How can I calculate the maximum value an input pull-up (or pull-down) resistor can be, to detect a lead-off condition for ECG? A pull-up resistor is used to pull the input high if the electrode becomes disconnected from the patient. 10MOhms is often used, but this greatly reduces the input impedance of the circuit, and the source impedance is normally 100-300kOhms (with mandatory patient protection resistors). It would be much better to use 100MOhms or even 1GOhm, but how can I determine the max value without using trial and error on a whole batch of amps. Is it directly proportional to the input bias current of the op-amp or in-amp, or the input impedance, or any other info from the datasheet?
The op-amps I am looking at in particular are OPA4316 and OPA4377, but I would like to know how to calculate it for any amp.
Thanks.