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Original question:

LOG112: how the reference voltage is setting

LOG112: input I1

Aftab Ashraf Av
Prodigy 195 points
Part Number: LOG112

I want to give the output from a photo detector,in that case how we  provide  input to I1.using current source? or current generator? .can u plz explain the reason also.

  • 0
    TI__Guru 61540 points

    The LOG112 computes the logarithm or log ratio of an input current relative to a reference current. VLOGOUT of the LOG112 is trimmed to 0.5V per decade of input current: VLOGOUT = (0.5V)*LOG (I1/I2) where I1 is phtodiofe current and I2 is a reference current; thus, for example if I1 is 100x higher than Iref, then Vout=1V.

    LOG112 also includes internal 2.5V voltage reference that may be used to generate a precision current reference using an external resistor or you may use an external current reference.  Therefore, if you use 1M resistor (Rref), this will set the reference current to 2.5uA and thus Vout will be 0.5V for I1=25uA and Vout=1.5V for I1=2.5mA (Vout=0.5V*Log(2.5mA/2.5uA) = 0.5V*Log(1000) = 0.5V*3 = 1.5V; for transient simulations you may use for I1 either current source or current generator.