1. Apparatus for calibrating output voltage levels of a current-integrating summing amplifier, comprising: a duplicate integrator circuit, wherein the duplicate integrator circuit replicates an integrator circuit of the current-integrating summing amplifier; a comparing circuit coupled to the duplicate integrator circuit, the comparing circuit comparing at least one output voltage level generated by the duplicate integrator circuit with a reference voltage level; and a feedback loop circuit coupled to the comparing circuit and the duplicate integrator circuit, the feedback loop circuit adjusting at least one bias signal of the duplicate integrator circuit so that the output voltage level generated by the duplicate integrator circuit matches the reference voltage level, wherein the bias signal is applied to the integrator circuit of the current-integrating summing amplifier thereby calibrating output signal components due to multiple input signals of the current-integrating summing amplifier.