Differential current measurements to determine ion current in the presence of leakage current

An apparatus for detecting smoke, comprising:

• an ionization chamber having a radiation source and comprising first and second electrodes, wherein the ionization chamber is open to smoke ingress;
• a pulse generator coupled to the first electrode of the ionization chamber, wherein the pulse generator output comprises voltage transitions from substantially zero volts to substantially a voltage;
• a capacitor coupled to the second electrode of the ionization chamber;
• a precharge voltage reference;
• a precharge switch coupled between the precharge voltage reference and the capacitor, wherein the precharge switch initially couples the precharge voltage reference to the capacitor for charging the capacitor to a first voltage;
• a voltage determination circuit coupled to the capacitor;
• and
• a pulse counter coupled to the pulse generator and counting a number of pulses therefrom, wherein the pulse counter counts the number of pulses from the pulse generator required to charge the capacitor to a second voltage.