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OPA642 Datasheet(PDF) 10 Page - Texas Instruments |
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OPA642 Datasheet(HTML) 10 Page - Texas Instruments |
10 / 16 page 10 ® OPA642 DAC TRANSIMPEDANCE AMPLIFIER High frequency DDC DACs require a low distortion output amplifier to retain their SFDR performance into real-world loads. A single-ended output drive implementation is shown in Figure 4. In this circuit, only one side of the complemen- tary output drive signal is used. The diagram shows the signal output current connected into the virtual ground summing junction of the OPA642, which is set up as a transimpedance stage or “I-V converter”. The unused cur- rent output of the DAC is connected to ground. If the DAC requires its outputs terminated to a compliance voltage other than ground for operation, then the appropriate voltage level may be applied to the non-inverting input of the OPA642. 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 OPA642 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: which will give a corner frequency ƒ-3dB of approximately: Figure 5 shows an example Sallen-Key low pass filter, in which the OPA642 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 band- width provided by the OPA642 in this configuration. This filter may be combined with the ADC driver suggestions to provide moderate (2-pole) Nyquist filtering, limiting noise and out of band components into the input of an ADC. This filter will deliver the exceptionally low harmonic distortion required by high SFDR A/D converters such as the ADS804 (12-bit, 10MSPS, 80dB SFDR). OPA642 High Speed DAC V O = IO RF R F C F GBP → Gain Bandwidth Product for the OPA642 C D I O I O FIGURE 4. Wideband, Low Distortion DAC Transimpedance Amplifier. ACTIVE FILTERS Most active filter topologies will deliver exceptional perfor- mance using the broad bandwidth and unity gain stability of the OPA642. Topologies employing capacitive feedback require a unity gain stable voltage feedback op amp. Sallen- Key filters simply use the op amp as a non-inverting gain stage inside an RC network. Either current or voltage feed- back op amps may be used in Sallen-Key implementations. FIGURE 5. 5MHz Butterworth Low Pass Active Filter. V O 402 Ω 505 Ω V I 124 Ω OPA642 100pF 402 Ω 150pF OPERATING SUGGESTIONS OPTIMIZING RESISTOR VALUES Since the OPA642 is a unity gain stable voltage feedback op amp, a wide range of resistor values may be used for the feedback and gain setting resistors. The primary limits on these values are set by dynamic range (noise and distortion) and parasitic capacitance considerations. For a non-inverting unity gain follower application, the feedback connection should be made with a 25 Ω resistor—not a direct short. This will isolate the inverting input capacitance from the output pin and improve the frequency response flatness. Usually, the feedback resistor value should be between 200 Ω and 1k Ω. Below 200Ω, the feedback network will present additional output loading which can degrade the harmonic distortion performance of the OPA642. Above 1k Ω, the typical parasitic capacitance (approximately 0.2pF) across the feedback resistor may cause unintentional band-limiting in the amplifier response. A good rule of thumb is to target the parallel combination of RF and RG (Figure 1) to be less than about 200Ω. The combined impedance RF || RG interacts with the inverting input capacitance, placing an additional pole in the feedback network and thus a zero in the forward response. Assuming a 2pF total parasitic on the inverting node, holding RF || RG < 200 Ω will keep this pole above 400MHz. By itself, this constraint implies that the feedback resistor RF can increase to several k Ω at high gains. This is acceptable as long as the pole formed by RF and any parasitic capacitance appearing in parallel is kept out of the frequency range of interest. ƒ –3dB = GBP / 2 πR FCD 1/ 2 πR FCF = GBP / 4 πR FCD () |
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