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ONET2591TA: How do you calculate the change in bandwidth based on changes to input capacitance for this TIA?

Part Number: ONET2591TA
Other Parts Discussed in Thread: OPA657, , OPA843, OPA846, OPA847

 I found an application note, sboa122.pdf that discusses how to calculate the maximum achievable bandwidth for TIAs.  It uses the OPA847, OPA846, OPA843 and OPA657 as examples.  Do these same equations apply to the ONET2591TA.   I have been told by our resident guru that they do not.   If not, are there tables or calculations available which provide the answer? 

  • Hi,

    Your guru is correct. The first four parts you listed are op-amps that can be configured in transimpedance circuits and therefore have equations governing the bandwidth based on the amplifiers parameters. The ONET2591TA is a much more specialized device and uses open loop amplifier architectures internally that do not follow the same rules as normal op-amps. It's close to a RF LNA than it is to a standard op-amp. The bandwidth for that device is fixed and the output is also a limiting output because it is generally designed to transmit digital NRZ signals.

    Regards,
  • Thanks for the quick feedback.

    The datasheet specifies 50 uApp as the input small-signal test condition. At 50 uApp, the output swing is 0.13 Vpp, which is prior to limiting. Figure 7 in the datasheet shows the small-signal transfer function.

    What I need to understand now is what happens to the small signal transfer function as the capacitance increases above the 0.6pF specified in the test conditions?

  • Hi,

    You are correct that the part does amplify linearly until it hits the limiting voltage at the output. The input capacitance wont effect the transimpedance gain, but has more of a risk to cause the part to be unstable. Unfortunately we never characterized these devices with higher capacitance input's because they are generally used with very low capacitance diodes.

    Regards,
  • Is there a way to predict or simulate that behavior or unstability at higher capacitance.  Specifically with a balanced photodiode configuration at 2pF.  My design requires a bandwidth of 600Mhz.