AD8330 Analog Devices, AD8330 Datasheet - Page 22

• Manufacturer Part Number: AD8330
• Description: Low Cost DC-150MHz Variable Gain Amplifier
• Manufacturer: Analog Devices

AD8330
The noise figure is just the decibel representation of the noise
factor, N
However, this is equivalent to
Let V
the source resistance. Then we have:
using (17). Thus, using the result (19) for a source resistance of
1 k , having a noise-spectral density of 4.08 nV/ Hz, we have:
Finally, converting this to decibels using:
we find the noise figure in this case to be 5.06 dB, which is some-
what lower than the value shown in Figure 10 for this operating
condition.
Noise as a Function of V
The chief consequence of lowering the basic gain using V
that the current-noise-spectral-density I
square root of the basic gain magnitude, G
Thus, at the maximum basic gain of 316, I
53.3 pA/ Hz, and if we recalculate the noise figure using the
procedures just explained, we find it has risen to 17.2 dB.
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 principle yield extremely
low distortion, a result of the fundamental linearization technique
that is an inherent aspect of these circuits.
In practice, however, the effect of device mismatches and junction
resistances in the core cell, and other mechanisms in its support-
ing 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 compo-
nents) will be quite variable. Where the highest linearity (and also
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 ultra-low noise LNAs).
N
FAC
N
N
N
N
I
NSD
NSD
=
FAC
FAC
FAC
FIG
V
FAC
be the voltage-noise-spectral-density kTRS due to
IN
V
, which is commonly defined as follows:
SIG
/
3 pA
10
1
Signal to noise ratio at output
{
Signal to noise ratio at the input pins
1
Signal to noise ratio at input
V
k
{
k
Signal to noise ratio at the source
log
NOISE IN
R
I
/
4 08
10
/
7 3
- -
.
- -
(
.
- -
Hz G
R
_
- -
N
I
nV
nV
FAC
+
R / R + R
DBS
R
S
/
/
S
BN
(
)
Hz
}
Hz
/
I
V
NSD
1 79
S
.
)
}
NSD
=
BN
increases with the
R V
NSD
:
I
R V
S
has risen to
NOISE IN
NSD
_
DBS
(20)
(21)
(22)
(23)
(24)
(25)
is
–22–
P1 dB and V1 dB
In addition to the nonlinearities that arise within the core of the
AD8330, at moderate output levels, a further 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 increas-
ing 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. (While 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 was shown in Figure 6, 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
waveform changes from the sinusoidal form of the test signal,
with an amplitude just below the clipping level, say, V
squarewave of precisely the same amplitude. The change in
power over this range is from (V
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
has been set to 2 V, the peak output is nominally 4 V (as noted
above, the actual value when loaded may differ due to the mismatch
between on-chip and external resistors), or 2.83 V rms for a sine
wave output, which corresponds to a power of 53.3 mW, that is,
17.3 dBm in 150 . Thus, the P1dB level, at 2 dB above clipping,
is 19.3 dBm.
While 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.
APPLICATIONS
The AD8330’s versatility, very constant ac response over a wide
range of gains, large signal dynamic range, output swing, single-
supply operation, and low power consumption will commend this
VGA to a diverse variety of applications. Only a few can be described
here, including the most basic uses and some unusual ones.
ADC Driving
The AD8330 is well-suited to driving a high speed converter.
There are now many available, but to illustrate the general fea-
tures we will use one of the least expensive, the AD9214, which
is available in three grades for operation at 65 MHz, 80 MHz,
and 105 MHz; the AD9214BRS-80 is a good complement to the
general capabilities of this VGA.
L
, continues to increase. In the most extreme case, the
CLIP
/ 2)
2
/R
L
to (V
load and V
CLIP
)
2
CLIP
/R
REV. A
L
, to a
, that
MAG