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AN-1026 Datasheet(PDF) 10 Page - Analog Devices |
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AN-1026 Datasheet(HTML) 10 Page - Analog Devices |
10 / 16 page AN-1026 APPLICATION NOTE Rev. 0 | Page 10 of 16 HARMONIC DISTORTION Low harmonic distortion in the frequency domain is important in both narrow-band and broadband systems. Nonlinearities in the drivers generate single-tone harmonic distortion and multitone, intermodulation distortion products at amplifier outputs. The same approach used in the noise analysis example can be applied to distortion analysis, comparing the harmonic distortion of the ADA4939 with 1 LSB of the AD9445’s ENOB of 12 bits with a 2 V full-scale output. One ENOB LSB is 488 μV in the noise analysis. The distortion data in the specifications table of the ADA4939 is given for a gain of 2, comparing second and third harmonics at various frequencies. Table 3 shows the harmonic distortion data for a gain of 2 and differential output swing of 2 V p-p. Table 3. Second and Third Harmonic Distortion of the ADA4939 Parameter Harmonic Distortion (dBc) HD2 @ 10 MHz −102 HD2 @ 70 MHz −83 HD2 @ 100 MHz −77 HD3 @ 10 MHz −101 HD3 @ 70 MHz −97 HD3 @ 100 MHz −91 The data shows that harmonic distortion increases with frequency and that HD2 is worse than HD3 in the bandwidth of interest (50 MHz). Harmonic distortion products are higher in frequency than the frequency of interest, so their amplitude can be reduced by system band-limiting. If the system had a brick-wall filter at 50 MHz, then only the frequencies higher than 25 MHz are of concern because all harmonics of higher frequencies are eliminated by the filter. Nevertheless, the system was evaluated up to 50 MHz because any filtering that is present may not sufficiently suppress the harmonics, and distortion products can alias back into the signal bandwidth. Figure 16 shows the harmonic distortion vs. frequency of the ADA4939 for various supply voltages with a 2 V p-p output. –60 –110 –105 –100 –95 –90 –85 –80 –75 –70 –65 1 10 100 FREQUENCY (MHz) HD2, VS (SPLIT SUPPLY) = ±2.5V HD3, VS (SPLIT SUPPLY) = ±2.5V HD2, VS (SPLIT SUPPLY) = ±1.65V HD3, VS (SPLIT SUPPLY) = ±1.65V VOUT, dm = 2V p-p HD2 ≈ –88dBc @ 50MHz Figure 16. Harmonic Distortion vs. Frequency The HD2 at 50 MHz is approximately −88 dBc, relative to a 2 V p-p input signal. To compare the harmonic distortion level to 1 ENOB LSB, this level must be converted to a voltage as shown in Equation 33. () p - p μV 80 10 p - p V 2 20 88 ≈ ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ = − HD2 (33) This distortion product is only 80 μV p-p, or 16% of 1 ENOB LSB. Thus, from a distortion standpoint, the ADA4939 is a good choice to consider as a driver for the AD9445 ADC. Because ADC drivers are negative feedback amplifiers, output distortion depends on the amount of loop gain in the amplifier circuit. The inherent open-loop distortion of a negative feedback amplifier is reduced by a factor of 1/(1 + LG), where LG is the available loop gain. The input (error voltage) of the amplifier is multiplied by a large forward voltage gain, A(s), then passes through the feedback factor, β, to the input, where it adjusts the output to minimize the error. Therefore, the loop gain of this type of amplifier is A(s) × β; as the loop gain (A(s), β, or both) decreases, harmonic distortion increases. Voltage feedback amplifiers, such as integrators, are designed to have large A(s) at dc and low frequencies, and then roll off as 1/f toward unity at a specified high frequency. As A(s) rolls off, loop gain decreases and distortion increases. Therefore, the harmonic distortion characteristic is the inverse of A(s). Current feedback amplifiers use an error current as the feedback signal. The error current is multiplied by a large forward transresistance, T(s), which converts it to the output voltage, then passes through the feedback factor, 1/RF, which converts the output voltage to a feedback current that tends to minimize the input error current. The loop gain of an ideal current feedback amplifier is therefore T(s) × (1/RF) = T(s)/RF. Like A(s), T(s) has a large dc value and rolls off with increasing frequency, reducing loop gain and increasing the harmonic distortion. Loop gain also depends directly upon the feedback factor, 1/RF. The loop gain of an ideal current feedback amplifier does not depend on a closed-loop voltage gain; therefore, harmonic distortion performance does not degrade as the closed-loop gain increases. In a real current feedback amplifier, loop gain does have some dependence on the closed-loop gain but not nearly to the extent that it does in a voltage feedback amplifier. This makes a current feedback amplifier, such as the ADA4927, a better choice than a voltage feedback amplifier for applications requiring high closed-loop gain and low distortion. |
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