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LDC1612-Q1: LDC1612 - Q1 - Drift in Raw Data in Environmental Conditions

Part Number: LDC1612-Q1
Other Parts Discussed in Thread: LDC1612

Hello TI Team,

We are a company from India using LDC1612 for detection of a moving metal wheel. We had used LDC1000 for same application and had faced lot of problems in reading Rp values at different temperatures. Hence we have developed our product using LDC1612. We had already sent queries to this blog and received some input also. However currently we are facing the same problem of drift in inductance values received by LDC1612 in outside environmental conditions. The environmental conditions are as follows 

Temperature - (20 degree celsius - 35 degree celcius).

*Interference of Railway Electrified area(25kVA) .

We are using the recommended c0g capacitors for LC circuit. The configuration of our device is as follows

LDC1 - Capacitor - 220pf(c0G, 1% Tolerance) , Inductance - 127uH - Spiral PCB Wound Coil, fRef - External Oscillator of 40 Mhz. Fref Divider Value - 2 , Sensor Drive Configuration(Reg - 0x1e) - 0x5500,  `   

LDC2 - Capacitor - 330pf(c0G, 1% Tolerance) , Inductance - 121uH - Spiral PCB Wound Coil, fRef - External Oscillator of 40 Mhz. Fref Divider Value - 2 , Sensor Drive Configuration(Reg - 0x1e) - 0x5500, 

I have attached the Data Log Excel Sheet for a period of almost 24 hours in the same field condition. According to your last input given to us there should very less  change in frequency . But as you can see there is a lot of change in the resultant data. What are the route causes which may resultant in this much changes. Since we are using two sensors at a distance of 140mm from each other, is there chances of interference? We have not imparted any EMI considerations into our design, can it be a resultant of the same? Kindly confirm what is the drift in temperature of LDC1612 over the same range mentioned in this message. We also request you to guide us through the mistakes in the configurations that we have implemented. Is there any special requirement to avoid these results?

We also have a doubt as to why our both the LDC Data0 are similar although there is a large change in capacitor value between each other. LDC_DEFAULT_CTR_CAP_FOR_TI.xlsx

Note - In file attached Column A, is the LDC1 Data0 and Column B is the LDC2 Data0 as received through I2C. Column C and Column D are the differences in the consecutive values in Data0 of the LDC0 and LDC1 respectively. The frequencies of the respective devices are also logged in the next column which is directly derived from the Data received. ( Ref Formula used - (Fref * Data0/ 2 ^28). We are in a critical situation where our product is developed after investment from the company and the application design is through. However we are facing a big challenge in clearing the above issue. I hope you reply to this message and guide us through future development. 

  • Hi Venkatesh,

    Can you probe both LDC's IN0A pins with an oscilloscope and share the screenshots? Your measured sensor frequencies are roughly 200kHz off from the calculated value for those LC values. I'd like to verify that the sensors are oscillating correctly. Is there a long distance between the sensor and the LDC1612?

    Also note that when calculating the sensor frequency from the LDC output codes, you should use the Fref value after the Fref divider has been applied, which is 20MHz in your case. 

    The sensor frequencies should be far enough apart that the two active sensors would not affect each other. 

    Best Regards,

  • Dear Kristin,

    kindly find the attached screenshots of IN0A and IN0B pin wave forms for both the LDCs. The distance between the sensors and LDC are maximum 50mm. The sensor frequency are far enough as per LC resonance frequency calculations. The frequency are almost near as per LDC raw value. 

  • Hi Kristen,

    We are awaiting for your reply and we need know at what frequency we have to measure the inductance. At present we are measuring the inductance in 1KHz.

    Kindly update As soon as possible.

  • Dear Kristen,

    We are awaiting for your reply........

    Remainder 1

  • Hello Venkatesh,

    I have been out of office with extremely limited access to email until today. I will have additional feedback for you today or tomorrow.

    Best Regards,

  • Hello Venkatesh,

    Can you clarify how the oscilloscope probe is connected in these images? Is is a differential probe connected across INxA and INxB?

    You should measure the inductance at the resonant frequency of the LC tank. This might be why your sensor frequencies are so different from the calculated values. If you have access to an impedance analyzer, I recommend measuring the LC tank's Rp across frequency to find the true resonant frequency. The resonant frequency is at the peak Rp. This value should match well with the digital output from the LDC.

    Best Regards,

  • Hello Venkatesh,

    I haven't heard from you in a while, so I'm assuming you were able to solve your issue. If this is not the case, please feel free to post a new thread or respond to this one.

    Best Regards,