The input filter R2, C7, C11 passes the excitation signal and suppresses higher harmonic content. The AC coupling removes any DC voltage which is important for the input circuitry in the TSD5402-Q1 device. Typically, a microcontroller generates the reference signal for the excitation amplifier. Two methods exist.

Modulated PWM is a very common design for the reference signal generation. The PWM frequency is set as high as possible. The desired excitation frequency modulates the duty cycle. A low-pass filter passes the signal of interest but removes the PWM harmonic content. PWM generators in microcontrollers use the system clock as the input. This means the system clock and desired resolution limits the maximum PWM frequency. Changing the modulation depth allows for fine adjustment of the output amplitude. Table 4-2 lists values for the PWM modulation when testing this design.

Digital to Analog Converter (DAC) is another possible design for the reference signal generation. This allows for fine tuning the signal parameters and liberates requirements for the low-pass filter. This design typically offers better performance and signal purity. However, not all systems and microcontrollers have the DAC available.