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OPA846IDBVRG4 Datasheet(PDF) 10 Page - Texas Instruments |
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OPA846IDBVRG4 Datasheet(HTML) 10 Page - Texas Instruments |
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10 / 29 page  OPA846 10 SBOS250E www.ti.com APPLICATIONS INFORMATION WIDEBAND, NONINVERTING OPERATION The OPA846 provides a unique combination of features. Low input voltage noise, along with a very low distortion output stage, gives one of the highest dynamic range op amps available. The very high Gain Bandwidth Product (GBP) can be used either to deliver high signal bandwidths at high gain, or to deliver very low distortion signals at moderate frequen- cies and lower gains. To achieve the full performance of the OPA846, 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 10V/V circuit used as the basis of the Electrical Characteristics and most of the Typical Characteristic curves. Most of the curves are charac- terized using signal sources with a 50 Ω driving impedance, and with a 50 Ω load impedance presented by the measure- ment equipment. In Figure 1, the 50 Ω resistor at the V IN terminal matches the source impedance of the test genera- tor, while the 50 Ω series resistor at the V O terminal provides a matching resistor for the measurement equipment load. Generally, the data sheet voltage swing specifications are at the output pin (VO in Figure 1), while the output power (dBm) specifications are at the matched 50 Ω load. The total 100Ω load at the output, combined with the 503 Ω total feedback network load, presents the OPA846 with an effective output load of 83 Ω for the circuit of Figure 1. guideline ensures that the noise added at the output due to the Johnson noise of the resistors does not significantly increase the total noise over that due to the 1.2nV/ √Hz input voltage noise for the op amp. Higher resistor values can certainly be used where the application requires it, but can start to add significantly to the output noise power as de- scribed in the Setting Resistor Values to Minimize Noise section. WIDEBAND INVERTING GAIN OPERATION Operating the OPA846 as an inverting amplifier has several benefits and is particularly appropriate when a matched input impedance is required. Figure 2 shows the inverting gain circuit used as the basis of the inverting mode of the Typical Characteristic curves. OPA846 +5V –5V V EE +V CC R S 50 Ω V O V I 50 Ω + 0.1 µF + 6.8 µF 6.8 µF R G 50 Ω R F 453 Ω 50 Ω Source 50 Ω Load 0.1 µF FIGURE 1. DC-Coupled, G = +10V/V, Bipolar Supply, Speci- fication and Test Circuit. Voltage-feedback op amps (unlike current-feedback designs) can use a wide range of resistor values to set the gain, although these resistors usually have low values to maintain a low total output noise. The circuit of Figure 1, and the specifications at other gains, use the constraint that RG be set to 50 Ω and R F adjusted to get the desired gain. Using this FIGURE 2. DC-Coupled, G = –20V/V, Bipolar Supply, Speci- fication and Test Circuit. OPA846 +5V –5V V CC V EE 91 Ω R S 50 Ω V O V I + 6.8 µF 0.1 µF + 6.8 µF 0.1 µF 0.1 µF R F 1k 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, gives both a signal band- width and noise advantage. RG acts as both the input termination resistor and the gain setting resistor for the circuit. Although the signal gain (VO/VI) for the circuit of Figure 2 is double that for Figure 1, the noise gains are in fact equal when the 50 Ω source resistor is included. This has the interesting effect of doubling the equivalent GBP of the amplifier. This can be seen by observing the 200MHz band- width for the inverting gain of –20. This implies a GBP of 4GHz, when in fact this extended bandwidth is given by the reduced noise gain when the matched source resistor is included. If the signal source is actually the low impedance output of another amplifier, RG is increased to the minimum load resistance value allowed for that amplifier and RF is then adjusted to achieve the desired gain. For stable operation of the OPA846, it is critical that this driving amplifier show very low output impedance at frequencies beyond the expected closed-loop bandwidth for the OPA846. |
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