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LOG114: Connecting a photodiode as the input current source.

Mike Nudelman
Intellectual 295 points

Part Number: LOG114

If connecting a photodiode as the current device, do I need to use the negative input current configuration as described in Figs. 7-9 in the datasheet? Meaning using additional parts?

Is there an appnote or sample sch for a single photodiode logarithmic TIA application?

  • 0
    Guru 140200 points

    Hi Morzh,

    figure 2 of datasheet shows how to proceed. When the photodiode is connected to the LOG114 with the shown polarity, no additional current converters are needed.

    Kai

  • 0
    TI__Guru* 93105 points
    Hi Morzh,

    The LOG114 has a unipolar inputs and current may only flow into the device inputs. The circuits shown in datasheet Figures 7 - 9 are only used when the current source's current direction must be reversed to conform with the LOG114 input current unidirectional requirement.

    The LOG114 may be configured to use a photodiode using the circuits shown in datasheet Figures 1 and 2, without needing the current reversing circuits of Figures 7 - 9. For Figure 1, the photodiode anode is connected to the LOG114 input Pin 3, in place of the current source shown in the figure. The diode's cathode is then connected to ground. Figure 2 already shows how to connect the photodiode when the LOG114 is powered by a single +5 V supply. These are single photodiode TIA applications having a voltage output that is a log function of the I1 to I2 current ratio, where I1 is the reference current and I2 is the photodiode input current.

    You can learn more about a LOG114 photodiode application by looking over the LOG114 EVM Users Guide found here:

    www.ti.com/.../sbou110.pdf

    Regards, Thomas
    Precision Amplifiers Applications Engineering
  • 0 in reply to Thomas Kuehl
    TI__Genius 11135 points
    Morzh

    We haven't heard back from you so we assume this answered your question. If you need more help just post another reply below.

    Thanks
    Dennis