AD8330 datasheet(22/28 Pages) AD

22 / 28 page

REV. A

–22–

AD8330

The noise figure is just the decibel representation of the noise

factor, NFAC, which is commonly defined as follows:

FAC

Signal to noise ratio at input

Signal to noise ratio at output

(20)

However, this is equivalent to

N FAC =

Signal to noise ratio at the source

Signal to noise ratio at the input pins

(21)

Let VNSD be the voltage-noise-spectral-density

√kTRS due to

the source resistance. Then we have:

R/ R + R

FAC

()

{}

()

{}

SIG

NSD

NOISE

NSD

(22)

using (17). Thus, using the result (19) for a source resistance of

1 k

Ω, having a noise-spectral density of 4.08 nV/√Hz, we have:

knV

FAC

Ω

()

Ω

()

17 3

14 08

179

(23)

Finally, converting this to decibels using:

N FIG =

()

log

NFAC

(24)

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 VDBS

The chief consequence of lowering the basic gain using VDBS is

that the current-noise-spectral-density INSD increases with the

square root of the basic gain magnitude, GBN:

INSD = 3 pA

GBN

(25)

Thus, at the maximum basic gain of

316, INSD has risen to

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).

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, RL, 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, say, VCLIP, to a

squarewave of precisely the same amplitude. The change in

power over this range is from (VCLIP/

√2)2/RL to (VCLIP)2/RL, 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 VMAG

has been set to 2 V, the peak output is nominally

±4V (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

Ω. 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.

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