Datasheet AD8306 (Analog Devices) - 10

AD8306
low frequency applications, a simple RC network forming a low- where VOUT is the demodulated and filtered RSSI output, pass filter should be added at the input for the same reason. VSLOPE is the logarithmic slope, expressed in V/dB, PIN is the If the limiter output is not required, Pin 9 (LMDR) should be input signal, expressed in decibels relative to some reference left open and Pins 12 and 13 (LMHI, LMLO) should be tied to level (either dBm or dBV in this case) and PO is the logarithmic VPS2 as shown in Figure 24. intercept, expressed in decibels relative to the same reference level. 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 For example, for an input level of –33 dBV (–20 dBm), the dBV number to get dBm Re 50 Ω. Figure 26 shows the typi- output voltage will be cal logarithmic linearity (log conformance) under the same VOUT = 0.02 V/dB × (–33 dBV – (–108 dBV)) = 1.5 V (3) conditions. The most widely used convention in RF systems is to specify power in dBm, that is, decibels above 1 mW in 50 Ω. Specifica-
2.5 100MHz
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
2
“power absorbed at the input”), but to the input voltage. The
50MHz
use of dBV, defined as decibels with respect to a 1 V rms sine wave,
10MHz
is more precise, although this is still not unambiguous because
1.5
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
1 RSSI OUTPUT – V
of power—more specifically, in dBm/50 Ω—we use both dBV and dBm in specifying the performance of the AD8306, showing
0.5
equivalent dBm levels for the special case of a 50 Ω environment. Values in dBV are converted to dBm re 50 Ω by adding 13.
0 Output Response Time and CF –120 –100 –80 –60 –40 –20 0 20
The RSSI output has a low-pass corner frequency of 3.5 MHz,
INPUT LEVEL – dBV
which results in a 10% to 90% rise time of 73 ns. For low fre- Figure 25. RSSI Output vs. Input Level at TA = +25°C for quency applications, the corner frequency can be reduced by Frequencies of 10 MHz, 50 MHz and 100 MHz adding an external capacitor, CF, between FLTR (Pin 10) and VLOG (Pin 16) as shown in Figure 24. For example, an exter-
5
nal 33 pF will reduce the corner frequency to 350 kHz, while
DYNAMIC RANGE
6
1dB
6
3dB 4
360 pF will set it to 35 kHz, in each case with an essentially
10MHz 86 93 50MHz 90 97
one-pole response.
3 100MHz 96 100 Using the Limiter 2
Figure 27 shows the basic connections for operating the limiter
1
and the log output concurrently. The limiter output is a pair of
100MHz 0
differential currents of magnitude, IOUT, from high impedance
10MHz
(open-collector) sources. These are converted to equal-amplitude
ERROR – dB –1 50MHz
voltages by supply-referenced load resistors, RLOAD. The limiter
–2
output current is set by RLIM, the resistor connected between
–3
Pin 9 (LMDR) and ground. The limiter output current is set
–4
according the equation:
–5
IOUT = –400 mV/RLIM (5)
–120 –100 –80 –60 –40 –20 0 20 INPUT LEVEL – dBV
and has an absolute accuracy of ± 5%. Figure 26. Log Linearity vs. Input Level at T The supply referenced voltage on each of the limiter pins will A = +25 °C, for Frequencies of 10 MHz, 50 MHz and 100 MHz thus be given by:
Transfer Function in Terms of Slope and Intercept
VLIM = VS –400 mV × RLOAD/RLIM (6) 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: VOUT = VSLOPE × (PIN – PO) (2) –10– REV. A