AD8330ACP-R2 datasheet(24/32 Pages) AD

24 / 32 page

AD8330

Rev. C | Page 24 of 32

impedance termination, which is the actual situation in this

case. Repeating the procedure

() () ()2

_ IN

NOISE

= 7.3 nV/√Hz

(19)

The noise figure is the decibel representation of the noise factor,

NFAC, commonly defined as follows:

output

SNR

input

SNR

FAC =

(20)

However, this is equivalent to

pins

input

the

SNR

source

the

SNR

FAC

(21)

Let VNSD be the voltage noise spectral density √kTRS due to the

source resistance. Using Equation 17, gives

(

)

{

}

()

{}

NSD

NOISE

NSD

SIG

FAC

(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

()

(

)

()

FAC

(23)

Finally, converting this to decibels using

NFIG = 10 log10(NFAC)

(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 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 such that

INSD = (3 pA/√Hz)(√GBN)

(25)

Therefore, at the minimum basic gain of ×0, INSD rises to

53.3 pA/√Hz. However, the noise figure rises to 17.2 db if it

is recalculated using the procedures in Equation16 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

junction 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 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 ultralow noise LNAs).

P1 dB and V1 dB

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 was 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, 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, 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 ±4 V (as

noted above, 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 that can be

expressed as 9.04 dBV (0 dBV corresponds to a 1 V sine wave).

Thus, the V1dB is at 11.04 dBV.

AD8330ACP-R2 数据表(PDF) 24 Page - Analog Devices

AD8330ACP-R2 数据表(HTML) 24 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

数据表下载