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LOG114EVM: Bandwidth considerations

Brett Klehn
Prodigy 10 points
Part Number: LOG114EVM
Other Parts Discussed in Thread: LOG114

I am running a photodiode where the minimum signal is around 1nA.  When I sweep my signal slowly, the response is perfect, very accurate and low noise.  When I speed things up, the rising edge looks OK but the falling edge is really slow.  I know this is typical of log amps, but are there any passive components to change out to get better bandwidth at low currents?  If I make my own PCB are there better ways to get more bandwidth or is the LOG114EVM kit already optimized and is the best I can get?

  • 0
    TI__Guru* 93105 points

    Hi Brett,

    Operating the LOG114 at very low input current levels produces the least bandwidth and slowest response times. This is evident from the SMALL−SIGNAL AC RESPONSE graphs in the datasheet Typical Characteristics section.

    The speed of the internal circuits of the log amplifier portion of the LOG114 are fixed and for the most part are not accessible. They have been optimized for precision and speed. Therefore, any possible speed enhancement would have to come about from a change in external conditions. Since a photodiode current change is assumed to be nonnegotiable possibly something might be tried to reduce its junction capacitance. Applying a reverse bias voltage to the cathode of a photodiode reduces the junction capacitance, but I have not tried this with a LOG114 circuit. There may be some speed gain if that could be accommodated in the design.

    Regards, Thomas

    Precision Amplifiers Applications Engineering