The input fail-safe circuitry on the MAX9206 is used to force the serial input high on the MAX9206 when the inputs are un-driven. The response time depends on the interconnect characteristics, and whether external pull-up circuitry is used. The input fail-safe circuitry is shown in Figure 1. Two resistors of value RIN1 are connected between the serial input pins to derive a common-mode voltage, and the intermediate node is pulled up to VCC through a resistor RIN2. When the sensed common-mode voltage rises above (VCC - 0.3V), a comparator forces the serial input high.
Assuming the V_CM = 1.1V, V_CC = 3.3V, the default values of RIN1 = 4kOhm and RIN2 = 150kOhm, and a total pin capacitance of 10pF, the time required to pull V_CM up to (VCC - 0.3V) can be approximated:
I = C*dV/dt and I = [V_CC - V(t)]/R_total, where R_total = RIN1 + RIN2
Re-arranging the terms in the equation:
dt = R_total * C_total * dV(t)/[V_CC - V(t)]
and integrating dV(t) from V_CM to (V_CC - 0.3V):
t = R_total * C_total * [-ln(V_CC - (V_CC - 0.3V)) - -ln(V_CC - V_CM)]
t = R_total * C_total * ln((V_CC - V_CM)/0.3)
t = 154kOhm * 10pF * ln(2.2/0.3) = 3.07us
If external pull-up resistors are added in parallel with the internal RIN1 and RIN2, the R_total term becomes the parallel combination of the external and internal networks. Note that the above calculation is just an approximation - measured results may vary accordingly.