SPRUJ53B April 2024 – September 2024 TMS320F28P550SJ , TMS320F28P559SJ-Q1
Some applications cannot tolerate having an RC network at the output of the amplifier. Amplifier output current flowing through the filter resistor creates a voltage offset that introduces output error. In this case, one can opt to filter the noise spikes by placing a feedback capacitor across the feedback loop. In fact, a simple single-pole low pass filter can be implemented by placing an external capacitance between PGA_INM and PGA_OUT in internal gain mode, which is shown in Figure 18-6.
This form of very simple filter is normally used in instances where a small amount of roll off is required. PGA feedback node RTAP can be brought on pin PGA_INM for external filtering by selecting PMUXSEL = 00, NMUXSEL = 01, and MMUXSEL = 01. Also, to enable this mode, PGA_FB_ON_PIN input/register needs to be set to 1.
The cutoff frequency for this type of filter can be calculated very easily by working out the frequency at which the reactance of the capacitor equals the resistance of the resistor Rib, which is defined by Equation 17:
The gain for this op amp circuit can be calculated in the normal way ignoring the effect of the capacitor: