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How to correctly calculate the Rfb value in AD5933

I was reading the datasheet of AD5933, and I did not understand a few things about the calculation for the Rfb value and other things.

This is the Rfb equation:

How do I find the value of Zmin?

I intend to power the AD5933 with 5V, use the 1st Output Range (3Vpp in this case) and PGA gain = 1.
With a frequency sweep of 1K Hz up to 100K Hz:
Internal oscillator: MCLK = 16.776 MHz
fSTART = 1000 Hz
∆f = 1000 Hz
Increments = 99

In addition, I will use a high pass filter with a cutoff frequency of 100Hz, a circuit with a current source controlled by voltage (1mA), and an instrumentation amplifier INA118 (Gain = 2). This circuit will use 4 Electrodes. Similar to this:

In AN-1252 APPLICATION NOTE, page 3 of 12, it is said that for range 1 (Adding external op amp) Zout is equal to "> 100 Ohms". And on page 5, The maximum ratio, ZMAX / ZMIN, for my case is x45. Right? If it is correct. How to calculate Zmax and Zmin for "> 100"?

Furthermore. What is "RCal" for? If I do not use it in any equation.

In the example on page 7, the Zmin value is 4.7K and the Zmax value is 47K. Why? I dont understand.

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  • Hi,

    1- the Rfb should be same value that the resistance connected to Vin pin, otherwise internal TIA will amplify your input signal.

    2-You can run all the circuit in single supply, +5V

    3- The impedance depends on the distance between electrodes... typically harm to harm is around 1kohms + impedance of your electrodes.

    4- For the current generator, I used this circuit,

    The RC network are 4.7Mohms,1uF capacitors

    The current injected is controlled using R7.

    Regards,

    Miguel

Reply
  • Hi,

    1- the Rfb should be same value that the resistance connected to Vin pin, otherwise internal TIA will amplify your input signal.

    2-You can run all the circuit in single supply, +5V

    3- The impedance depends on the distance between electrodes... typically harm to harm is around 1kohms + impedance of your electrodes.

    4- For the current generator, I used this circuit,

    The RC network are 4.7Mohms,1uF capacitors

    The current injected is controlled using R7.

    Regards,

    Miguel

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