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OPA2650U Datasheet(PDF) 10 Page - Texas Instruments |
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OPA2650U Datasheet(HTML) 10 Page - Texas Instruments |
10 / 13 page 10 ® OPA2650 FREQUENCY RESPONSE COMPENSATION Each channel of the OPA2650 is internally compensated to be stable at unity gain with a nominal 60 ° phase margin. This lends itself well to wideband integrator and buffer applications. Phase margin and frequency response flatness will improve at higher gains. Recall that an inverting gain of –1 is equivalent to a gain of +2 for bandwidth purposes, i.e., noise gain = 2. The external compensation techniques devel- oped for voltage feedback op amps can be applied to this device. For example, in the non-inverting configuration, placing a capacitor across the feedback resistor will reduce the gain to +1 starting at f = (1/2 πR FCF). Alternatively, in the inverting configuration, the bandwidth may be limited with- out modifying the inverting gain by placing a series RC network to ground on the inverting node. This has the effect of increasing the noise gain at high frequencies, thereby limiting the bandwidth for the inverting input signal through the gain-bandwidth product. At higher gains, the gain-bandwidth of this voltage feedback topology will limit bandwidth according to the open-loop frequency response curve. For applications requiring a wider bandwidth at higher gains, consider the dual current feed- back model, OPA2658. In applications where a large feed- back resistor is required (such as photodiode transimpedance circuits), precautions must be taken to avoid gain peaking due to the pole formed by the feedback resistor and the capacitance on the inverting input. This pole can be compen- sated by connecting a small capacitor in parallel with the feedback resistor, creating a cancelling zero term. In other high-gain applications, use of a three-resistor “T” connec- tion will reduce the feedback network impedance which reacts with the parasitic capacitance at the summing node. PULSE SETTLING TIME High speed amplifiers like the OPA2650 are capable of extremely fast settling time with a pulse input. Excellent frequency response flatness and phase linearity are required to get the best settling times. As shown in the specifications table, settling time for a 2V step at a gain of +1 for the OPA2650 is extremely fast. The specification is defined as the time required, after the input transition, for the output to settle within a specified error band around its final value. For a 2V step, 1% settling corresponds to an error band of ±20mV, 0.1% to an error band of ±2mV, and 0.01% to an error band of ±0.2mV. For the best settling times, particu- larly into an ADC capacitive load, little or no peaking in the frequency response can be allowed. Using the recommended RISO for capacitive loads will limit this peaking and reduce the settling times. Fast, extremely fine scale settling (0.01%) requires close attention to ground return currents in the supply decoupling capacitors. For highest performance, con- sider the OPA642 which offers considerably higher open loop DC gain. DIFFERENTIAL GAIN AND PHASE Differential Gain (dG) and Differential Phase (dP) are among the more important specifications for video applications. The percentage change in closed-loop gain over a specified change in output voltage level is defined as dG. dP is defined as the change in degrees of the closed-loop phase over the same output voltage change. dG and dP are both specified at the NTSC sub-carrier frequency of 3.58MHz. dG and dP increase closed-loop gain and output voltage transition. All measurements were performed using a Tektronix model VM700 Video Measurement Set. DISTORTION The OPA2650’s harmonic distortion characteristics into a 100 Ω load are shown versus frequency and power output in the typical performance curves. Distortion can be signifi- cantly improved by increasing the load resistance as illus- trated in Figure 5. Remember to include the contribution of the feedback resistance when calculating the effective load resistance seen by the amplifier. 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 crosstalk 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 6 illustrates the measured effect of crosstalk in the OPA2650U. SPICE MODELS Computer simulation of circuit performance using SPICE is often useful when analyzing the performance of analog circuits and systems. This is particularly true for Video and RF amplifier circuits where parasitic capacitance and induc- tance can have a major effect on circuit performance. SPICE models are available on a disk from the Burr-Brown Appli- cations Department. –60 –70 –80 –90 10 20 50 100 200 500 1k Load Resistance ( Ω) (G = +1, f O = 5MHz) 2f O 3f O FIGURE 5. 5MHz Harmonic Distortion vs Load Resistance. |
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