Sensor Ganging--Optimize Power in a Capacitive Sensing System — Part II

August 13, 2015 //By Shruti Hanumanthaiah and Subbarao Lanka
Sensor Ganging--Optimize Power in a Capacitive Sensing System — Part II
Shruti Hanumanthaiah and Subbarao Lanka continues to consider the challenges of sensor ganging and how to optimize power in a capacitive sensing system.

In Part I , we briefly covered how to optimize power consumption in capacitive sensing system, utilizing sensor ganging with a capacitive sensing controller, and use cases of sensor ganging. In part II, we will cover trade-offs between response time and power consumption, as well as other issues resolved by a ganged sensor.

Response Time vs. Power Consumption

Before discussing the advantages of sensor ganging, let us discuss the tradeoffs between response time and power consumption in a capacitive sensing system. The response time of a system is defined as the time required by a system to give a valid output from the time the corresponding input is received.

In capacitive sensing applications, it is not easy to optimize both power consumption response time as it takes a considerable amount of time to scan sensors. The refresh interval can be varied to optimize either power consumption or response time. But it is not always possible to get the desired level of optimization by controlling the refresh interval alone. If the device is optimized for power consumption, then response time will be slower, and the opposite is also true.

Another technique is used to optimize response time and power consumption. When a user interacts with the device, the capacitive sensing system can be optimized to respond quickly and when the user does not touch the buttons, the device can be optimized for power consumption.

To achieve the optimization of these two key parameters – power consumption and response time – the refresh interval needs to be adjusted. For optimizing response time, the sensors need to be scanned at a high rate and hence the refresh interval needs to be small. Let us call this quick scan mode. For optimizing power consumption, the sensors need to be scanned at a lower rate as the device needs to sleep for more time, hence the refresh interval needs to be larger. Let us call

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