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OPA691IDBVRG4 Datasheet(PDF) 11 Page - Texas Instruments |
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OPA691IDBVRG4 Datasheet(HTML) 11 Page - Texas Instruments |
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11 / 31 page  OPA691 11 SBOS226D www.ti.com APPLICATIONS INFORMATION WIDEBAND CURRENT FEEDBACK OPERATION The OPA691 gives the exceptional AC performance of a wideband current feedback op amp with a highly linear, high power output stage. Requiring only 5.1mA quiescent current, the OPA691 will swing to within 1V of either supply rail and deliver in excess of 160mA at room temperature. This low output headroom requirement, along with supply voltage independent biasing, gives remarkable single (+5V) supply operation. The OPA691 will deliver greater than 200MHz bandwidth driving a 2VPP output into 100Ω on a single +5V supply. Previous boosted output stage amplifiers have typi- cally suffered from very poor crossover distortion as the output current goes through zero. The OPA691 achieves a comparable power gain with much better linearity. The pri- mary 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 at low gains, with improved DC accuracy, consider the high slew rate, unity-gain stable, voltage feed- back OPA690. Figure 1 shows the DC-coupled, gain of +2, dual power- supply circuit configuration used as the basis of the ±5V Electrical Characteristic tables and Typical Characteristic 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 specifications 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 to ensure 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 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 Characteristic tables and Typical Characteristic curves. Though not a “rail-to-rail” design, the OPA691 requires mini- mal input and output voltage headroom compared to other very wideband current feedback op amps. It will deliver a 3VPP 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 2VPP 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 combi- nation 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 ad- justed 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 120mA output current. A demanding 100 Ω load to a mid-point bias is used in this characterization circuit. The new output stage used in the OPA691 can deliver large bipolar output currents into this mid-point 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. OPA691 +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 + 6.8 µF 0.1 µF 6.8 µF 0.1 µF 0.1 µF OPA691 +5V DIS V S/2 806 Ω 100 Ω V O V I +V S 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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