We are developing 64 channels EEG recording boards which are already operating.
Actually we are quite satisfied with the acquisition signal quality.
Now we are having problems in the use of the LEAD IMPEDANCE MEASURE using the AC LEAD OFF FUNCTION. The measure is pretty good if we use single resistors connected between each input and the common reference (BIAS OUT), but it is not acceptable when the measure is done on the patient because also the BIAS OUT is connected to the patient with an unknown resistance.
The error on the patient impedance measure is great because all the internal current sources have the same signal phase. This cause the summation of all the current at the common ground reference (BIAS OUT) node and alter the result of the impedance measurement , even if the positive and negative input are enabled at different time.
This problem could be strongly reduced if one half of the current sources would be in opposite phase in respect of the other half.
We have tried to overcome the problem using the DC LEAD OF function by using the Input current source LOFF_FLIP=0 or 1 alternatively . In this case the output signal is not a square wave as we expect but a positive or negative pulse of the duration of a single sample and then the signal become flat until the next change in the LOFF_FLIP. Beside the pulse phase does not always correspond to the LOFF_FLIP setting.
We are not using any high pass filter and the inputs are connected with resistor to the common reference (BIAS OUT).
Changing the input current source phase using the DC LEAD OFF excitation with the LOFF_FLIP bit we thought to generate a square wave signal. How can we do that?
Thanks for any help.