AD8310 Analog Devices, AD8310 Datasheet - Page 11

• Manufacturer Part Number: AD8310
• Description: Fast Response, DC - 440 Mhz, Voltage Out, 90 DB Logarithmic Amplifier
• Manufacturer: Analog Devices

Figure 26. Connections for Isolation of “Source” Ground
from Device Ground
In applications where the ground plane may not be an equipoten-
tial (possibly due to noise in the ground plane), the “low” input
of an unbalanced source should generally be ac-coupled through
a separate connection the “low” associated with the source.
Furthermore, it is good practice in such situations to break the
ground loop by inserting a small resistance to ground in the “low”
side of the input connector (Figure 26).
Figure 27 shows the output versus the input level for sine
inputs at 10 MHz, 50 MHz, and 100 MHz; Figure 28 shows
the logarithmic conformance under the same conditions.
REV. A
GENERATOR
Figure 27. Output vs. Input Level at 10 MHz, 50 MHz, and
100 MHz
Figure 28. Log-Conformance Errors vs. Input Level at
10 MHz, 50 MHz, and 100 MHz
COMMON
SIGNAL
INPUT
3.0
2.5
2.0
1.5
1.0
0.5
–1
–2
–3
–4
–5
–120
–120
5
4
3
2
1
0
0
INTERCEPT
(–87dBm)
4.7
(–87dBm)
–100
–100
52.3
0.01 F
0.01 F
C2
C1
BOARD-LEVEL
GROUND
–80
–80
INLO COMM OFLT VOUT
INPUT LEVEL – dBV
INHI ENBL BFIN VPOS
INPUT LEVEL – dBV
3dB DYNAMIC RANGE
1dB DYNAMIC RANGE
–60
–60
AD8310
–40
–40
NC
NC
NC = NO CONNECT
–20
–20
50MHz
100MHz
10MHz
OPTIONAL
(+13dBm)
4.7
(+13dBm)
C4
0.01 F
0
0
100MHz
10MHz
50MHz
V
(2.7–5.5V)
V
20
20
S
OUT
(RSSI)
–11–
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 approxi-
mately 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 28, 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 27). For the
AD8310 the intercept is calibrated to be –108 dBV (–95 dBm).
Using the slope and intercept, the output voltage can be calcu-
lated for any input level within the specified input range using
the equation:
where V
is the logarithmic slope, expressed in V/dB, P
expressed in decibels relative to some reference level (either
dBm or dBV in this case) and P
pressed in decibels relative to the same reference level.
For example, for an input level of –33 dBV (–20 dBm), the out-
put voltage will be
dBV vs. dBm
The most widely used convention in RF systems is to specify
power in dBm, that is, decibels above 1 mW in 50 . Specifi-
cation of log amp input level in terms of power is strictly a
concession 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
dBV and dBm in specifying the performance of the AD8310,
showing equivalent dBm levels for the special case of a 50
environment. Values in dBV are converted to dBm re 50
adding 13 dB.
Effect of Waveform Type on Intercept
Input signals of equal rms power, but differing crest factors, will
produce different results at the log amp’s output.
Differing signal waveforms shift the effective value of the inter-
cept. Graphically, this looks like a vertical shift in the log amp’s
transfer function. The logarithmic slope, however, is not affected.
For example, consider the case of the AD8310 being alternately
fed by an unmodulated sine wave and by a single CDMA channel
of the same rms power. The output voltage will differ by the
equivalent of 3.55 dB (71 mV) over the complete dynamic range
of the device (the output for the CDMA input being lower).
V
OUT
OUT
= 0.024 V/dB
is the demodulated and filtered RSSI output, V
V
OUT
= V
(–33 dBV – (–108 dBV)) = 1.8 V
SLOPE
0
is the logarithmic intercept, ex-
(P
IN
– P
IN
0
)
is the input signal,
AD8310
—we use both
SLOPE
by