Analog switch leakage current can silently corrupt precision reference circuits, especially at elevated temperatures. Three SPST/SPDT switch topologies for a switchable 1.25V reference (gain of 1 or 2) are analyzed using real Nexperia part specs. Circuit A (SPST switch in series with the feedback path) yields the smallest worst-case error (2.5 mV max), while Circuit B suffers up to 12.5 mV error due to on-state leakage through the feedback resistor. Circuit C (SPDT mux switching the op-amp's negative input) performs well at gain-of-1 (20 µV) but hits 10 mV at gain-of-2. Op-amp input bias current is shown to be negligible compared to switch leakage. Practical mitigations include reducing feedback resistor values, limiting the operating temperature range, or choosing switches with lower leakage specs (e.g., ON Semi FSA4157 or Vishay DG4157 with 40 nA max on-state leakage).
