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Datenblatt-Suchmaschine für elektronische Bauteile |
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Datenblatt-Suchmaschine für elektronische Bauteile |
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AD8330 Datenblatt(PDF) 25 Page - Analog Devices |
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AD8330 Datenblatt(HTML) 25 Page - Analog Devices |
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25 / 32 page  Data Sheet AD8330 Rev. H | Page 25 of 32 The noise figure is the decibel representation of the noise factor, NFAC, commonly defined as follows: Output at SNR Input at SNR NFAC = (20) However, this is equivalent to Pins Input the at SNR Source the at SNR = NFAC (21) Let VNSD be the voltage noise spectral density √kTRS due to the source resistance. Using Equation 17 gives ( ) { } ( ) { } NSD S IN NOISE I S I S IN NOISE IN NSD S I I SIG FAC V R V R R R R V V V R R 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 Hz nV/ 08 . 4 kΩ 1 Hz / nV 3 . 7 kΩ 1 = = FAC N (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 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, 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 square wave 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 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. |
Ähnliche Teilenummer - AD8330_16 |
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Ähnliche Beschreibung - AD8330_16 |
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