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OPA847IDBVR Datasheet(PDF) 10 Page - Texas Instruments |
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OPA847IDBVR Datasheet(HTML) 10 Page - Texas Instruments |
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10 / 30 page  OPA847 10 SBOS251E www.ti.com APPLICATIONS INFORMATION WIDEBAND, NONINVERTING OPERATION The OPA847 provides a unique combination of a very low input voltage noise along with a very low distortion output stage to give one of the highest dynamic range op amps available. Its very high gain bandwidth product (GBP) can be used to either deliver high signal bandwidths at high gains, or to deliver very low distortion signals at moderate frequencies and lower gains. To achieve the full performance of the OPA847, careful attention to PC board layout and compo- nent selection is required, as discussed in the following sections of this data sheet. Figure 1 shows the noninverting gain of a +20V/V circuit used as the basis for most of the Typical Characteristics. Most of the curves are characterized using signal sources with a 50 Ω driving impedance and with measurement equipment pre- senting a 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 voltage swing speci- fications are at the output pin (VO in Figure 1) while output power specifications are at the matched 50 Ω load. The total 100 Ω load at the output combined with the 790Ω total feedback network load presents the OPA847 with an effec- tive output load of 89 Ω for the circuit of Figure 1. Voltage-feedback op amps, unlike current-feedback designs, can use a wide range of resistor values to set their gain. The circuit of Figure 1, and the specifications at other gains, use an RG set to 39.2Ω and RF adjusted to get the desired gain. Using this guideline ensures that the noise added at the output due to the Johnson noise of the resistors does not significantly increase the total over that due to the 0.85nV/ √Hz input voltage noise for the op amp itself. This RG is suggested as a good starting point for design. Other values are certainly acceptable, if required by the design. WIDEBAND, INVERTING GAIN OPERATION There can be significant benefits to operating the OPA847 as an inverting amplifier. This is particularly true when a matched input impedance is required. Figure 2 shows the inverting gain of a –40V/V circuit used as a starting point for the Typical Characteristics showing inverting mode performance. Driving this circuit from a 50 Ω source, and constraining the gain resistor (RG) to equal 50Ω, gives both a signal bandwidth and a noise advantage. RG, in this case, acts as both the input termination resistor and the gain setting resistor for the circuit. Although the signal gain for the circuit of Figure 2 is double that for Figure 1, their noise gains are nearly equal when the 50 Ω source resistor is included. This has the interesting effect of approximately doubling the equivalent GBP for the amplifier. This can be seen by observing that the gain of –40 bandwidth of 240MHz shown in the Typical Characteristics implies a gain bandwidth product of 9.6GHz, giving a far higher bandwidth at a gain of –40 than at a gain of +40. While the signal gain from RG to the output is –40, the noise gain for bandwidth setting purposes is 1 + RF/(2 • RG). In the case of a –40V/V gain, using an RG = RS = 50Ω gives a noise gain = 1 + 2kΩ/100Ω = 21. This inverting gain of –40V/V therefore has a frequency response that more closely matches the gain of a +20 frequency re- sponse. If the signal source is actually the low impedance output of another amplifier, RG should be increased to be greater than the minimum value allowed at the output for that amplifier and RF adjusted to get the desired gain. It is critical for stable operation of the OPA847 that this driving amplifier show a very low output impedance through frequencies exceeding the expected closed-loop bandwidth for the OPA847. OPA847 +5V –5V –V S +V S 50 Ω V O V DIS V I 50 Ω + 0.1 µF + 6.8 µF 6.8 µF R G 39.2 Ω R F 750 Ω 50 Ω Source 50 Ω Load 0.1 µF FIGURE 1. Noninverting G = +20 Specification and Test Circuit. FIGURE 2. Noninverting G = –40 Specification and Test Circuit. OPA847 +5V –5V +V S –V S 95.3 Ω 50 Ω V O V I + 6.8 µF 0.1 µF + 6.8 µF 0.1 µF 0.01 µF R F 2k Ω R G 50 Ω 50 Ω Source 50 Ω Load V DIS |
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