Bias Current

Bias current is only applicable to mutual capacitance sensors. When measuring mutual capacitance sensors, a sample and hold amplifier drives the receive electrode to remove any contributions of the parasitic capacitance to ground. This ensures that only the charge associated with the mutual capacitance is accumulated on the sample capacitor during the measurement. The bias current parameter provides the ability to optimize a design for power consumption if the system allows for it.

If power consumption is not a concern, the bias current may be defaulted to the highest value and left alone.

If power must be optimized, it is possible to begin reducing the bias current one level at a time while observing the impact on the measurement when touched. If there is too much parasitic capacitance to ground, the achievable deltas may be reduced and the measurement may dip below the LTA due to a touch, rather than rise above it. In this case, the parasitic capacitance is overwhelming the drive capability of the sample and hold amplifier and the bias current needs to be increased.

Implementation Notes
For some systems with little parasitic capacitance to ground, varying the bias current may not have a large effect on performance. When adjusting the bias current, be sure to test all touch use-cases to verify functionality.

Affected Software Parameters
The Bias_Current parameter corresponds to the ui8BiasControl member of the tSensor type in the CapTIvate Touch Library.