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OPA681U Datasheet(PDF) 11 Page - Burr-Brown (TI) |
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OPA681U Datasheet(HTML) 11 Page - Burr-Brown (TI) |
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11 / 21 page  ® OPA681 11 APPLICATIONS INFORMATION WIDEBAND CURRENT FEEDBACK OPERATION The OPA681 gives the exceptional AC performance of a wideband current feedback op amp with a highly linear, high power output stage. Requiring only 6mA quiescent current, the OPA681 will swing to within 1V of either supply rail and deliver in excess of 135mA guaranteed at room temperature. This low output headroom requirement, along with supply voltage independent biasing, gives remarkable single (+5V) supply operation. The OPA681 will deliver greater than 200MHz bandwidth driving a 2Vp-p output into 100 Ω on a single +5V supply. Previous boosted output stage amplifiers have typically suffered from very poor crossover distortion as the output current goes through zero. The OPA681 achieves a comparable power gain with much better linear- ity. The primary advantage of a current feedback op amp over a voltage feedback op amp is that AC performance (bandwidth and distortion) is relatively independent of sig- nal gain. For similar AC performance at low gain, with improved DC accuracy, consider the high slew rate, unity gain stable, voltage feedback OPA680. Figure 1 shows the DC coupled, gain of +2, dual power supply circuit configuration used as the basis of the ±5V Specifications and Typical Performance Curves. For test purposes, the input impedance is set to 50 Ω with a resistor to ground and the output impedance is set to 50 Ω with a series output resistor. Voltage swings reported in the speci- fications are taken directly at the input and output pins while load powers (dBm) are defined at a matched 50 Ω load. For the circuit of Figure 1, the total effective load will be 100 Ω || 804 Ω = 89Ω. The disable control line (DIS) is typically left open to guarantee normal amplifier operation. One optional component is included in Figure 1. In addition to the usual power supply de-coupling capacitors to ground, a 0.1 µF capacitor is included between the two power supply pins. In practical PC board layouts, this optional added capacitor will typically improve the 2nd harmonic distortion perfor- mance by 3dB to 6dB. Figure 2 shows the AC-coupled, gain of +2, single supply circuit configuration used as the basis of the +5V Specifica- tions and Typical Performance Curves. Though not a “rail- to-rail” design, the OPA681 requires minimal input and output voltage headroom compared to other very wideband current feedback op amps. It will deliver a 3Vp-p output swing on a single +5V supply with greater than 150MHz bandwidth. The key requirement of broadband single supply operation is to maintain input and output signal swings within the usable voltage ranges at both the input and the output. The circuit of Figure 2 establishes an input midpoint bias using a simple resistive divider from the +5V supply (two 806 Ω resistors). The input signal is then AC coupled into this midpoint voltage bias. The input voltage can swing to within 1.5V of either supply pin, giving a 2Vp-p input signal range centered between the supply pins. The input impedance matching resistor (57.6 Ω) used for testing is adjusted to give a 50 Ω input match when the parallel combination of the biasing divider network is included. The gain resistor (RG) is AC-coupled, giving the circuit a DC gain of +1—which puts the input DC bias voltage (2.5V) on the output as well. The feedback resistor value has been adjusted from the bipolar supply condition to re-optimize for a flat frequency response in +5V, gain of +2, operation (see Setting Resistor Values to Optimize Bandwidth). Again, on a single +5V supply, the output voltage can swing to within 1V of either supply pin while delivering more than 80mA output current. A demanding 100 Ω load to a midpoint bias is used in this characterization circuit. The new output stage used in the OPA681 can deliver large bipolar output currents into this midpoint load with minimal crossover distortion, as shown by the +5V supply, 3rd harmonic distortion plots. FIGURE 1. DC-Coupled, G = +2, Bipolar Supply, Specifi- cation and Test Circuit. FIGURE 2. AC-Coupled, G = +2, Single Supply Specifica- tion and Test Circuit. OPA681 +5V + DIS –5V –V S +V S 50 Ω Load 50 Ω 50 Ω V O V I 50 Ω Source R G 402 Ω R F 402 Ω + 6.8µF 0.1µF 6.8µF 0.1µF 0.1µF OPA681 +5V DIS V S/2 806 Ω 100 Ω V O V I +V S 57.6 Ω 806 Ω R F 499 Ω R G 499 Ω 0.1µF 0.1µF 6.8µF + 0.1µF |
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