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LDC1614EVM: How to determine ideal offset to improve dynamic range

Patrick O'Neill
Prodigy 35 points
Part Number: LDC1614EVM
Other Parts Discussed in Thread: LDC1314EVM, LDC1614

Hello,

Our team is wondering how to determine the right amount of offset we should be using to improve the dynamic range of our coil sensors. Based on the information in this blog post:

https://e2e.ti.com/blogs_/b/analogwire/posts/inductive-sensing-improve-the-enob-of-a-multichannel-ldc-by-4-bits-in-3-simple-steps

the combination of gain and offset can significantly improve the resolution of the LDC device series. Since the LDC 1614 does not have a gain option, we were wondering if we could use the offset nonetheless to improve our dynamic range. For context, the magnitude of signal in terms of code difference between when the metal is nearby or not is on the order of about 100. 

Any help would be greatly appreciated, whether it be with respect to offset or any other ideas that might improve our resolution. 

Thanks! 

  • 0
    Prodigy 35 points

    Each channel also has its own coil that operates at a slightly different frequency than the others, and the difference in output in the presence of target is how we quantify the signal while also compensating for temperature and other noise factors.

  • 0 in reply to Patrick O'Neill
    TI__Mastermind 40295 points

    Patrick,

    Thank you for the post, and your interest in TI products.
    The offset is valuable for the LDC131x series because the device converts at a 16 bit resolution, but only displays 12 bits. So the gain/offset adjustment can help extract more resolution from your data.
    The offset adjustment doesn't get as much attention for the LDC161x family because it has 28 bits of resolution, for data conversion and the output.
    The LDC161x  optimum offset will really be dependent the sensors' frequency shifts (100x) and that channel's  frequency ratio fsensorx/fref.

    Without knowing more details, my guess would be to make the OFFSETx for each channel so the 100x fluctuation is around the output mid-scale, or as close to mid-scale as possible. 

    I hope this helps. Please let me know if you have any more questions.
    Just so you know, today is a holiday for us in the US, and we will be back in the office on Monday.

    Regards,
    John

  • 0 in reply to John Miller - Sensing
    Prodigy 35 points

    Hi John,

    Thank you for your reply. Our team has gone ahead and purchased and LDC1314EVM to see if we can reap the benefits of the gain and offset parameters, but in the mean time I would appreciate any insight you can provide for our system.

    External oscillator frequency 38.4 MHz (this gets all channels under the 3.3 MHz deglitch filter)
    Raw Code Output 22934880
    Sensor Frequency 3.280849
    Sensor Inductance 5.006923

    Would you say that our sensors are operating near the output mid-scale? If not, what kind of offset should we be looking at to achieve this?

    As a beginner in the field I appreciate your patience with all of this!

    Patrick

  • 0 in reply to Patrick O'Neill
    TI__Mastermind 40295 points

    Patrick,

    Could you lease confirm the Raw code value in your table?
    Also, your inductance value is in uH?

    John

  • 0 in reply to John Miller - Sensing
    Prodigy 35 points

    Hi John, 

    I can confirm that number the raw code number in the table. The average across all four sensors for a collection of about 20 discrete measurements in time is of 22936680. As for the inductance value, it is indeed in uH.

    Patrick

  • 0 in reply to Patrick O'Neill
    TI__Mastermind 40295 points

    Patrick,

    I was unable to get a raw data code output anywhere near what you are reporting.
    Can you give a little more detail on your settings how you are getting that value?

    regards,
    John

  • 0 in reply to John Miller - Sensing
    Prodigy 35 points

    Hi John,

    We are using the LDC with pretty much out-of-the box settings, connected to 4-layer planar coils and that have a capacitor of 470 pF. We have not changed any of the registers, settle count, or reference count, and are running the iDrive current at 30 for all channels.

    If there is any way we could have a longer form discussion in addition to this thread, I'd be happy to show you more! Thanks again.

    Patrick

  • 0 in reply to Patrick O'Neill
    TI__Mastermind 40295 points

    Patrick,

    Have you tried using our Excel-based sensor design tool available in the LDC1614 product folder?
    If not, you can download it from here.
    It may serve as a useful tool to validate some of the results you are seeing from the EVM. 


    One of the worksheets is dedicated to parameters and settings for a design based on the LDC1614.
    As a first step, with a little trial-and-error you can create a sensor design close to what is on the EVM using the worksheet named Spiral_Inductor_Designer.
    That tab also allows you to specify the target parameters and distance to the sensor. 
    Once you have modelled the sensor (and EVM capacitor) on the first worksheet, you should see the same frequency as on your EVM.
    You may need to get some of the info , like the resonating cap, from the LDC1614 EVM User Guide.

    The results of this tab will be fed to the worksheet LDC131x_LDC161x_Config in the field labelled as Free Space Sensor Frequency. 
    Further down LDC131x_LDC161x_Config you can see device settings as well as some outputs based on those settings and the sensor frequency.
    Hopefully this will help you validate your settings and the results you are seeing.

    We have some additional info on the Excel tool that might be useful on the E2E Inductive Sensing FAQ page and in some of our Inductive Sensing Blog pages.

    Please give it a try and let me know if you have any questions.

    Regards,
    John

  • 0 in reply to John Miller - Sensing
    Prodigy 35 points

    Hi John,

    Thank you for your note. I am familiar with the tool and will explore it further as you've detailed above.

    Will keep you updated on my progress!

    Patrick