AD8309 Analog Devices, AD8309 Datasheet - Page 18

• Manufacturer Part Number: AD8309
• Description: 5 - 500 Mhz, 100 DB Demodulating Logarithmic Amplifier With Limiter Output
• Manufacturer: Analog Devices

AD8309
Modulated Limiter Output
The limiter output stage of the AD8309 also provides an analog
multiplication capability: the amplitude of the output square
wave can be controlled by the current withdrawn from LMDR
(Pin 9). An analog control input of 0 V to +1 V is used to gener-
ate an exactly-proportional current of 0 mA to 10 mA in the npn
transistor, whose collector is held at a fixed voltage of 400 mV
by the internal bias in the AD8309. When the input signal is
above the limiting threshold, the output will then be a square-
wave whose amplitude is proportional to the control bias.
Effect of Waveform Type on Intercept
The AD8309 fundamentally responds to voltage and not to
power. A direct consequence of this characteristic is that input
signals of equal rms power, but differing crest factors, will pro-
duce different results at the log amp’s output.
The effect of differing signal waveforms is to shift the effective
value of the log amp’s intercept. Graphically, this looks like a
vertical shift in the log amp’s transfer function. The device’s
logarithmic slope however is not affected. For example, consider
0.1 F
10
Figure 39. Variable Limiter Output Programming
1
2
3
4
5
6
7
8
COM2
VPS1
PADL
INHI
INLO
PADL
COM1
ENBL
ENABLE
INLO
INHI
+V
EXT
AD8309
SIG
SIG
S
LMDR
VLOG
LMLO
PADL
PADL
VPS2
FLTR
LMHI
15
13
12
10
16
14
11
9
0
R1
10
0mA TO
10mA
0.1 F
0.01 F
0.01 F
VARIABLE
OUTPUT
4.7
C2
R2
C1
2N3904
RSSI
0.1 F
C3
R/L
52.3
18
A
B
Figure 40. Evaluation Board Schematic
AD8031
0.1 F
1
2
3
4
5
6
7
8
0V TO +1V
COM2
VPS1
PADL
INHI
INLO
PADL
COM1
ENBL
AD8309
8.2k
1.8k
V
S
–18–
LMDR
VLOG
LMLO
PADL
PADL
VPS2
FLTR
LMHI
the case of the AD8309 being alternately fed by an unmodu-
lated sine wave and by a single CDMA channel of the same rms
power. The AD8309’s output voltage will differ by the equiva-
lent of 3.55 dB (71 mV) over the complete dynamic range of the
device (the output for a CDMA input being lower).
Table III shows the correction factors that should be applied to
measure the rms signal strength of a various signal types. A sine
wave input is used as a reference. To measure the rms power of
a square wave, for example, the mV equivalent of the dB value
given in the table (20 mV/dB times 3.01 dB) should be sub-
tracted from the output voltage of the AD8309.
Table III. Shift in AD8309 Output for Signals with Differing
Crest Factors
Signal Type
Sine Wave
Square Wave or DC
Triangular Wave
GSM Channel (All Time Slots On) +0.55 dB
CDMA Channel
PDC Channel (All Time Slots On) +0.58 dB
Gaussian Noise
Evaluation Board
An evaluation board, carefully laid out and tested to demon-
strate the specified high speed performance of the AD8309 is
available. Figure 40 shows the schematic of the evaluation board
which fairly closely follows the basic connections schematic
shown in Figure 30. For ordering information, please refer to
the Ordering Guide. Links, switches and component settings for
different setups are described in Table IV.
15
13
12
10
16
14
11
9
C7 (OPEN)
402
R6
LK1
0.1 F
402
C4
R8
R7
402
(OPEN)
4.7
0
R5
R3
L1
R4
(OPEN)
Correction Factor
(Add to Output Reading)
0 dB
–3.01 dB
+0.9 dB
+3.55 dB
+2.51 dB
0.01 F
0.01 F
C5
C6
V
LMHI
LMLO
+V
RSSI
S
REV. B