SNOA949 May 2016 LDC1312 , LDC1312-Q1 , LDC1314 , LDC1314-Q1 , LDC1612 , LDC1612-Q1 , LDC1614 , LDC1614-Q1
Inductive sensing is a contactless technique for applications ranging from position or motion measurement of a conductive target to detection of spring compression or extension. Depending on the specific application, there are different system requirements regarding sensitivity, responsiveness, and power. Power consumption is a key parameter for many applications, including wearables, consumer electronics, and some automotive applications. This application note covers techniques to reduce power consumption for TI’s LDC1312, LDC1314, LDC1612, and LDC1614 inductive-to-digital converters and various experiments to verify the effectiveness of these power-reducing techniques.
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The multichannel LDC devices utilize an LC tank-based sensor to determine the change in inductance by measuring (sampling) the change in oscillation frequency of the sensor. The sensor oscillation frequency (fOSC) is compared to an independent reference clock (fREF) to produce an output sample that represents fOSC as a fraction of fREF. This fraction is represented as a digitized output code which then can be used to determine position or proximity of a moving conductive target. The power consumption of these LDC devices is typically on the order of a few milliamps when in continuous sampling mode. One of the advantages of inductive sensing technology over a competitive Hall effect sensor is that Hall effect sensors typically lack a low-power mode which would make it more difficult to decrease the power consumption when compared to the LDC sensing technology.
For low power applications, duty-cycling the LDC is a technique that can be used to reduce the power consumption. When the LDC can sample much faster than the application requires, the device can be put into a lower power mode while it is not in the data conversion process. The device is only active when it is performing a measurement conversion to minimize the total amount of current flowing through the device and therefore reduce overall power consumption. The LDC devices have two low power modes that can be used for duty-cycling. The first one is sleep mode where the LDC retains its registers contents except for those in the data registers. The second one is shutdown mode where the LDC loses all its register contents and the shutdown (SD) pin on the chip itself needs to be toggled. For consistency, the rest of this document will refer to sleep mode as the low power mode when discussing duty-cycling.
One of the main tradeoffs associated with this duty-cycling technique is responsiveness. The higher the sampling rate, the more responsive the system is, but at the cost of higher average power. Many applications do not need a fast response time and can benefit from the lower power consumption that comes with a lower sampling rate. For example, a human-machine interface (HMI) may only need a sampling rate of 20 SPS or lower, which is much lower than the peak 13 kSPS of the multichannel LDC devices. The total conversion time of a measurement for a given sampling rate can also affect power consumption depending on the resolution requirements. This concept corresponds to the duty cycle of the device at a particular sampling rate.