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OPA687 Datasheet(PDF) 8 Page - Texas Instruments |
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OPA687 Datasheet(HTML) 8 Page - Texas Instruments |
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8 / 17 page  8 OPA687 SBOS065A APPLICATIONS INFORMATION WIDEBAND, NON-INVERTING OPERATION The OPA687 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 perfor- mance of the OPA687, careful attention to PC board layout and component selection is required as discussed in the remaining sections of this data sheet. Figure 1 shows the non-inverting gain of +20 circuit used as the basis for most of the Typical Performance Curves. Most of the curves were characterized using signal sources with 50 Ω driving impedance, and with measurement equipment presenting a 50 Ω load impedance. In Figure 1, the 50Ω shunt resistor at the VI terminal matches the source imped- ance of the test generator, while the 50 Ω series resistor at the VO terminal provides a matching resistor for the measure- ment equipment load. Generally, data sheet voltage swing specifications 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 OPA687 with an effective 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 will guarantee that the noise added at the output due to Johnson noise of the resistors will not signifi- cantly increase the total over that due to the 0.95nV/ √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. OPA687 +5V –5V –V S +V S 50 Ω V O 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. Non-Inverting G = +20 Specifications and Test Circuit. WIDEBAND, INVERTING GAIN OPERATION There can be significant benefits to operating the OPA687 as an inverting amplifier. This is particularly true when a matched input impedance is required. Figure 2 shows the inverting gain circuit used as a starting point for the Typical Performance Curves showing inverting mode performance. OPA687 +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 0.1 µF R F 2k Ω R G 50 Ω 50 Ω Source 50 Ω Load Driving this circuit from a 50 Ω source, and constraining the gain resistor, RG, to equal 50Ω, will give both a signal bandwidth and noise advantage. RG, in this case, is acting 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 equal when the 50 Ω source resistor is included. This has the interesting effect of doubling the equivalent GBP for the amplifier. This can be seen in comparing the G = +12 and G = –20 small-signal frequency response curves. Both show approximately 500MHz bandwidth with 3dB peaking, but the inverting configuration of Figure 2 is giving 4.4dB higher signal gain. The noise gains are ap- proximately equal in this case. 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 of that amplifier and RF adjusted to get the desired gain. It is critical for stable operation of the OPA687 that this driving amplifier show a very low output impedance through frequencies exceeding the expected closed-loop bandwidth for the OPA687. WIDEBAND, HIGH SENSITIVITY, TRANSIMPEDANCE DESIGN The high Gain Bandwidth Product (GBP) and low input voltage and current noise for the OPA687 make it an ideal wideband transimpedance amplifier for low to moderate transimpedance gains. Very high transimpedance gains (> 100k Ω) will benefit from the low input noise current of a FET-input op amp such as the OPA655. Unity gain stability in the op amp is NOT FIGURE 2. Inverting G = –40 Specifications and Test Circuit. |
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