Datasheet

AD8310
Rev. F | Page 17 of 24
()
+V
S
(2.7V–5.5V)
Step 4: Calculate L
M
The matching inductor required to provide both L
IN
and L
O
is
the parallel combination of these.
O
IN
O
IN
M
LL
LL
L
+
=
(7)
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
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
=
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
=
S
IN
S
IN
R
R
R
R
GAIN log10log20 (8)
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
M
M
+
+
=Δ
1
1
log20dB
10
(9)
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.
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
can be adjusted to produce a dc output of 15 dB above 0 at
24 mV/dB, which is 360 mV.
0.01μF
4.7Ω
52.3Ω
NC = NO CONNECT
C1
0.01μF
NC
INHI ENBL BFIN VPOS
INLO COMM OFLT VOUT
AD8310
V
OUT
(RSSI)
SIGNAL
INPUT
10kΩ
C2
0.01μF
25kΩ
VR1
10kΩ
R
S
VR2
100kΩ
FOR V
POS
= 3V, R
S
= 500kΩ
FOR V
POS
= 5V, R
S
= 850kΩ
24mV/dB ±10%
1234
8765
01084-033
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 (for example, 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 by either
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.
V
S
(2.7V–5.5V)
0.01μF
4.7Ω
52.3Ω
NC = NO CONNECT
C1
0.01μF
NC
INHI ENBL BFIN VPOS
INLO COMM OFLT VOUT
AD8310
V
OUT
100mV/dB
SIGNAL
INPUT
C2
0.01μF
8765
R
SLOPE
12.1kΩ
1234
01084-034
Figure 34. Raising the Slope to 100 mV/dB
The resistor, R
SLOPE
, is set according to the equation
Slope
R
SLOPE
mV/dB24
1 −
k22.9 Ω
= (10)