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LOG114

Teodor Neagoe
Intellectual 260 points
Other Parts Discussed in Thread: LOG114

I am looking for answers and suggestions on TI’s LOG114 (Log Amp). LOG114 provides best results when the input is current (for example: photodiode output). However, for my application, we have to use it with voltage source. We would like to monitor the photodiode’s output (pulse) in two ways:

-one goes to LOG114 and its output goes to the DSP for monitoring the integrated power level;

-another one goes to  a connector so that the user can connect it to a scope to monitor the pulse output (50MHz Bandwidth required).

I am thinking of converting the current output of the photodiode to voltage (via a transimpedance amp), then split in two ways, one to the LOG114 and another one to another op-amp (low noise, precision op-amp) that will connect to the the external connector.

Do you have any advise on this approach? Are there any application notes about LOG114 used with voltage input? Are there any better parts to recommend for this application?

Teodor

  • 0
    Prodigy 60 points

    What is the power range expected for each output?  You could convert to voltage, then source the voltage to the log amp with a high value resistor (1Meg), but then the LOG amp error at low currents may increase due to voltage offsets.  Possibly you could calibrate and compensate in the DSP.

  • 0
    Genius 5430 points

    Teodor;

    The LOG114 must be used with an input from a current source; this is true for all log amps that use diode feedback. The LOG114 has nowhere near 50MHz bandwidth but you could low-pass filter the input to get an output representing the average input current.

  • 0 in reply to Neil Albaugh
    Prodigy 60 points

    What is the issue if fed through a high value resistor?  The datasheet suggests using 2.5Meg from the reference voltage as a reference current.  A photodiode has high shunt resistance, but some can be as low as 1Meg.  One issue I see is that the +/- 4mV offset will create an error in the dark current range.