD8310
Step 4: Calculate L
M
The matching inductor required to provide both
L
IN
and
L
O
is
the parallel combination of these.
L
M
L
IN
L
O
=
(
L
IN
+
L
O
)
SIGNAL
INPUT
0.01µF
4.7Ω
+V
S
(2.7V–5.5V)
VR2
100kΩ
R
S
C2
0.01µF
52.3Ω
C1
0.01µF
8
7
6
5
INHI ENBL BFIN VPOS
FOR V
POS
= 3V, R
S
= 500kΩ
FOR V
POS
= 5V, R
S
= 850kΩ
25kΩ
AD8310
INLO COMM OFLT VOUT
1
2
3
4
With
L
IN
= 1.8 µH and
L
O
= 356 nH, the value of L
M
to complete
this example of a match of 50 Ω at 100 MHz is 297.2 nH.
The nearest standard value of 270 nH can be used with only a
slight loss of matching accuracy. The voltage gain at resonance
depends only on the ratio of impedances, as given by
⎛
R
IN
GAIN
=
20 log
⎜
⎜
R
S
⎝
⎞
⎛
⎟ =
10 log
⎜
R
IN
⎜
R
⎟
⎝
S
⎠
⎞
⎟
⎟
⎠
NC
10kΩ
NC = NO CONNECT
V
OUT
(RSSI)
24mV/dB ±10%
Figure 33. Slope and Intercept Adjustments
INCREASING THE SLOPE TO A FIXED VALUE
It is also possible to increase the slope to a new fixed value and,
therefore, to increase the change in output for each decibel of
input change. A common example of this is the need to map the
output swing of the AD8310 into the input range of an analog-
to-digital converter (ADC) with a rail-to-rail input swing.
Alternatively, a situation might arise when only a part of the
total dynamic range is required—say, just 20 dB—in an
application where the nominal input level is more tightly
constrained and a higher sensitivity to a change in this level is
required. Of course, the maximum output is limited either by
the load resistance and the maximum output current rating of
25 mA or by the supply voltage (see the Specifications section).
The slope can easily be raised by adding a resistor from VOUT
to BFIN, as shown in Figure 34. This alters the gain of the
output buffer, by means of stable positive feedback, from its
normal value of 4 to an effective value that can be as high as 16,
corresponding to a slope of 100 mV/dB.
0.01µF
C2
0.01µF
8
7
6
5
SLOPE AND INTERCEPT ADJUSTMENTS
Where system (that is, software) calibration is not available, the
adjustments shown in Figure 33 can be used, either singly or in
combination, to trim the absolute accuracy of the AD8310.
The log slope can be raised or lowered by VR1; the values
shown provide a calibration range of ±10% (22.6 mV/dB to
27.4 mV/dB), which includes full allowance for the variability in
the value of the internal resistances. The adjustment can be
made by alternately applying two fixed input levels, provided by
an accurate signal generator, spaced over the central portion of
the dynamic range, for example, −60 dBV and –20 dBV.
Alternatively, an AM-modulated signal at about the center of
the dynamic range can 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
4.7Ω
V
S
(2.7V–5.5V)
∆
dB
=
20 log
10
1
+
M
1
+
M
(3)
SIGNAL
INPUT
INHI ENBL BFIN VPOS
52.3Ω
C1
0.01µF
NC
NC = NO CONNECT
V
OUT
100mV/dB
The log intercept is adjustable by VR2 over a −3 dB range with
the component values shown. VR2 is adjusted while applying an
accurately known CW signal, preferably 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 0 at 24 mV/dB,
which is 360 mV.
Figure 34. Raising the Slope to 100 mV/dB
The resistor,
R
SLOPE
, is set according to the equation
R
SLOPE
=
9.22 k
Ω
24 mV/dB
Slope
1
+
Rev. D | Page 17 of 24
01084-034
For example., using a generator output of −40 dBm with a 70%
modulation depth (M = 0.7), the decibel range is 15 dB, because
the signal varies from −47.5 dBm to −32.5 dBm.
AD8310
INLO COMM OFLT VOUT
1
2
3
4
R
SLOPE
12.1kΩ
01084-033
VR1
10kΩ
AD [ ANALOG DEVICES ]
