SPRUIZ1B July 2023 – August 2024 TMS320F28P650DH , TMS320F28P650DK , TMS320F28P650SH , TMS320F28P650SK , TMS320F28P659DH-Q1 , TMS320F28P659DK-Q1 , TMS320F28P659SH-Q1
A representative circuit with the OSDETECT implementation consists of the signal source with series resistance RS, shunt capacitor CP, the equivalent OSDETECT resistance ROSDETECT and voltage VOSDETECT is shown in Figure 18-30 and can be used as a basis to calculate the signal level going in to the sampling capacitor. ROSDETECT and VOSDETECT are the equivalent input resistance and voltage source contributed by the OSDETECT circuit with values shown in Table 18-12 for the different configuration settings. Refer to Figure 18-30 when deriving the input signal to S/H if signal source VS is driving while the OSDETECT feature is enabled.
The input impedance RS and CP are integral parts of the signal source or can have been implemented in the design to precondition the signal or to control signal settling time to meet S/H requirements. The input path has to be considered when using the OSDETECT feature, as this affects the conversion results. For instance, driving an input signal when this feature is enabled connects signal VS to the OSDETECT circuit through RS and affects the ADC results. Larger CP values (in the order greater than hundreds of pF) require using higher ACQPS to make sure the signal at the input has settled prior to conversion.
To enable the circuit:
Interpret the results based on what is driving on the input side and what are the values of RS and CP. If the VS signal can be disconnected from the input pin, the circuit can be used to detect open and shorted input pins as described in the following sections.