Impedance Measurement
Page 24 of the Intan datasheet shows a neat hack to provide an impedance measurement. A square 1kHz signal is provided by a pin, it gets split into two; copy (a) gets conditioned and goes to the Intan pin elec_test going out of the chip, and copy (b) gets rectified and fed into a cap, to turn on the impedance pin test_en on the Intan. The pins conn_all and VESD on the intan are permanently connected to ground.