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OPA692 Datasheet(PDF) 11 Page - Texas Instruments |
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OPA692 Datasheet(HTML) 11 Page - Texas Instruments |
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11 / 30 page  OPA2691 11 SBOS224D www.ti.com APPLICATIONS INFORMATION WIDEBAND CURRENT-FEEDBACK OPERATION The OPA2691 gives the exceptional AC performance of a wideband current-feedback op amp with a highly linear, high- power output stage. Requiring only 5.1mA/ch quiescent cur- rent, the OPA2691 will swing to within 1V of either supply rail and deliver in excess of 160mA tested at room temperature. This low output headroom requirement, along with supply voltage independent biasing, gives remarkable single (+5V) supply operation. The OPA2691 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 OPA2691 achieves a compa- rable power gain with much better linearity. 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 signal gain. For similar AC performance with improved DC accuracy, consider the high slew rate, unity-gain stable, voltage-feedback OPA2690. Figure 1 shows the DC-coupled, gain of +2, dual power- supply circuit configuration used as the basis of the ±5V Electrical Characteristics and Typical Characteristics. 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 electrical characteristics 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 || 804Ω = 89Ω. The disable control line (DIS) is typically left open (SO-14 only) to ensure normal amplifier operation. One optional component is included in Figure 1. In addition to the usual power-supply decoupling capacitors to ground, a 0.01 µF capacitor is included between the two power-supply pins. In practical printed circuit board (PCB) layouts, this optional added capacitor will typically improve the 2nd-harmonic distortion performance by 3dB to 6dB. Figure 2 shows the AC-coupled, gain of +2, single-supply circuit configuration used as the basis of the +5V Electrical Characteristics and Typical Characteristics. Though not a rail-to-rail design, the OPA2691 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 operation, gain of +2, operation (see the Setting Resistor Values to Optimize Bandwidth section). Again, on a single +5V supply, the output voltage can swing to within 1V of either supply pin while delivering more than 75mA output current. A demanding 100 Ω load to a midpoint bias is used in this characterization circuit. The new output stage used in the OPA2691 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, Specifica- tion and Test Circuit. FIGURE 2. AC-Coupled, G = +2, Single-Supply Specification and Test Circuit. 1/2 OPA2691 +5V + DIS –5V 50 Ω Load 50 Ω 50 Ω V O V I 50 Ω Source R G 402 Ω R F 402 Ω + 6.8 µF 0.1 µF 6.8 µF 0.1 µF 0.01 µF +V S –V S 1/2 OPA2691 +5V +V S DIS V S/2 806 Ω 100 Ω V O V I 57.6 Ω 806 Ω R F 453 Ω R G 453 Ω 0.1 µF 0.1 µF 6.8 µF + 0.1 µF |
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