Inductive sensing: Top three LDC1000 questions


It’s been a few weeks since we unveiled the LDC1000, the industry’s first inductance-to-digital converter (LDC), and the response has been phenomenal. I’ve never seen a new analog product generate so much buzz and interest so quickly.

At the same time, we launched the Inductive Sensing Forum here on the TI E2E™ Community to provide customers with a place to ask questions and get support. Here are the top three questions we’ve gotten so far, both on the forum and from our sales and distribution teams.  

1.  What is the maximum distance I can sense with the LDC1000?

The sensing distance of the LDC1000 depends on the diameter of the coil and the core material. As a rule of thumb, the maximum sensing distance is 50-100% of the coil diameter with an air core. With ferrite backing on a PCB coil, the maximum sensing distance can be extended to 70-150% of the coil diameter (ferrites boost the inductance of the coil).

Figures 1 and 2 below show LDC outputs versus distance for an air core PCB coil and a PCB coil with a ferrite backing. We collected the data with a 16-mm diameter 2-layer PCB coil

                                                      Figure 1: Rp versus distance for air core and ferrite PCB coils                                                                     

Figure 2: Inductance versus distance for air core and ferrite PCB coils

2.  Is the external clock input mandatory for LDC1000 operation? The EVM uses a 6-MHz clock, while the datasheet specifies 8 MHz.

The external clock is needed to measure inductance; it is not needed if the LDC1000 is used only to measure Rp (eddy current losses).The external clock input can be either a clock on TBCLK pin or a crystal connected between XIN and XOUT.

The maximum spec on the external clock is 8 MHz. Any frequency below 8 MHz is acceptable. However, going to lower frequency will decrease the resolution and is recommended to be in the range of 5-8MHz. On the LDC1000EVM, the 6-MHz signal on the TBCLK pin is derived from the MSP430, which runs on a 24-MHz clock.

3.  What is the total supply current of the LDC1000, including the LC tank?

Current consumption of the coil depends on several factors, such as the Q factor of the coil, the amplitude at which the tank is oscillating (this is programmable in the LDC1000), and the operating point of the tank with respect to distance.

The Q factor of the coil determines the amount of energy that needs to be replenished every cycle to maintain a sustained oscillation.

You can approximate the dependence of current on oscillation amplitude (VP-P) as:

Note that VP-P can be 4V, 2V or 1V, depending on the register setting in the ‘LDC Configuration register’ addr=0x04.

Rp of the tank changes with distance of the target and forms the above Q factor formula. Q is directly proportional to Rp. Hence,

  • Higher the Q-> Higher the Rp => Lower the current
  • Closer the target->lower the Rp => Higher the current

You can check out more questions and answers at our Inductive Sensing Forum. And if you have a question that hasn’t been asked yet, I hope you’ll post it there so we can answer it!

What a chance to win an LDC demo kit and $3,000?

Have you heard about our Sensing Design Contest? In partnership with EDN, we’re challenging you to come up with the most creative, useful, and interesting application ideas for the LDC1000. 

Are there devices that don’t have a sensor but should? Are there sensor implementations that could be done better?

There are no boundaries on industry or use case. From satellite sensors and deep sea oil rigs to industrial and consumer gadgets, we want to hear it all! Get your creative juices flowing, and let your imagination do the rest.

Note that contestants must be members of EDN and live in the United States (excluding Puerto Rico) or Canada (excluding Quebec).

All entries must be received by Oct. 18, 2013. Enter today!