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Some question about FDC2214

Gabriel Wang1
Genius 4715 points
Other Parts Discussed in Thread: FDC2214

Dear Expert

For FDC2214:

1. Our application only considers single-channel Mode, so subsequent related issues are also discussed in single-channel Mode.

2. We pay most attention to the response time (the time between the input change and the data transmission from IIC to MCU) and ENOB

1. In single-channel mode, the time of Amplitude Correction is not reflected in the data manual. Please help to ask whether this value can be checked.

 

2. In single-channel mode, the VALID fREFx RANGE should be less than 35MHz. If 40MHz crystal oscillator is used externally, should the value of fREFx be adjusted actively or set automatically inside the chip?

 

3. How to calculate the response time more accurately in single-channel mode?

The way I think about it is:

(1) Sensor Activation Time

 

(2) Conversion Time

 

(3) Amplitude Correction Time（We don't know）

(4) IIC Transition Time of 28bit data (limited to 400kHz)

4. How is ENOB calculated?

Data sheet for FDC2214

 

That is

It can be deduced as:

 

Where did this value come from?

5. In addition, ENOB in the upper computer interface of FDC2214 is as follows:

 

The corresponding derivation is as follows:

 

And where did this value come from?

6. In addition, the manual of the chip calls itself 28bit. How did you get this?

  • 0
    TI__Mastermind 26335 points

    Hello Gabriel,

    Thanks for considering to use Texas Instruments.  We are looking into your question.  However, as we are short staffed due to the holidays, it will take a while for us to respond to all of your inquiries. 

  • 0 in reply to Patrick Simmons
    TI__Mastermind 26335 points
    1. At the moment we do not have many details about the amplitude correction. When our part expert returns after the holidays, we may be able to give more detail on that.  This post for a device leveraging similar circuitry suggests it is a few cycles.  In the absence of any information, a conservative estimate might be 2^2 or 2^3 cycles.
    2. You should specify the frefx actively by setting the CLOCK_DIVIDERS_CHx register. That register allows you to divide by any number between 2^0 and 2^9.  However to comply with fref spec of 35MHz  you will need to divide by at least 2.
    3. First response would be tsx+tcx+I2C read period, subsequent would be amplitude correction + tcx + I2C read period.
    4. This number is a tradeoff between time and accuracy. The smallest number you could use for CHx_RCOUNT is 8, but worst case it would be 0xFFFF. A larger ENOB will lead to a longer conversion time.  Analogous to averaging, more conversion converges toward the true mean value.
    5. From table 20 and the example provided, it looks like ENOB might be CHx_RCOUNT*16. 0x200=512, 512*16 =8192.  I would use ENOB = log10(CHx_COUNT*16)/log10(2)
    6. I think ENOB = 21 is a typo. Will verify when our part expert returns.
    7. DATA registers collectively have 28 bits. ENOB specifies the number of conversion cycles, not the resolution of the circuit that compares your two signals.  The sensor frequency does affect the resolution though.  Higher frequencies will have missing codes and less resolution.  Maximum resolution is expected to occur for fsensor between 400kHz and 6MHz.
  • +1 in reply to Patrick Simmons
    TI__Mastermind 18545 points

    Hello Gabriel, 

    The automatic amplitude correction timing is very short as it uses data from the previously run sample collection and just makes a quick change to the device settings. We don't have exact timing on this but it is negligible compared to most sample rates.
    Additionally, the auto amplitude correction setting is turned off for this device.  When the amplitude corrects, it leads to a slight shift in data and this is harmful to high resolution applications. So as long as you are following the datasheet suggested register settings, you won't have to worry about the amplitude correction timing.

    Best Regards, 

    Justin Beigel