AD8310
5
4
3
2
ERROR (dB)
dBV vs. dBm
±3dB DYNAMIC RANGE
±1dB DYNAMIC RANGE
10MHz
1
0
–1
–2
50MHz
–3
–4
–5
–120
–100
(–87dBm)
–80
–60
–40
100MHz
–20
INPUT LEVEL (dBV)
0
(+13dBm)
20
Figure 30. Log Conformance Errors vs. Input Level at 10 MHz,
50 MHz, and 100 MHz
The most widely used convention in RF systems is to specify
power in dBm, decibels above 1 mW in 50 Ω. Specification of
the log amp input level in terms of power is strictly a concession
to popular convention. As mentioned previously, log amps do
not
respond to power (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. However, this is still
ambiguous, because waveform is also involved in the response
of a log amp, which, for a complex input such as a CDMA
signal, does not follow the rms value exactly. Because most users
specify RF signals in terms of power (more specifically, in
dBm/50 Ω) both dBV and dBm are used to specify the perform-
ance of the AD8310, showing 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 dB.
Table 4. Correction for Signals with Differing Crest Factors
Signal Type
Sine wave
Square wave or dc
Triangular wave
GSM channel (all time slots on)
CDMA channel (forward link, nine
channels on)
CDMA channel (reverse link)
PDC channel (all time slots on)
__________________________________________
1
TRANSFER FUNCTION IN TERMS OF SLOPE AND
INTERCEPT
The transfer function of the AD8310 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 AD8310, slope is nominally 24 mV/dB. Therefore, a
10 dB change at the input results in a change at the output of
approximately 240 mV. The plot of log conformance shows the
range over which the device maintains its constant slope. The
dynamic range of the log amp is defined as the range over
which the slope remains within a certain error band, usually
±1 dB or ±3 dB. In Figure 30, for example, the ±1 dB dynamic
range is approximately 95 dB (from +4 dBV to −91 dBV).
The intercept is the point at which the extrapolated linear
response would intersect the horizontal axis (see Figure 29).
For the AD8310, 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 following equation:
V
OUT
=
V
SLOPE
× (
P
IN
−
P
O
)
01084-030
Correction Factor
1
(dB)
0
−3.01
0.9
0.55
3.55
0.5
0.58
Add to the measured input level.
INPUT MATCHING
Where higher sensitivity is required, an input matching network
is useful. Using a transformer to achieve the impedance trans-
formation also eliminates the need for coupling capacitors,
lowers the offset voltage generated directly at the input, and
balances the drive amplitude to INLO and INHI.
The choice of turns ratio depends somewhat on the frequency.
At frequencies below 50 MHz, the reactance of the input
capacitance is much higher than the real part of the input
impedance. In this frequency range, a turns ratio of about 1:4.8
lowers the input impedance to 50 Ω, while raising the input
voltage lowers the effect of the short-circuit noise voltage by the
same factor. The intercept is also lowered by the turns ratio; for
a 50 Ω match, it is reduced by 20 log
10
(4.8) or 13.6 dB. The total
noise is reduced by a somewhat smaller factor, because there is a
small contribution from the input noise current.
where:
V
OUT
is the demodulated and filtered RSSI output.
V
SLOPE
is the logarithmic slope expressed in V/dB.
P
IN
is the input signal expressed in dB relative to some reference
level (either dBm or dBV in this case).
P
O
is the logarithmic intercept expressed in dB relative to the
same reference level.
For example, for an input level of −33 dBV (−20 dBm), the
output voltage is
V
OUT
= 0.024 V/dB × (−33 dBV − (−108 dBV)) = 1.8 V
Rev. D | Page 15 of 24
AD [ ANALOG DEVICES ]
