I have several questions, but let me first start off with a basic question based on the 4127 datasheet.
On page 6, figure 4, there is a 10k pot with a 100meg resistor used to bias the input current on pin 4. My question is why would you need to bias the input current as well as finely adjust the reference current. It seems like there should only be 2 things to adjust, the level shifting and the gain setting (thus there should only be two resistors to adjust?) What would happen if I just ignored the 10k pot with 100meg resistor in my circuit?
Second, lets say that I DO need to adjust all 3 parameters to finely calibrate the transfer function to the one desired. I am then lost when I look at the datasheet for the log112 or even the Log114. Using these datasheets (and simulations) I can determine the values of all the external components to use, but even then it seems to be missing the calibration part. What if my resistor values are slightly off, then I should need trimmers to finely adjust the transfer function to the one I want, but the data sheet doesnt say anything about fine adjustments to things such as the bias current. Now I know that gain cannot be adjusted (which is perfectly fine), but level shifting still needs to be addressed. Can someone explain to me why the new Log112 doesnt have notes on fine calibration (maybe it's because users are expected to level and gain adjust using an external stage?)?
The transfer function I am aiming for is Vout=-10 log (Vin) with input voltages from .1V to 10V, thus outputs would range from -10 to 10 Volts. Using the Log112, wit a transfer function of Vout=.5 log (I1/I2), I plan on making I1 my reference (2.5 volts through a 250k resistor making I1=10 microamp). Then, I can obtain I2 from my input voltages by using a 100k resistor (thus .1V->1 microamp while 10V->100 microamp). I can also add an external amp with a gain of 20, so then I arrive as Vout=-10 log (Vin)! Perfect. But unlike the 4127 (which is what I've been using), I am not sure of the adjustment procedures to ensure that my input resistor is exactly 100k or that my reference current is exactly 10 microamps.
Thanks for the help, I really appreciate it. Sorry if it is a bit confusing I can add more clarification if needed.
For the Log112 and the Log114 you could simply put a potentiometer (configured as a rheostat) in series with your input resistance and/or your reference resistance to allow for adjustment. However, using precision resistors (.1% or better) for your external resistors would be a far better solution than a series potentiometer in terms of drift over time and temperature. I should also point out that there is no need for an additional external amplifier to achieve your desired transfer function with the Log112 or the Log114 as both parts contain internal (uncomitted) amplifiers for this purpose (A3 in the LOG112, and A4/A5 in the LOG114).
I will look into your question on the 4127 and post the answer here if I'm able to find an answer.
Analog Applications Engineer
PA Linear Apps
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