Hi Prabhu, I have carefully reviewed your LOG114 circuit and don't see anything grossly incorrect about it. Operating in voltage input mode its not a LOG114 strong-point, but it is doable if only three decades of log performance is needed. I calculate the ideal performance would be as follows: Input voltage range: 1.6 mV to 960 mV Reference current (I1): 1.0 uA, set by R173 2.5 MEG, V REF = 2.5 V Input current range (I2): 1.6 uA to 960 uA, for 1k input resistor R174 Output voltage at V LOGOUT pin: V LOGOUT = 0.375 log 10 (I1 / I2) V LOGOUT(min) = 0.375 log 10 (1.0e-6 / 1.6 e-6) = -76.5 mV V LOGOUT(max) = 0.375 log 10 (1.0e-6 / 960e-6) = -1.12 V Output voltage at VO4: A4 voltage gain: A(V/V) = -R176 / R175 = -100e3 /24e3 = -2.1 V/V VO4 (min) = A(V/V) V LOGOUT(min) = (-2.1 V/V) (-76.5 mV) = 160.7 mV VO4 (max) = A(V/V) V LOGOUT(max) = (-2.1 V/V) (-1.12 V) = 2.349 V When I review the results you have listed in the Excel spreadsheet I see that any input voltage below about 40 mV results in the negative output voltage at VO4, which is not correct. However, with an input level somewhere between 50 and 100 mV the log amplifier starts to produce positive output voltage levels that are closer to the calculated values. For example: Vin V LOGOUT VO4 Measured delta +40 mV -600.7 mV 1.262 V 0.116 V -1.146 V +100 mV -750.0 mV 1.575 V 1.612 V +37 mV +200 mV -862.9 mV 1.812 V 1.860 V +48 mV +448 mV -994.2 mV 2.088 V 2.148 V +60 mV So we can see the issue is really at the low end of the input scale. There are some small improvements that could be made by following the datasheet. Equation 16 provides a method to improve the log accuracy by setting I REF to a level within the span of the input current range. But I don't think that is at the root of the problem. There is an issue that I have ran into before with LOG114 users that I need to ask you about. Page 2 of the datasheet shows in the section where the package and pins drawing resides that states, "exposed thermal die pad on underside must be connected to V-." Is that pad connected to VCCO3P3AM (V-) on your PC board? I have worked with the LOG114 much over the past several years and have always found that it performs as expected when everything it requires is set up correctly. Regards, Thomas PA - Linear Applications Engineering
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