Mitigating Switched Capacitor ADC Load Transient Drift in Integrated References
Mitigating switched-capacitor reference droop requires a low-ESR reservoir capacitor sized to charge-redistribution ratios and Kelvin-connected to analog ground.
Reservoir Capacitor Sizing
Design Calculation
An analytical calculation used in power distribution and decoupled circuit networks determines the minimum capacitance needed to prevent excessive voltage drop during transient events. This reservoir capacitor sizing takes into account both the peak load current and the switching duration. If the capacitance is too small, the reference voltage will drop below the noise margin.
Designers must use the worst-case current demand to establish the safety boundary.
Circuit Mechanism
When a switched-capacitor load is activated, it draws charge from the nearest energy storage element. Through reservoir capacitor sizing, engineers balance the charge transferred to the load against the energy stored in the reservoir. A larger capacitor holds more charge but requires more physical space.
This balance is critical in compact sensor designs where board area is limited.
Optimization Choice
Selecting the right capacitance value requires a trade-off between board space and voltage stability. Designers often choose a standard value that exceeds the calculated minimum by a factor of two. This margin accounts for component tolerances and temperature drift.
It protects the circuit from unexpected fluctuations.
Layout Effect
Parasitic inductance from long circuit board traces can defeat the purpose of a large capacitor. Placing the reservoir capacitor directly next to the pins reduces this trace inductance.