An offset-free LPF for /spl pi//4-shift QPSK signal generator

With progress of on-chip digital signal processing capability, analog signal processing circuit blocks associated with A-to-D converters (ADCs) and/or D-to-A converters (DACs) are required to satisfy increasingly stringent performance criteria. For example, low-pass filters (LPFs) with low dc offset for anti-aliasing and/or smoothing are required. Conventional on-chip LPFs such as switched-capacitor filters and active-RC LPFs exhibit offset voltages of tens of millivolts depending on specific circuit architecture. These offset voltages are too large for some applications. Also, low-voltage operation implies low offset to maintain dynamic range. This paper describes an intrinsically offset-free LPF for use with a /spl pi//4-shift QPSK signal generator for the Personal Handy Phone System (PHS).