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OPA842IDBVT Datasheet(PDF) 10 Page - Texas Instruments |
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OPA842IDBVT Datasheet(HTML) 10 Page - Texas Instruments |
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10 / 19 page  OPA842 10 SBOS267A www.ti.com APPLICATIONS INFORMATION WIDEBAND VOLTAGE-FEEDBACK OPERATION The OPA842’s combination of speed and dynamic range is easily achieved in a wide variety of application circuits, providing that simple principles of good design practice are observed. For example, good power-supply decoupling, as shown in Figure 1, is essential to achieve the lowest possible harmonic distortion and smooth frequency response. Proper PC board layout and careful component selection will maximize the performance of the OPA842 in all applications, as discussed in the following sections of this data sheet. Figure 1 shows the gain of +2 configuration used as the basis for most of the typical characteristics. Most of the curves were characterized using signal sources with 50 Ω driving impedance and with measurement equipment presenting 50 Ω load impedance. In Figure 1, the 50Ω shunt resistor at the VI terminal matches the source impedance of the test generator while the 50 Ω series resistor at the V O terminal provides a matching resistor for the measurement equipment load. Generally, data sheet specifications refer to the voltage swing at the output pin (VO in Figure 1). The 100Ω load, combined with the 804 Ω total feedback network load, pre- sents the OPA842 with an effective load of approximately 90 Ω in Figure 1. In the inverting case, just the feedback resistor appears as part of the total output load in parallel with the actual load. For the 100 Ω load used in the typical characteristics, this gives a total load of 80 Ω in this inverting configuration. The gain resistor is set to get the desired gain (in this case 200 Ω for a gain of –2) while an additional input matching resistor (RM) can be used to set the total input impedance equal to the source if desired. In this case, RM = 66.5Ω in parallel with the 200 Ω gain setting resistor gives a matched input imped- ance of 50 Ω. This matching is only needed when the input needs to be matched to a source impedance, as in the characterization testing done using the circuit of Figure 2. The OPA842 offers extremely good DC accuracy as well as low noise and distortion. To take full advantage of that DC precision, the total DC impedance looking out of each of the input nodes must be matched to get bias current cancella- tion. For the circuit of Figure 2, this requires the 147 Ω resistor shown to ground on the noninverting input. The calculation for this resistor includes a DC-coupled 50 Ω source imped- ance along with RG and RM. Although this resistor will provide cancellation for the bias current, it must be well decoupled (0.1 µF in Figure 2) to filter the noise contribution of the resistor and the input current noise. As the required RG resistor approaches 50Ω at higher gains, the bandwidth for the circuit in Figure 2 will far exceed the bandwidth at that same gain magnitude for the noninverting circuit of Figure 1. This occurs due to the lower “noise gain” for the circuit of Figure 2 when the 50 Ω source impedance is included in the analysis. For instance, at a signal gain of –8 (RG = 50Ω, RM = open, RF = 402Ω) the noise gain for the circuit of Figure 2 will be 1 + 402 Ω/(50Ω + 50Ω) = 5 due to the addition of the 50 Ω source in the noise gain equation. This gives considerable higher bandwidth than the noninverting gain of +8. Using the 200MHz gain bandwidth product for the OPA842, an inverting gain of –8 from a 50 Ω source to a 50 Ω R G will give approximately 40MHz band- width, whereas the noninverting gain of +8 will give 25MHz. FIGURE 1. Gain of +2. High-frequency application and characterization circuit. OPA842 +5V –5V –V S +V S R S 50 Ω V O V IN 50 Ω + 2.2 µF + 2.2 µF 0.1 µF R G 402 Ω R F 402 Ω 50 Ω Source 50 Ω Load 0.1 µF WIDEBAND INVERTING OPERATION Operating the OPA842 as an inverting amplifier has several benefits and is particularly useful when a matched 50 Ω source and input impedance is required. Figure 2 shows the inverting gain of –2 circuit used as the basis of the inverting mode typical characteristics. FIGURE 2. Inverting G = –2 Specifications and Test Circuit. OPA843 +5V –5V 50 Ω V O V I + 0.1 µF 2.2 µF + 0.1 µF 2.2 µF R M 66.5 Ω R T 147 Ω R F 402 Ω 50 Ω Source 50 Ω Load 0.1 µF R G 200 Ω |
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