Abstract: A method and a circuit are provided for detecting low level ionic currents. An ionization detection voltage is applied to a spark plug and a resulting output voltage signal is sent to a threshold device. The output voltage signal is developed by a low offset operational amplifier. First and second serially connected Zener diodes carry current in a normal diode direction when a spark event occurs and carry current in their Zener breakdown mode upon recovery from the spark event. A second operational amplifier and its associated feedback circuit generate a guard voltage which is applied to the junction of the two Zener diodes. The guard voltage is regulated to track the input voltage to the circuit at the cathode of the first diode. Because the guard voltage is essentially the same as the input voltage, there is no leakage current flow through the first diode.