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Hi, i've a problem with an evaluation circuit clone, sometimes there is like a glitch in the impedance curve. For example if i use a 560ohm as calibration resistance, with continuos sweep, sometimes the impedance trace shows a glitch only for one freq but not the same freq (e.s 200ohm @ 30kHz). Maybe a problem caused by the power supply, the layout or what?
Do you have a screen shot of this issue ? This is with the 16MHz crystal ?
thanks,here the screen shot of the problem, the 16MHz maybe good enought, i've tried with two power supply for the 5V ( digital) and 12V for the analog but nothing.I've to check if there is a problem with analog and digital ground. thanks again.
Data: 17-feb-2012 16.59
A: "Matteo Fattori"<email@example.com>
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Try breaking it into two excitation blocks, 1KHz to 20KHz and 20 KHz +.
For sub 5KHz we generally suggest reducing MCLK as per the User Guides recommendations.
Thanks but the problem came again,now i will try with an external 16MHz clock.
How did you zoom in to the plot in figure 5?
I used the download to excel tab and used the data on excel to generate a graph.Any luck with the external xtal oscillator that is on the board ?
Hi, i've a question about the choice of Rfeedback (Rfb or sometimes called Rref): there is a way to use the right value of Rfb with a knowed Z to measure?I've see in one thesis that the suggestion is use the ratio Rfb/Zunk between 0,066-0,2. In others words Zunk must be 15/2 times the Rfb. I don't find this spec in the datasheet and specially the spec doesn't match with my test.
Now the question there is a way to calculate the error for a gived Zunk, and Rfb?With a gived Rfb wich is the measure span of Zunk ?
Rfb sets the gain in the I-V amplifier.
Ideally, you should choose a Rfb value that generates less noise than the distortion of your impedance.
Remember, you are measuring harmonics so you don't want to hide harmonics due to the resistor noise.
Could you refer the thesis so I can read?
Sorry if I am repeating a known source. Macias' thesis is a nice and detailed application for AD5933 bioimpedance measurements:
Do you know if there is any limitations for the impedance measured? For example, the AD5933 datasheet tells me how to calculate the minimal impedance I can measure, but with this circuit, is there a maximum? Because if I try to measure an impedance that is the double of the one used for calibration it doesn't work.
You can use this GUi to roughly calculate Max and min impedances based on Rfb.
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