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LDC1614EVM: Changes in the standard deviation

May
Genius 9880 points
Part Number: LDC1614EVM

Hi ,

Customer  running a pre-study using LDC1614EVM to try out a sensor solution and have noticed a changes in the standard deviation. Below is the details.

"

One of my tests was to try to decrease the frequency of the solution from about 1 MHz, achieved by using a 10nF capacitor, by switching to a 100nF capacitor. The frequency shift 1000kHz -> 385kHz was achieved, but unfortunately, the standard deviation was increased from 0.0002 to 0.0017, an increase of about 8 times.

I found the increase negative and cannot find any reason for the negative change. I see it as crucial for my design to have this understanding. Have you seen this before?

Are you able to help me sort this error out?

The setup is currently a lab-setup which very well may contain weaknesses. If weaknesses in the setup is causing the problem, I need to understand them.

"

Regards,
Maynard

  • +1
    TI__Mastermind 40295 points

    Maynard,

    Some approximate calculations of the sensor Q using Equation 11 in the data sheet make it look like increasing the sensor C from 10nF to 100nF leads to an increase in the sensor Q by a factor of 3.16 (t=SQRT(10)).

    Increasing Q will in turn cause the sensor's start-up transient to settle more slowly, and thereby increasing the sensor's settling time.

    This could be the issue if the customer is cycling through measurements on each of the device channels. This wasn't mentioned as a condition, but since this is a multi-channel device, I will assume it could be.

    Section 8.1.4.1 - Settling Time in the data sheet gives some advice on setting the device settling time window to account for sensor settling time via the SETTLECOUNT parameter and the reference frequency (equation 9):

    where x is the channel number 0-3.

    The SETTLECOUNT parameter is in turn based on the sensor Q, reference frequency, and the sensor frequency. 
    ----

    ---

    Equation 10 shows that for an increase in the sensor's C (and in the sensor Q), that channel's corresponding SETTLECOUNT should be increased to allow for the sensor's longer settling times. 

    Regards,
    John