ad8306-evalz Analog Devices, Inc., ad8306-evalz Datasheet - Page 10

• Manufacturer Part Number: ad8306-evalz
• Description: 5 Mhz-400 Mhz 100 Db High Precision Limiting-logarithmic Amplifier
• Manufacturer: Analog Devices, Inc.

AD8306
low frequency applications, a simple RC network forming a low-
pass filter should be added at the input for the same reason.
If the limiter output is not required, Pin 9 (LMDR) should be
left open and Pins 12 and 13 (LMHI, LMLO) should be tied to
VPS2 as shown in Figure 24.
Figure 25 shows the output versus the input level in dBV, for
sine inputs at 10 MHz, 50 MHz and 100 MHz (add 13 to the
dBV number to get dBm Re 50 . Figure 26 shows the typi-
cal logarithmic linearity (log conformance) under the same
conditions.
Transfer Function in Terms of Slope and Intercept
The transfer function of the AD8306 is characterized in terms
of its Slope and Intercept. The logarithmic slope is defined as
the change in the RSSI output voltage for a 1 dB change at the
input. For the AD8306 the slope is calibrated to be 20 mV/dB.
The intercept is the point at which the extrapolated linear re-
sponse would intersect the horizontal axis. For the AD8306 the
intercept is calibrated to be –108 dBV (–95 dBm). Using the
slope and intercept, the output voltage can be calculated for any
input level within the specified input range using the equation:
Figure 25. RSSI Output vs. Input Level at T
Frequencies of 10 MHz, 50 MHz and 100 MHz
Figure 26. Log Linearity vs. Input Level at T
Frequencies of 10 MHz, 50 MHz and 100 MHz
V
OUT
2.5
1.5
0.5
–1
–2
–3
–4
–5
5
4
3
2
1
0
2
1
0
–120
–120
= V
SLOPE
–100
–100
–100
(P
–80
–80
IN
INPUT LEVEL – dBV
INPUT LEVEL – dBV
– P
–60
–60
O
DYNAMIC RANGE
10MHz
50MHz
100MHz
)
–40
–40
10MHz
50MHz
–20
100MHz
–20
86
90
96
1dB
10MHz
A
A
0
0
= +25 C for
= +25 C, for
100MHz
93
97
100
3dB
50MHz
20
20
(2)
–10–
where V
V
input signal, expressed in decibels relative to some reference
level (either dBm or dBV in this case) and P
intercept, expressed in decibels relative to the same reference
level.
For example, for an input level of –33 dBV (–20 dBm), the
output voltage will be
The most widely used convention in RF systems is to specify
power in dBm, that is, decibels above 1 mW in 50 . Specifica-
tion of log amp input level in terms of power is strictly a conces-
sion to popular convention; they do not respond to power (tacitly
“power absorbed at the input”), but to the input voltage. The
use of dBV, defined as decibels with respect to a 1 V rms sine wave,
is more precise, although this is still not unambiguous because
waveform is also involved in the response of a log amp, which,
for a complex input (such as a CDMA signal) will not follow the
rms value exactly. Since most users specify RF signals in terms
of power—more specifically, in dBm/50 —we use both dBV
and dBm in specifying the performance of the AD8306, showing
equivalent dBm levels for the special case of a 50
Values in dBV are converted to dBm re 50
Output Response Time and C
The RSSI output has a low-pass corner frequency of 3.5 MHz,
which results in a 10% to 90% rise time of 73 ns. For low fre-
quency applications, the corner frequency can be reduced by
adding an external capacitor, C
VLOG (Pin 16) as shown in Figure 24. For example, an exter-
nal 33 pF will reduce the corner frequency to 350 kHz, while
360 pF will set it to 35 kHz, in each case with an essentially
one-pole response.
Using the Limiter
Figure 27 shows the basic connections for operating the limiter
and the log output concurrently. The limiter output is a pair of
differential currents of magnitude, I
(open-collector) sources. These are converted to equal-amplitude
voltages by supply-referenced load resistors, R
output current is set by R
Pin 9 (LMDR) and ground. The limiter output current is set
according the equation:
and has an absolute accuracy of 5%.
The supply referenced voltage on each of the limiter pins will
thus be given by:
SLOPE
V
I
V
OUT
OUT
LIM
is the logarithmic slope, expressed in V/dB, P
OUT
= –400 mV/R
= V
= 0.02 V/dB
is the demodulated and filtered RSSI output,
S
–400 mV
LIM
(–33 dBV – (–108 dBV)) = 1.5 V
LIM
R
LOAD
, the resistor connected between
F
F
, between FLTR (Pin 10) and
/R
LIM
OUT
, from high impedance
O
by adding 13.
is the logarithmic
LOAD
environment.
. The limiter
IN
is the
REV. A
(3)
(5)
(6)