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ADA2200: Replacing RCLK signal with DAC output

Question from customer about the ADA2200 :

I need to excite my sensor with a sine wave. If I understand correctly, RCLK is only producing a squarewave, which is very unpleasant because it is quite tricky to filter out all the harmonics to obtain a pure sine wave.  The harmonics can induce a phase shift in my measurement. I want to avoid the complexity of designing and building an analog filter to remove the harmonics.

 

Can I use the DAC of my microcontroller to produce a sinewave and excite my sensor with that signal?

Answer:

Yes, you can generate a sinewave with a DAC and use that as an exitation signal instead of using RCLK.  It is important to phase-lock the sample clock of the DAC to the ADA2200 CLKIN signal.  In your case you could use the microcontroller to drive the CLKIN signal and control the DAC, which should make synchronization straightforward.  The input sampling rate of the ADA2200 is 64x the frequency of RCLK.  Selecting a DAC sample rate that matches the input sampling rate would result in very low harmonics with minimal anti-aliasing filtering on the DAC output.

Comments
Anonymous
  • I didn’t understand the physics when I am in the 9th standard and after completing the engineering now this time I didn’t get it. so my friend told me to visit http://topratedessayservice.com/ this site RCLK is only producing square wave I learnt from this site this is too good for me.

  • Hi Brian,

    No news ? You tried to use the DUE ?

  • Thanks Brian. It works. Here is my code for Arduino DUE (I am using Arduino DUE because I've seen in an other post that you have used it also).

    The digital pin 13 control the reset line.

    I tied it LOW at the begining of the program and tied it HIGH after 5 period of CLKIN. I am not sure it is the best way to do it !

    Also can you check the configuration for both Q and I using SPI ?

  • As you have noted, when an external clock is used to drive the CLKIN pin, the device will operate with or without setting the "Clock Source Select" bit. It is preferred to set the bit high when using an external clock.  Setting the bit high tri-states the buffer on the XOUT pin.  This lowers the device power consumption and eliminates any noise that may be generated by the XOUT pin.

    Regarding the synchronization issue.  Have you tried connecting both the CLKIN pins and the RESET pins of the 2 (I and Q) ADA2200s?  I think if you tie those pins together, hold the parts in reset, start the input clock, then deassert reset, the I and Q devices will come out of reset synchronously.

  • So it seems to work with or without setting the bit  "Clock source select" in the register "Analog pin configuration"  : fair enough.

    My goal is to build a real lock-in, so able to give amplitude and phase. I put a second ADA2200 and I feed the CLKIN input with the same output of the arduino Due as the first ADA2200, later on I will set the bit in the register "Demod control". For now if I reset the second ADA2200, the phase shift between the sine wave and RCLK is changed randomly. I use RCLK of the first ADA2200 as an input to the Arduino and set i=0 for every rising edge of RCLK. I am not able to synchronise the two ADA2200.

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