[Blocking input] Wrong resistor value
Blocking input should change the value of one resistor to reduce degradation of diode in the optoisolator.
Taking EDA schematics document as reference and going to page 10. R222 resistor, which is 100R must be replaced for 1K. Thus, a maximum value of 22 mA is driven into the LED of the optoisolator.
Do the same for R227,R219, R217, R215 and R226.
DC REJECTION FILTER*
Coming back to page 10, the DC rejection filter consist of a capacitance between C27 and C28 and a resistance between R226 and R213 in parallel with the dynamic resistance of the HCPL-0601 diode. Due to the dynamic resistance of the diode, the corner frequency will be slighty dependant on its working point. R213 desensitize the dependency effect from the diode.
So, after C27 and C28 the filtering is done and the corner frequency, given the values of all the components is explained below.
> Case A: diode not conducting
>
> If the diode is not conducting, its dynamical resistance is high, so
the parameters of the high pass filter are:
>
> C = 9,4 nF
> R = 1K + 4K7//1M = 5K7
> fc = 1/(2*pi*R*C) = 2,97 KHz
>
> Case B: diode conducting at 24V
> C = 9,4 nF
> R = 1K + 4K7//75 = 1K073.
> fc = 1/(2*pi*R*C) = 15,75KHz KHz
>
> this second is the worst case for the corner frequency.
CONSIDERATION ABOUT THE BLOCKING INPUT STAGE*
This stage was thought as being isolated from another source in the design stage. Later on, once the layout was almost finished, safety concerns arose about being isolated in the input stage. Finally, it was decided that this stage should not be isolated and the reference should be ground.
The main asset of this stage was isolation, however it won't be used that input isolation in V2. This input should be replaced for a similar one like in the front, with a 74LVC14AD which has good latch up, it's faster and has a better hysteris. However, instead of using a PIN diode, should be used a Schottky one for avoiding long fall time due to recovery isues. If using a PIN, a 1K resistor instead of a 100 should be used to avoid long recovery.