Abstract: The toner concentration of a xerographic reproduction device is maintained by use of a patch sensor, wherein the photoconductor's test patch is toned while the photoconductor voltage is substantially zero, and while the developing field is provided by a development electrode voltage source whose polarity is opposite that which is used during reproduction.

Abstract: A circuit providing digital representation of an analog error voltage present as an output of a bridge circuit where the digital representation is independent of supply voltage variations to the bridge circuit. The circuit shows a capacitor designed to ramp from an initial voltage representative of the zero point of the analog error voltage range to a full-scale error voltage in the amount of time it takes for a digital counter to count from zero to full scale. The ramping voltage is compared to the error voltage in order to decode error count whenever the ramping voltage reaches a value just in excess of the error voltage.