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AD5933 I2C SCL Minimum Frequency

Question asked by Lunatic91 on Aug 14, 2015
Latest reply on May 2, 2017 by BerlinAnalog

Hello All,

 

I was wondering if there is a minimum frequency for the AD5933's I2C clock to run at. I am using it for EIS measurements and I need to output waves across the entire range of the device. I am using the AD9834 to divide down the clock for frequencies lower than 1kHz, but at my lowest frequencies, 1Hz, 0.5Hz and 0.1Hz, I'm returning data data that is always very close to the calibration impedance. At the moment, I am trying to get this to work on a resistor as my Zunknown.

 

I had originally solved this same issue with 10Hz and 5 Hz by lowering the MCLK's to the AD5933 at those frequencies, but at 1Hz my MCLK will either return values close to the calibration impedance or very close to zero when I lower my MCLK past a certain point. Normally when a result is the same as the calibration impedance, it means that the the impedance the chip found on the calibration resistor is the same it found on the unknown resistor, which makes the result almost the same as the calibration impedance.

 

My Zunknown = 110kohms and my Zcal = 120kohms. Some sample MCLK's I used are shown below. Keep in mind that 10 Hz and 5 Hz are working correctly for me, but 1, 0.5 and 0.1Hz are not.

 

Excitation Frequency (Hz)     MCLK (Hz)

10                                             25000

5                                               25000

1                                               16385

0.5                                              7000

0.1                                              7000

 

I've verified that the Vpp of the signal is within reason at the Vin, also. I have a good feeling this is a clock issue. My hunch that if I lower the MCLK from the AD9834 too far, the I2C bus on the AD5933 will not be able to communicate with the uC I2C clock line, which is running at 100kHz. Any tips, help or information will be greatly appreciated!

 

Thanks for the help,

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