Digital isolation system with ADC offset calibration including coarse offset

摘要

A digital capacitive isolation barrier system is provided that is suitable for use in a telephone or modem where the locally powered circuits must be effectively isolated from the public telephone system, while permitting data transfer across the barrier. In particular, an automatic ADC offset calibration system is provided for determining the magnitude of the ADC offset signal required in the system during a calibration operation, and for providing the ADC offset signal during normal operation of the isolation barrier system. The offset calibration system includes a coarse offset signal generator which provides ;elected increments of offset voltage to the ADC outside of the outgoing data signal channel, In order to increase the calibration range and to avoid injecting large offset voltages into the outgoing data channel. Fixed bias signals are also provided for the ADC and for a DAC in the system. In a preferred embodiment, the ADC is located on the isolated side of the isolation barrier, while the integrator and register that determine and hold the offset signal are located on the powered side of the isolation barrier.