Datasheet

AD8330 Data Sheet
Rev. F | Page 24 of 32
The noise figure is the decibel representation of the noise factor,
N
FAC
, commonly defined as follows:
outputatSNR
inputatSNR
N
FAC
=
(20)
However, this is equivalent to
pinsinputtheatSNR
sourcetheatSNR
=N
FAC
(21)
Let V
NSD
be the voltage noise spectral density √kTRS due to the
source resistance. Using Equation 17 gives
( ){ }
( )
{ }
NSDS
IN
NOISE
I
S
I
SIN
NOISE
IN
NSDS
II
SIG
FAC
V
R
VR
RRRVV
VRR
R
V
N
_
_
//
//
=
+
+
=
(22)
Then, using the result from Equation 19 for a source resistance
of 1 kΩ, having a noise-spectral density of 4.08 nV/√Hz produces
(
)
( )
( )
( )
79.1
HznV/08.4kΩ1
Hz/nV3.7kΩ1
==
FAC
N
(23)
Finally, converting this to decibels using
N
FIG
= 10 log
10
(N
FAC
) (24)
Thus, the resultant noise figure in this example is 5.06 dB,
which is somewhat lower than the value shown in Figure 53 for
this operating condition.
Noise as a Function of V
DBS
The chief consequence of lowering the basic gain using V
DBS
is
that the current noise spectral density I
NSD
increases with the
square root of the basic gain magnitude, G
BN
such that
I
NSD
= (3 pA/√Hz)(√G
BN
) (25)
Therefore, at the minimum basic gain of ×0, I
NSD
rises to
53.3 pA/√Hz. However, the noise figure rises to 17.2 db if it
is recalculated using the procedures in Equation 16 through
Equation 24.
Distortion Considerations
Continuously variable gain amplifiers invariably employ
nonlinear circuit elements; consequently, it is common for their
distortion to be higher than well-designed fixed gain amplifiers.
The translinear multiplier principles used in the AD8330, in
theory, yield extremely low distortion, a result of the funda-
mental linearization technique that is an inherent aspect of
these circuits.
In practice, however, the effect of device mismatches and junc-
tion resistances in the core cell, and other mechanisms in its
supporting circuitry inevitably cause distortion, further aggravated
by other effects in the later output stages. Some of these effects
are very consistent from one sample to the next, while those due
to mismatches (causing predominantly even-order distortion
components) are quite variable. Where the highest linearity
(and lowest noise) is demanded, consider using one of the X-
AMP products such as the AD603 (single-channel), AD604
(dual-channel), or AD8332 (wideband dual-channel with
ultralow noise LNAs).
P1dB and V1dB
In addition to the nonlinearities that arise within the core of the
AD8330, at moderate output levels, another metric that is more
commonly stated for RF components that deliver appreciable
power to a load is the 1 dB compression point. This is defined
in a very specific manner: it is that point at which, with increasing
output level, the power delivered to the load eventually falls to a
value that is 1 dB lower than it would be for a perfectly linear
system. (Although this metric is sometimes called the 1 dB gain
compression point, it is important to note that this is not the
output level at which the incremental gain has fallen by 1 dB).
As shown in Figure 49, the output of the AD8330 limits quite
abruptly, and the gain drops sharply above the clipping level.
The output power, on the other hand, using an external resistive
load, R
L
, continues to increase. In the most extreme case, the
waveform changes from the sinusoidal form of the test signal,
with an amplitude just below the clipping level, V
CLIP
, to a
square wave of precisely the same amplitude. The change in
power over this range is from (V
CLIP
/√2)
2
/R
L
to (V
CLIP
)
2
/R
L
, that
is, a factor of 2, or 3 dB in power terms. It can be shown that for
an ideal limiting amplifier, the 1 dB compression point occurs
for an overdrive factor of 2 dB.
For example, if the AD8330 is driving a 150 Ω load and V
MAG
is
set to 2 V, the peak output is nominally ±4 V (as noted previously,
the actual value, when loaded. can differ because of a mismatch
between on-chip and external resistors), or 2.83 V rms for a sine
wave output that corresponds to a power of 53.3 mW, that is,
17.3 dBm in 150 Ω. Thus, the P1dB level, at 2 dB above
clipping, is 19.3 dBm.
Though not involving power transfer, it is sometimes useful
to state the V1dB, which is the output voltage (unloaded or
loaded) that is 2 dB above clipping for a sine waveform. In the
above example, this voltage is still 2.83 V rms, which can be
expressed as 9.04 dBV (0 dBV corresponds to a 1 V sine wave).
Thus, the V1dB is at 11.04 dBV.