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OPA687N Datasheet(PDF) 9 Page - Burr-Brown (TI) |
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OPA687N Datasheet(HTML) 9 Page - Burr-Brown (TI) |
9 / 16 page 9 ® OPA687 required for application as a transimpedance amplifier. Fig- ure 3 shows one possible transimpedance design example that would be particularly suitable for the 155Mbit data rate of an OC-3 receiver. Designs that require high bandwidth from a large area detector with relatively low transimpedance gain will benefit from the low input voltage noise for the OPA687. The amplifier’s input voltage noise is peaked up, at the output, over frequency by the diode source capaci- tance and can, in many cases, become the dominant output noise contribution. The key elements to the design are the expected diode capacitance (CD) with the reverse bias volt- age (–VB) applied, the desired transimpedance gain, RF, and the GBP for the OPA687 (3600MHz). With these three variables set (and including the parasitic input capacitance for the OPA687 added to CD), the feedback capacitor value (CF) may be set to control the frequency response. FIGURE 3. Wideband, High Sensitivity, OC-3 Transimpedance Amplifier. To achieve a maximally flat 2nd-order Butterworth fre- quency response, the feedback pole should be set to: 1/(2 πR FCF) = √(GBP/(4πRFCD)) Adding the common-mode and differential-mode input ca- pacitance (1.2 + 2.5)pF to the 1pF diode source capacitance of Figure 3 (CD), and targeting a 12kΩ transimpedance gain using the 3600MHz GBP for the OPA687, will require a feedback pole set to 71MHz to get a maximum bandwidth design. This will require a total feedback capacitance of 0.16pF. Using this maximum bandwidth, maximally flat frequency response target will give an approximate –3dB bandwidth set by: f–3dB = √(GBP/2πRFCD)Hz The example of Figure 3 will give approximately 100MHz flat bandwidth using the 0.16pF feedback compensation capacitor. This bandwidth will easily support an OC-3 re- ceiver with exceptional sensitivity. If the total output noise is bandlimited to a frequency less than the feedback pole frequency, a very simple expression for the equivalent input noise current can be derived as: Where: iEQ = Equivalent input noise current if the output noise is bandlimited to f < 1/(2 πR FCD) iN = Input current noise for the op amp inverting input eN = Input voltage noise for the op amp CD = Total Inverting Node Capacitance f = Bandlimiting frequency in Hz (usually a post filter prior to further signal processing) Evaluating this expression up to the feedback pole frequency at 71MHz for the circuit of Figure 3, gives an equivalent input noise current of 3.0pA/ √Hz. This is somewhat higher than the 2.5pA/ √Hz for just the op amp itself. This total equivalent input current noise is being slightly increased by the last term in the equivalent input noise expression. It is essential in this case to use a low voltage noise op amp. For example, if a slightly higher input noise voltage, but other- wise identical, op amp were used instead of the OPA687 in this application (say 2.0nV/ √Hz), the total input-referred current noise would increase to 4.0pA/ √Hz. Low input voltage noise is required for the best sensitivity in these wideband transimpedance applications. This is often un- specified for dedicated transimpedance amplifiers with a total output noise for a specified source capacitance given instead. It is the relatively high input voltage noise for those components that cause higher than expected output noise if the source capacitance is higher than expected. LOW GAIN COMPENSATION FOR IMPROVED SFDR A new external compensation technique may be used at low signal gains to retain the full slew rate and noise benefits of the OPA687, while maintaining the increased loop gain and the associated improvement in distortion offered by the decompensated architecture. This technique shapes the loop gain for good stability while giving an easily controlled second-order low pass frequency response. This technique was used for the circuit on the front page of the data sheet in a differential configuration to achieve extremely high SFDR through high frequencies. That circuit is set up for a differential gain of 8.5V/V from a differential input signal to the output. Using the transformer shown will improve the noise figure and translate from a single to a differential ii kT R e R eC f EQ N F N F ND =+ + + () 2 2 2 4 2 3 π R F 12k Ω 12k Ω 0.1 µF 100pF Supply Decoupling Not Shown λ OPA687 +5V –5V –V B C F 0.16pF 1pF Photodiode |
Similar Part No. - OPA687N |
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Similar Description - OPA687N |
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