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OPA2690 Datasheet(PDF) 22 Page  BurrBrown (TI) 


OPA2690 Datasheet(HTML) 22 Page  BurrBrown (TI) 
22 / 30 page OPA2690 SBOS238D − JUNE 2002 − REVISED DECEMBER 2004 www.ti.com 22 DISTORTION PERFORMANCE The OPA2690 provides good distortion performance into a 100 Ω load on ±5V supplies. Relative to alternative solutions, it provides exceptional performance into lighter loads and/or operating on a single +5V supply. Generally, until the fundamental signal reaches very high frequency or power levels, the 2ndharmonic dominates the distortion with a negligible 3rdharmonic component. Focusing then on the 2ndharmonic, increasing the load impedance improves distortion directly. Remember that the total load includes the feedback network; in the noninverting configuration (see Figure 1), this is sum of RF + RG, while in the inverting configuration it is just RF. Also, providing an additional supplydecoupling capacitor (0.1 µF) between the supply pins (for bipolar operation) improves the 2ndorder distortion slightly (3dB to 6dB). Operating differentially also lowers 2ndharmonic distortion terms (see the plot on the front page). In most op amps, increasing the output voltage swing increases harmonic distortion directly. The new output stage used in the OPA2690 actually holds the difference between fundamental power and the 2nd and 3rdharmonic powers relatively constant with increasing output power until very large output swings are required (>4VPP). This also shows up in the 2tone, 3rdorder intermodulation spurious (IM3) response curves. The 3rdorder spurious levels are extremely low at low output power levels. The output stage continues to hold them low even as the fundamental power reaches very high levels. As the Typical Characteristics show, the spurious intermodulation powers do not increase as predicted by a traditional intercept model. As the fundamental power level increases, the dynamic range does not decrease significantly. For 2 tones centered at 20MHz, with 10dBm/tone into a matched 50 Ω load (i.e., 2VPP for each tone at the load, which requires 8VPP for the overall 2tone envelope at the output pin), the Typical Characteristics show 46dBc difference between the test tone powers and the 3rdorder intermodulation spurious powers. This exceptional performance improves further when operating at lower frequencies or powers. NOISE PERFORMANCE High slew rate, unitygain stable, voltagefeedback op amps usually achieve their slew rate at the expense of a higher input noise voltage. The 5.5nV/ √Hz input voltage noise for the OPA2690 is, however, much lower than comparable amplifiers. The inputreferred voltage noise, and the two inputreferred current noise terms, combine to give low output noise under a wide variety of operating conditions. Figure 15 shows the op amp noise analysis model with all the noise terms included. In this model, all noise terms are taken to be noise voltage or current density terms in either nV/ √Hz or pA/√Hz. 4kT R G R G R F R S 1/2 OPA2690 I BI E O I BN 4kT = 1.6E − 20J at 290 _K E RS E NI 4kTR S √ 4kTR F √ Figure 15. Op Amp Noise Analysis Model The total output spot noise voltage can be computed as the square root of the sum of all squared output noise voltage contributors. Equation 6 shows the general form for the output noise voltage using the terms shown in Figure 15. E O + ENI 2 ) I BNRS 2 ) 4kTR S NG2 ) I BIRF 2 ) 4kTR FNG Dividing this expression by the noise gain (NG = (1 + RF/RG)) will give the equivalent inputreferred spot noise voltage at the noninverting input, as shown in Equation 7. E N + E NI 2 ) I BNRS 2 ) 4kTR S ) I BIRF NG 2 ) 4kTR F NG Evaluating these two equations for the OPA2690 circuit and component values (see Figure 1) gives a total output spot noise voltage of 12.3nV/ √Hz and a total equivalent input spot noise voltage of 6.1nV/ √Hz. This is including the noise added by the bias current cancellation resistor (175 Ω) on the noninverting input. This total inputreferred spot noise voltage is only slightly higher than the 5.5nV/ √Hz specification for the op amp voltage noise alone. This will be the case as long as the impedances appearing at each op amp input are limited to the previously recommend maximum value of 300 Ω. Keeping both (RF RG) and the noninverting input source impedance less than 300 Ω will satisfy both noise and frequency response flatness considerations. As the resistorinduced noise is relatively negligible, additional capacitive decoupling across the bias current cancellation resistor (RB) for the inverting op amp configuration of Figure 12 is not required. (6) (7) 
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