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OPA65250 Datasheet(PDF) 10 Page - Texas Instruments |
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OPA65250 Datasheet(HTML) 10 Page - Texas Instruments |
10 / 25 page OPA656 10 SBOS196G www.ti.com APPLICATIONS INFORMATION WIDEBAND, NONINVERTING OPERATION The OPA656 provides a unique combination of a broadband, unity gain stable, voltage-feedback amplifier with the DC precision of a trimmed JFET-input stage. Its very high Gain Bandwidth Product (GBP) of 230MHz can be used to either deliver high signal bandwidths for low-gain buffers, or to deliver broadband, low-noise transimpedance bandwidth to photodiode-detector applications. To achieve the full perfor- mance of the OPA656, careful attention to printed circuit board (PCB) layout and component selection is required as discussed in the remaining sections of this data sheet. Figure 1 shows the noninverting gain of +2 circuit used as the basis for most of the Typical Characteristics. Most of the curves were characterized using signal sources with 50 Ω driving im- pedance, and with measurement equipment presenting a 50 Ω load impedance. In Figure 1, the 50 Ω shunt resistor at the V I 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. Gener- ally, 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 100Ω load at the output combined with the 500 Ω total feedback network load, presents the OPA656 with an effective output load of 83 Ω for the circuit of Figure 1. WIDEBAND, INVERTING GAIN OPERATION The circuit of Figure 2 shows the inverting gain of –1 test circuit used for most of the inverting Typical Characteristics. In this case, an additional resistor RM is used to achieve the 50 Ω input impedance required by the test equipment using in characterization. This input impedance matching is optional in a circuit board environment where the OPA656 is used as an inverting amplifier at the output of a prior stage. FIGURE 1. Noninverting G = +2 Specifications and Test Circuit. Voltage-feedback op amps, unlike current feedback prod- ucts, can use a wide range of resistor values to set their gain. To retain a controlled frequency response for the noninverting voltage amplifier of Figure 1, the parallel combination of RF || RG should always < 200Ω. In the noninverting configu- ration, the parallel combination of RF || RG will form a pole with the parasitic input capacitance at the inverting node of the OPA656 (including layout parasitics). For best perfor- mance, this pole should be at a frequency greater than the closed loop bandwidth for the OPA656. For this reason, a direct short from output to inverting input is recommended for the unity gain follower application. In this configuration, the output sees the feedback resistor as an additional load in parallel with the 100 Ω load used for test. It is often useful to increase the RF value to decrease the loading on the output (improving harmonic distortion) with the constraint that the parallel combination of RF || RG < 200Ω. For higher inverting gains with the DC precision provided by the FET input OPA656, consider the higher gain bandwidth product OPA657. Figure 2 also shows the noninverting input tied directly to ground. Often, a bias current canceling resistor to ground is included here to null out the DC errors caused by input bias current effects. This is only useful when the input bias currents are matched. For a JFET part like the OPA656, the input bias currents do not match but are so low to begin with (< 5pA) that DC errors due to input bias currents are negligible. Hence, no resistor is recommended at the noninverting inputs for the inverting signal path condition. WIDEBAND, HIGH SENSITIVITY, TRANSIMPEDANCE DESIGN The high GBP and low input voltage and current noise for the OPA656 make it an ideal wideband transimpedance ampli- fier for low to moderate transimpedance gains. Higher transimpedance gains (> 100k Ω) will benefit from the low input noise current of a FET input op amp such as the OPA656. One transimpedance design example is shown on the front page of the data sheet. Designs that require high bandwidth from a large area detector will benefit from the low input voltage noise for the OPA656. This input voltage noise FIGURE 2. Inverting G = –1 Specifications and Test Circuit. OPA656 +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 250 Ω R F 250 Ω 50 Ω Source 50 Ω Load 0.1 µF OPA656 +5V –5V +V S –V S R M 57.6 Ω 50 Ω V O V I + 6.8 µF 0.1 µF + 6.8 µF 0.1 µF R F 402 Ω R G 402 Ω 50 Ω Source 50 Ω Load |
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