Can anyone recommend the best circuit for skin to electrode impedance measurement using the AD5933.

Question

Hello, 
 I am trying to build a portable system to measure skin to electrode impedance using the AD5933. Is there a recommend circuit i should follow? 
 Do i need to be reducing the current to make it safe to connect to a person? 
 Any circuit diagrams, tips or suggestions would be greatly appreciated. 
 Kind Regards, 
 James

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musach · Answer

Hi James, 
 If helps, one of the electrodes is connected to a Howland current source (controlled by the AD5933) and the other extreme to a bias voltage , typically Vdd/2. 
 An In-amp is used to convert from current to voltage and feedback the AD5933. 
 The electrode impedance is quite high so, ensure that the current source has high output impedance. 
 Regards, 
 Miguel

nicsky · Answer

I wouls think the App note that describes "measuring Grounded impedances" would work well, 
 You need to make sure that the Return voltage is correctly biased to match the imput circuit of the Ad5933 
 The IA does this nicely with the ref cct. You can set it to the right level, 
 AC coupling the output will keep the DC bias at zero irrespective of the output selected. 
 The only thing left is t use a Isolator such as the 4Ch isolator with built in Pwr reg. I forgot th part. 
 This gives 2W and Isolation is good enough for you i think. If you need more you could use Te ADUM2401 and a max845 plus halo transformer. 
 I have used this for medical devices and it will pass regs

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Can anyone recommend the best circuit for skin to electrode impedance measurement using the AD5933.