Is it necessary to use 68 kOhm resistors with the AD2S1205?
Two 68 kOhm resistors on the SIN/COS inputs are required by the AD2S1205 to dectect all wire break condition, for example complete resolver disconnection. If these two resistors are not used the AD2S1205 may not trigger LOS fault (Loss of Signal) however 1, 2 and 3 wire break should be detectable.
Please be aware that upon complete resovler disconnection the differential DC signal may present on the SIN/COS inputs and this signal is the cause of the missing LOS fault. The 68 kOhm resistor simply reduces amplitude of the DC signal such as the LOS fault can be triggered.
It should be noted that these 68 kOhm may be not populated if all wire break detection is not required by the system.
Can a higher value resistor (100- 200 kOhm) be used instead and still successfully trigger LOS?
Any resistor value between 50KΩ - 100KΩ can be used to minimize the DC component so that LOS will be triggered upon signal disconnection. The tolerance of the resistor is not critical, 5% tolerance components is sufficient.