SPRUJ53B April 2024 – September 2024 TMS320F28P550SJ , TMS320F28P559SJ-Q1
The PGA supports chopping feature, which is a continuous-time modulation technique in which the signal and offset are modulated to different frequencies. The motivation behind adding this feature is primarily to reduce input referred offset to a very small number (in micro volt order). Not only chopping is dynamic technique that continuously reduces offset, but also this technique removes low frequency 1/f noise as well as offset drift over temperature or time.
The chopping feature is demonstrated in Figure 18-9. In broad terms, an input voltage first passes through a chopper driven by a clock at frequency fch; so, the voltage is converted to a square-wave voltage. Then, the modulated signal along with the offset are amplified by the fully differential amplifier. The second chopper then demodulates the amplified input signal back to DC, while the offset is shifted to the frequency of the clock fch harmonics. The DC offset can be filtered out by a low-pass filter.
This device supports ADC-assisted method. In this method, the chopping control signal comes from the on-chip ADC module. Moreover, Start of Conversion (SOC) in the ADC module can be used to implement chopping. In this method, PGA output is sampled by the on-chip ADC module. Note that the ADC-assisted method relies on ADC module for filtering the output instead of using external low-pass filter. To remove the offset, an average of the output signal can be taken in ADC module. This functionality is provided in post-processing block in ADC module. (See Chapter 15). The ADC-assisted chopping stands out from purely analog chopping methods due to the ability to offer higher accuracy and precision.
Figure 18-10 shows the general diagram of ADC-assisted chopping mode, in which a logic block is used to generate the necessary signal to handle chopping. To configure the ADC-assisted chopping, write to the PGA_CHOP field in the MUXSEL register. To configure control signal for ADC-assisted chopping, write to the CHOP_EXTCTRL field in the PGACTL register.
This logic takes care of different scenarios, like two ADCs using the same PGA at different time intervals. For a given PGA, the ADC assisted chopping feature is supported on ADCs which are connected to PGA_OUT_INT for that PGA. Table 18-4 shows the connectivity between PGA and ADC modules.
PGA Connectivity to ADC | ADC |
---|---|
PGA1_OUT_INT | A21, B21 |
PGA2_OUT_INT | C21, B22 |
PGA3_OUT_INT | C22, A22 |