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Log112 - voltage inputs - "limited to 3 decades"

texian3
Prodigy 10 points
Other Parts Discussed in Thread: LOG112, LOG114

Page 9 of the LOG112 data sheet has a paragraph labeled "Voltage Inputs".  In the paragraph, it states, "the dynamic range is limited to approximately three decades of input voltage etc."  How would I determine where the 3 decades would be centered around based on the 7 decade range of the Log112.  (i.e. is there an analogous graph to Fig. 14 on p. 11).

  • 0
    TI__Guru* 93105 points

    Hello Texian,

    I'd like to provide a little more information why the LOG112 has much more limited dynamic range when driver by a voltage source than a current source. The information actually comes from the LOG114 datasheet:

    "This limitation exists because of the magnitude of the required input voltage and size of the corresponding series resistor. For 10nA of input current, a 10V voltage source and a 1GΩ resistor would be required. Voltage and current noise from these sources must be considered and can limit the usefulness of this technique."

    Since the best dynamic range is achieved across 3 decades when driven by a voltage source and series resistor I would set up the resistor based on the middle decade. For example, set I2 the reference current equal to 1 uA and RREF would be 2. 5 MEG. Let's say the middle decade for I1 is 1 uA to 10 uA. Then the lower decade would be 100 nA to 1 uA, and the upper decade would be 10 uA to 100 uA. If our external voltage source is 1.0 V and I select I1 to be 1 uA, then the series resistor associated with I1 would be 1 MEG. 

    When I calculate the VLOGOUT for a current in each decade from the LOG112 transfer function VLOGOUT = (0.5 V) log (I1 /I2):

    100 nA       VLOGOUT = (0.5 V) log (100 nA / 1 uA) = -0.5 V

    1 uA            VLOGOUT = (0.5 V) log (1 uA / 1 uA) = 0 V

    10 uA          VLOGOUT = (0.5 V) log (10 uA / 1 uA) = +0.5 V

    Putting any current within the 3 decades into the VLOGOUT equation will provide the resulting output voltage. 

    I used 1 uA for I1 in my example, but you can use whatever reasonable value you would like within the decade of your choice.

    Regards, Thomas

    PA - Linear Applications Engineering