Hi Prabhu, The PowerPad on the LOG114 package connects to the die substrate. Tying it to the most negative circuit potential V- keeps the individual transistors on the die electrically isolated. Connecting the PowerPad to ground, instead of the -3.3 V supply, biases the substrate +3.3 V higher than the normal operating condition. That can lead to unexpected current flow between circuits on the die and can degrade, or disrupt, normal operation. Since the problems are most evident at low input current levels I expect that is what might be occurring in your circuit. Another factor that we see with very low input current amplifiers has to do with PC board contamination. Residual solder flux remaining on the circuit board can provide, unexpected high resistance current paths in the amplifier circuit. Leakage current can travel these paths and interfere with the intended circuit currents such as that from an input sensor. The LOG114 is usable with very low input currents and stray currents can contribute significant errors when it is operated with low input current levels. We find that simply cleaning the PC board with alcohol and a brush isn't sufficient. We run our PC boards through an ultrasonic cleaner at least one time. We have found that a second run through the cleaner can yield further improvements in reducing leakage currents. Regards, Thomas PA - Linear Applications Engineering
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