An electrical circuit includes a sensor configured to generate a current signal comprising a first portion comprising a contribution from a target source and/or a second portion comprising a contribution from sources other than the target source, a trans-impedance amplifier that amplifies the current signal and generate a low noise signal, and a high pass filter that converts the low noise signal into an AC signal having a positive amplitude, a negative amplitude, and a zero cross-over point between the positive and negative amplitudes. The circuit also includes a positive integrating amplifier that receives the positive amplitude of the AC signal and generates a positive integrated value over an integration period, and a negative integrating amplifier that receives the negative amplitude of the AC signal and generates a negative integrated value over the integration period. The circuit further includes at least one analog-to-digital converter that receives the integrated values.
