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OPA2673 Datasheet(PDF) 28 Page - Texas Instruments |
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OPA2673 Datasheet(HTML) 28 Page - Texas Instruments |
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28 / 40 page  Noise Performance Distortion Performance E = O E +(I R ) +4kTR NI BN S S +(I R ) +4kTR NG BI F F 2 2 2 4kT R G R G R F R S 1/2 OPA2673 I BI E O I BN 4kT=1.6E 20J - at290 K ° E RS E NI Ö4kTR S Ö4kTR F OPA2673 SBOS382 – JUNE 2008...................................................................................................................................................................................................... www.ti.com loads greater than 2pF can begin to degrade the performance of the OPA2673. Long PCB traces, Wideband current-feedback op amps generally have unmatched cables, and connections to multiple a higher output noise than comparable devices can easily cause this value to be exceeded. voltage-feedback op amps. The OPA2673 offers an Always consider this effect carefully, and add the excellent balance between voltage and current noise recommended series resistor as close as possible to terms to achieve low output noise. The inverting the OPA2673 output pin (see the Board Layout current noise (35pA/ √Hz) is lower than earlier Guidelines section). solutions, whereas the input voltage noise (2.4nV/ √Hz) is lower than most unity-gain stable, wideband voltage-feedback op amps. This low input voltage noise is achieved at the price of higher The OPA2673 provides good distortion performance noninverting input current noise (5.2pA/ √Hz). As long into a 100 Ω load on ±6V supplies. Generally, until the as the ac source impedance from the noninverting fundamental signal reaches very high frequency or node is less than 100 Ω, this current noise does not power levels, the second harmonic dominates the contribute significantly to the total output noise. The distortion with a negligible third harmonic component. op amp input voltage noise and the two input current Focusing then on the second harmonic, increasing noise terms combine to give low output noise under a the load impedance improves distortion directly. wide variety of operating conditions. Figure 84 shows Remember that the total load includes the feedback the op amp noise analysis model with all noise terms network—in the noninverting configuration (see included. In this model, all noise terms are taken to Figure 76), this network is the sum of RF + RG; in the be noise voltage or current density terms in either inverting configuration, it is RF. Also, providing an nV/ √Hz or pA/√Hz. additional supply decoupling capacitor (0.01 µF) between the supply pins (for bipolar operation) The total output spot noise voltage can be computed improves the second-order distortion slightly (3dB to as the square root of the sum of all squared output 6dB). noise voltage contributors. Equation 16 shows the general form for the output noise voltage using the In most op amps, increasing the output voltage swing terms given in Figure 84. directly increases harmonic distortion. The Typical Characteristics show the second harmonic increasing at a little less than the expected 2x rate, whereas the third harmonic increases at a little less than the (16) expected 3x rate. Where the test power doubles, the difference between it and the second harmonic decreases less than the expected 6dB, while the difference between it and the third harmonic decreases by less than the expected 12dB. This factor also shows up in the two-tone, third-order intermodulation spurious (IM3) response curves. The third-order spurious levels are extremely low at low-output power levels. The output stage continues to hold them low even as the fundamental power reaches very high levels. As the Typical Characteristics show, the spurious intermodulation powers do not increase as predicted by a traditional intercept model. As the fundamental power level increases, the dynamic range does not decrease significantly. For two tones centered at 40MHz, with 10dBm/tone into a matched 50 Ω load (that is, 2VPP Figure 84. Op Amp Noise Analysis Model for each tone at the load, which requires 8VPP for the overall two-tone envelope at the output pin), the Dividing this expression by the noise gain [NG = (1 + Typical Characteristics show 69dBc difference RF/RG)] gives the equivalent input-referred spot noise between the test-tone power and the third-order voltage at the noninverting input, as shown in intermodulation spurious levels. This exceptional Equation 17. performance improves further when operating at lower frequencies. 28 Submit Documentation Feedback Copyright © 2008, Texas Instruments Incorporated Product Folder Link(s): OPA2673 |
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