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LOG112 vs. LOG114 Response Time

Lucy Fanelli
Prodigy 20 points
Other Parts Discussed in Thread: LOG114, LOG112

Hello,

I'm using the log 112 as a current sensor.  I set I2 to 70.7nA as the reference current, and my input current I1 varies between 100pA and 50uA.  I'm finding that I need a compensation capacitor Cc of at least a 4.7nF to achieve a linear response at very small input currents, and this value capacitor causes the response time to be far too slow (I need less than a 4ms response time for changes of about a decade).  The bandwidths listed on the datasheet of the log114 seem much higher, though it is for a much larger reference current (1uA).  There also isn't an external compensation capacitor for the log114.  Is the log114 a faster part, that might help solve my problem?

Thanks,

Lucy

  • 0
    TI__Guru* 93105 points

    Hi Lucy,

    The slow response behavior that you are observing with the LOG112 is normal for log amplifiers; the lower the input current, the lower the bandwidth. This is evident Electrical Characteristics table BW, 3 dB.

    The LOG114 is a newer log amplifier designe on a much faster semiconductor process. If you compare the BW, 3 dB for the LOG114 compared to the LOG112 you can see that for comparable input current levels its bandwidth is about 2 orders of magnitude higher than the LOG112. Therefore, the bandwidth at equivalent low input current levels will be much higher with the LOG114. 

    The LOG112 does require frequency compensation as the data sheet explains on page 8, while the LOG114 doesn't require any frequency compensation to achieve its bandwidth.

    Step response times for the LOG114 are under a microsecond for all the conditions listed in Electrical Characteristics table. I expect that even with a 100 pA input the response time will be well under your 4 ms requirement.

    Regards, Thomas

    PA - Linear Applications Engineering