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LDC1000EVM uC Sampling Rate range

Koyo Kegasa
Expert 1260 points
Other Parts Discussed in Thread: LDC1000EVM, ADS1256EVM-PDK

I set the LDC1000EVM parameter as follows.

Rp max = 3078 ohm
Rp min = 1796 ohm
f sensor = 2688.820 kHz

I get the log data with the default sampling frequency = 1kHz all right.

But, when I set the uC Sampling Rate to 10kHz, I got the same data 6 to 12 times in the log file.

As I measure the rotaing smooth surface rotar, the value shoud change continuously. 

Please tell me how to set the uC Sampling Rate which truely measure the inductance at that sample rate, not repeating the same data several times.  

By the way, Is it possible to set uC Sampling Rate at 70 kHz maximum with the LDC1000EVM and GUI software?

part of the log_data_file

04:09:14:13:39:55:273.600 27601 139.6 13.787 uH 27601 3511
04:09:14:13:39:55:273.700 27601 139.7 13.787 uH 27601 3511
04:09:14:13:39:55:273.800 27601 139.8 13.787 uH 27601 3511
04:09:14:13:39:55:273.900 27601 139.9 13.787 uH 27601 3511
04:09:14:13:39:55:273.000 27601 140 13.787 uH 27601 3511
04:09:14:13:39:55:274.100 27601 140.1 13.787 uH 27601 3511
04:09:14:13:39:55:274.200 27677 140.2 13.787 uH 27677 3511
04:09:14:13:39:55:274.300 27677 140.3 13.787 uH 27677 3511
04:09:14:13:39:55:274.400 27677 140.4 13.787 uH 27677 3511
04:09:14:13:39:55:274.500 27677 140.5 13.787 uH 27677 3511
04:09:14:13:39:55:274.600 27677 140.6 13.787 uH 27677 3511
04:09:14:13:39:55:274.700 27677 140.7 13.787 uH 27677 3511
04:09:14:13:39:55:274.800 27717 140.8 13.803 uH 27717 3513
04:09:14:13:39:55:274.900 27717 140.9 13.803 uH 27717 3513
04:09:14:13:39:55:274.000 27717 141 13.803 uH 27717 3513
04:09:14:13:39:55:275.100 27717 141.1 13.803 uH 27717 3513
04:09:14:13:39:55:275.200 27717 141.2 13.803 uH 27717 3513
04:09:14:13:39:55:275.300 27717 141.3 13.803 uH 27717 3513
04:09:14:13:39:55:275.400 27660 141.4 13.866 uH 27660 3521
04:09:14:13:39:55:275.500 27660 141.5 13.866 uH 27660 3521
04:09:14:13:39:55:275.600 27660 141.6 13.866 uH 27660 3521
04:09:14:13:39:55:275.700 27660 141.7 13.866 uH 27660 3521
04:09:14:13:39:55:275.800 27660 141.8 13.866 uH 27660 3521
04:09:14:13:39:55:275.900 27660 141.9 13.866 uH 27660 3521
04:09:14:13:39:55:275.000 27484 142 13.976 uH 27484 3535
04:09:14:13:39:55:276.100 27484 142.1 13.976 uH 27484 3535
04:09:14:13:39:55:276.200 27484 142.2 13.976 uH 27484 3535
04:09:14:13:39:55:276.300 27484 142.3 13.976 uH 27484 3535
04:09:14:13:39:55:276.400 27484 142.4 13.976 uH 27484 3535

Hope to get any help soon!

  • 0
    TI__Genius 13650 points

    Hello Koyo,

    can you confirm that the LDC itself is configured to sample fast enough to support this? How many samples per second do you expect from the LDC? Please refer to section 'Output data rate' on page 12 of the LDC1000 datasheet to calculate the output data rate for your particular clock speed and response time settings.

  • 0 in reply to Ben Kasemsadeh
    Expert 1260 points

    Thank you very much!  So, changing the Sample rate on the front page isn't good enough.

    " Output data rate=( fsensor)/(Response Time/3), Sample per second(SPS) "

    fsensor = 2688.820 kHz

    LDC Configuration Response Time = 6144 cycles

    Output data rate = 2688820/(6144/3) = 1312.9 sps

    That's why I got the same data, when I change the data rate to 10kHz.

    So, if I set the LDC Configuration Response Time = 768 cycles

    Output data rate = 2688820/(768 /3) = 10503 sps > 10ksps

    Thank you very much!

    I'll change the configuration and try again!

  • 0 in reply to Koyo Kegasa
    TI__Genius 13650 points

    Great, I'm glad I could help. I will close this thread. Feel free to post again if you still have any issues.

  • 0 in reply to Ben Kasemsadeh
    Expert 1260 points

    I have some more questions.

    It looks like LDC1000 EVM GUI 1.0.4.1 allow Continuous Logging >= 0.1mS

    I tried to set uC Sampling Rate 0.05ms 20000.000Hz.

    But, when I push the LogData button, alart below is displayed.

    " Continuous Loggging < 0.1ms is not supported at the moment "

    (1) Is it the limitation of the microprocessor on ADS1256EVM-PDK?

    (2) Are there any way to overcome this limitation?

    If possible, I'd like to try 0.025ms 40000.00Hz continuous logging.

    (3) Do I have to use faster microcontroller to achieve this goal?

    It is possible if I set the Response time to 192.

    2688820/(192/3) = 42012.8125 > 40kHz

    (4) When I reduce the response time by one step, resolution bit number is also reduced by 1 bit?

    Response Time 6144 cycle = 16 bit resolution

    Response Time 3072 cycle = 15 bit resolution

    Response Time 1536 cycle = 14 bit resolution

    Response Time 768 cycle = 13 bit resolution

    Response Time 384 cycle = 12 bit resolution

    Response Time 192 cycle = 11 bit resolution

    Am I correct?

  • 0 in reply to Koyo Kegasa
    TI__Genius 13650 points

    Hi Koyo,

    1) The sample rate is a firmware limitation of the MSP430 in conjunction with the used firmware architecture implementation. 

    2) I'm afraid we currently don't support faster sampling rates on our EVM. However, with a different MCU and/or different firmware implementation, you should be able to achieve the maximum data rate of the LDC1000.

    3) Would a C2000 work in your application?

    4) not exactly, even at a 192 cycle response time, the LDC takes a 16 bit reading. By choosing a slow response time, you are allowing the LDC to average data for a longer time and thereby decrease noise and improve ENOB. The ENOB is highly dependent on the Q of the chosen chosen sensor, target material, and distance. 

  • 0 in reply to Ben Kasemsadeh
    Expert 1260 points

    I only know MSP430 series now.  I never tried C2000 yet.  

    So, if I reduce the response time, resolution and S/N will decrese.  

    I tried sevelal setting to measeure my rotar rotatin at 1400rpm.

    (1)10ksps 0.1mS=100uS interval setting
    LDC Configuration Response Time 768 cycles

    Sampling Speed is more than enough.  

    Noise level is big, Resolution is not so bad.. 

    (2)5ksps 0.2mS=200uS interval setting
    LDC Configuration Response Time 1536 cycles、

    Enough Sampling Speed. 

    Noise level is smaller.  Resolution is better.

    (3)2ksps 0.5mS=500uS interval setting

    LDC Configuration Response Time 3072 cycles

    Not enough sampling speed.

    Noise level is very low.  Resolution is very good.

    (4)1ksps 1mS=1000uS interval setting

    LDC Configuration Response Time 6144 cycles

    Sampling speed is too low.

    Noise level is very low.  Resolution is very good.

    So, for my application, (2) 5ksps is the best.

    Thank you very much for your help!