The 4-20 mA control-loop stability must be evaluated for any XTR11x design. A 10‑nF decoupling capacitor between V+ and I_O is recommended for most applications. As this capacitance appears in parallel with the load resistance R_LOAD from a stability perspective, the capacitor and resistor form a filter corner that can limit the bandwidth of the system. Therefore, for HART applications, use a bypass capacitance of 2 nF to 3 nF instead.

For applications with EMI and EMC concerns, use a bypass capacitor with sufficiently low ESR to decouple any ripple voltage from the V_LOOP supply. Otherwise, the ripple voltage couples onto the 4‑mA to 20‑mA current source, and appears as noise across R_LOAD after the current-to-voltage conversion.

Additionally, stability concerns apply to the V_REF reference buffer when driving capacitive loads. Figure 8-4 shows that two filtering capacitors are required, one C_HF of 10 pF to 0.5 µF and another C_LF of 2.2 µF to 22 µF. Either a series isolation resistance R_ISO or a snubber R_COMP is used, depending on application requirements.

(1) Required compensation components.

Figure 8-4 Stable Operation With Capacitive Load on V_REF