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OPA2658U Datasheet(PDF) 11 Page - Burr-Brown (TI) |
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OPA2658U Datasheet(HTML) 11 Page - Burr-Brown (TI) |
11 / 13 page 11 ® OPA2658 CAPACITIVE LOADS The OPA2658’s output stage has been optimized to drive low resistive loads. Capacitive loads, however, will decrease the amplifier’s phase margin which may cause high fre- quency peaking or oscillations. Capacitive loads greater than 5pF should be buffered by connecting a small resistance, usually 10 Ω to 35Ω, in series with the output as shown in Figure 5. This is particularly important when driving high capacitance loads such as flash A/D converters. In general, capacitive loads should be minimized for opti- mum high frequency performance. Coax lines can be driven if the cable is properly terminated. The capacitance of coax cable (29pF/foot for RG-58) will not load the amplifier when the coaxial cable or transmission line is terminated with its characteristic impedance. COMPENSATION The OPA2658 is internally compensated and is stable in unity gain with a phase margin of approximately 62 °, and approximately 64 ° in a gain of +2V/V when used with the recommended feedback resistor value. Frequency response for other gains are shown in the Typical Performance Curves. The high-frequency response of the OPA2658 in a good layout is very flat with frequency. DISTORTION The OPA2658’s Harmonic Distortion characteristics into a 100 Ω load are shown versus frequency and power output in the Typical Performance Curves. Distortion can be further improved by increasing the load resistance as illustrated in Figure 6. Remember to include the contribution of the feedback resistance when calculating the effective load re- sistance seen by the amplifier. Narrowband communication channel requirements will ben- efit from the OPA2658’s wide bandwidth and low intermodulation distortion on low quiescent power. If output signal power at two closely spaced frequencies is required, third-order nonlinearities in any amplifier will cause spuri- ous power at frequencies very near the two funda- mental frequencies. If the two test frequencies, f1 and f2, are specified in terms of average and delta frequency, f O = (f1 + f2)/2 and ∆f = f2 – f1, the two, third-order, close-in spurious tones will appear at fO ±3 • ∆f. The two FIGURE 5. Driving Capacitive Loads. C L R L 50 Ω R S 10 Ω to 35Ω 402 Ω 402 Ω 1/2 OPA2658 tone, third-order spurious plot shown in Figure 7 indicates how far below these two equal power, closely spaced, tones the intermodulation spurious will be. The single tone power is at a matched 50 Ω load. The unique design of the OPA2658 provides much greater spurious free range than what a two- tone third-order intermodulation intercept specification would predict. This can be seen in Figure 7 as the spurious free range actually increases at the higher output power levels. –55 –60 –65 –70 –75 –80 –85 5MHz HARMONIC DISTORTION vs LOAD RESISTANCE (G = +2) Load Resistance ( Ω) 10 100 1k G = +2, V O = 2Vp-p, fO = 5MHz 3f O 2f O FIGURE 6. 5MHz Harmonic Distortion vs Load Resistance. –65 –70 –75 –80 –85 –90 –18 –16 –14 –12 –10 –8 –6 –4 –2 0 2 4 TWO TONE, THIRD-ORDER SPURIOUS LEVELS Single Tone Power (dBm) 20MHz 10MHz 5MHz CROSSTALK Crosstalk is the undesired result of the signal of one channel mixing with and reproducing itself in the output of the other channel. Crosstalk occurs in most multichannel integrated circuits. In dual devices, the effect of crosstalk is measured by driving one channel and observing the output of the undriven channel over various frequencies. The magnitude of this effect is referenced in terms of channel- to-channel isolation and expressed in decibels. "Input referred" points to the fact that there is a direct correlation between gain and crosstalk, there- fore at increased gain, crosstalk also increases by a factor equal to that of the gain. Figure 8 illustrates the measured effect of crosstalk in the OPA2658U. FIGURE 7. Third-Order Intercept Point vs Frequency. |
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