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AD8605 Howland Current Source unstable for bioimpedance

Question asked by eastercountry on Jan 20, 2017
Latest reply on Jan 27, 2017 by musach



I have build an impedance plethysmography sensor using the AD5933. Furtrhermore i used an AFE for four wire measurement. The Vout pin from AD5933 controls the Howland Current Source with AD8605.

Rgain=2k & R=12k ; Vcc=+3v3; i_out=1mA; f=10-100kHz

The electrodes are attached at the forearm. Unfortaneately this configuration is unstable. The direct output from AD8605 sometimes cutting GND or VCC or the amplitude of the output voltage swings with 2-5 Hz

I have already tried many things: R=1k-100k, without or with feedback capacitor 5pF,100pF. But it has no problems with resistors until 1k. Does somebody has any idea?


best regards





Howland Current Source