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LOG114EVM: Log ratio amplification and bandwidth vs current

Sean O'Byrne
Prodigy 10 points
Part Number: LOG114EVM
Other Parts Discussed in Thread: LOG114,

On the front page of the log114 datasheet is a schematic for a log-ratio amplifier using current from two photodiodes.  This is the circuit I would like to test.  However, there seems to be only 1 input and a fixed reference current in the manual for the log114EVM evaluation module.  Is there any way of testing a log-ratio amplification circuit on this board?

Also, the log102 data sheet shows bandwidth for a range of input currents, but the log114 data sheet  has only a gain bandwidth product of 50 MHz at 1 uA.  Is there any information about the bandwidth of the log amplifier for different currents? Thanks.

  • 0
    Prodigy 10 points

    OK, for the second question I was just looking at the wrong part of the data sheet... sorry for the dumb question.  But I'd still like to know whether there was some way to make this module work for a log-ratio amplification measurement.

    Sean

  • +1 in reply to Sean O'Byrne
    TI__Guru* 93105 points

    Hi Sean,

    Thanks for letting me know you figured out the bandwidth question. 

    The LOG114 EVM was designed for a single +5 V, single current input application such as photodiode. It was developed to emphasize the log conformity improvement had when the inputs are high such as when in the 1 to 10 mA range so some significant layout area devoted to that. When I review the PC board layout it really doesn't lend itself well to adding another photodiode input. It would appear that if log ratio is important that the board would need to be optimized for equivalent physical placement of two identical photodiodes. I just don't see that happening with the EVM layout. 

    Regards, Thomas

    Precision Amplifiers Applications Engineering