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Moteur de recherche de fiches techniques de composants électroniques |
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Moteur de recherche de fiches techniques de composants électroniques |
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OPA842 Fiches technique(PDF) 12 Page - Texas Instruments |
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OPA842 Fiches technique(HTML) 12 Page - Texas Instruments |
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12 / 19 page  OPA842 12 SBOS267A www.ti.com THREE OP AMP DIFFERENCING (Instrumentation Topology) The primary drawback of the single op amp differential amplifier is its relatively low input impedances. Where high impedance is required at the differential input, a standard instrumentation amplifier (INA) topology may be built using the OPA842 as the differencing stage. Figure 4 shows an example of this, in which the two input amplifiers are pack- aged together as a dual voltage-feedback op amp, the OPA2822. This approach saves board space, cost, and power compared to using two additional OPA842 devices, and still achieves very good noise and distortion perfor- mance due to the moderate loading on the input amplifiers. requires its outputs terminated to a compliance voltage other than ground for operation, then the appropriate voltage level may be applied to the noninverting input of the OPA842. FIGURE 4. Wideband 3-Op Amp Differencing Amplifier. OPA842 Power-supply decoupling not shown. V O V 1 R G 500 Ω V 2 OPA2822 +5V +5V –5V –5V OPA2822 500 Ω 500 Ω R F1 500 Ω 500 Ω 500 Ω R F1 500 Ω In this circuit, the common-mode gain to the output is always 1, due to the four matched 500 Ω resistors, whereas the differential gain is set by (1 + 2RF1/RG), which is equal to 2 using the values in Figure 4. The differential to single-ended conversion is still performed by the OPA842 output stage. The high-impedance inputs allow the V1 and V2 sources to be terminated or impedance matched as required. If the V1 and V2 inputs are already truly differential, such as the output from a signal transformer, then a single matching termination resistor may be used between them. Remember, however, that a defined DC signal path must always exist for the V1 and V2 inputs; for the transformer case, a center-tapped secondary connected to ground would provide an optimum DC operating point. DAC TRANSIMPEDANCE AMPLIFIER High-frequency Digital-to-Analog Converters (DACs) require a low-distortion output amplifier to retain their SFDR perfor- mance into real-world loads. A single-ended output drive implementation is shown in Figure 5. In this circuit, only one side of the complementary output drive signal is used. The diagram shows the signal output current connected into the virtual ground-summing junction of the OPA842, which is set up as a transimpedance stage or “I-V converter.” The unused current output of the DAC is connected to ground. If the DAC FIGURE 5. Wideband Low-Distortion DAC Transimpedance Amplifier. OPA842 High-Speed DAC V O = ID RF R F C F GBP → Gain Bandwidth Product (Hz) for the OPA842 C D I D I D The DC gain for this circuit is equal to RF. At high frequen- cies, the DAC output capacitance will produce a zero in the noise gain for the OPA842 that may cause peaking in the closed-loop frequency response. CF is added across RF to compensate for this noise-gain peaking. To achieve a flat transimpedance frequency response, this pole in the feed- back network should be set to: 1 24 ππ RC GBP RC FF FD = (1) which will give a corner frequency f–3dB of approximately: f GBP RC dB FD − = 3 2 π (2) ACTIVE FILTERS Most active filter topologies will have exceptional performance using the broad bandwidth and unity-gain stability of the OPA842. Topologies employing capacitive feedback require a unity-gain stable, voltage-feedback op amp. Sallen-Key filters simply use the op amp as a noninverting gain stage inside an RC network. Either current- or voltage-feedback op amps may be used in Sallen-Key implementations. See Figure 6 for an example Sallen-Key low-pass filter, in which the OPA842 is set up to deliver a low-frequency gain of +2. The filter component values have been selected to achieve a maximally flat Butterworth response with a 5MHz, –3dB bandwidth. The resistor values have been slightly adjusted to compensate for the effects of the 150MHz bandwidth provided by the OPA842 in this configuration. This filter may be com- bined with the ADC driver suggestions to provide moderate (2- pole) Nyquist filtering, limiting noise, and out-of-band harmon- ics into the input of an ADC. This filter will deliver the exceptionally low harmonic distortion required by high SFDR ADCs such as the ADS850 (14-bit, 10MSPS, 82dB SFDR). |
Numéro de pièce similaire - OPA842 |
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Description similaire - OPA842 |
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