An integrated CMOS baseline restorer based on a switched current-conveyor

We present a fully integrated CMOS gated baseline restorer for shaper amplifiers with shaping times of up to 10 μs. As is well known a baseline restorer can be thought of as a switched C-R network. A switch enables the C-R differentiator along the baseline and excludes it on the signal. At moderate count rates, time constant CR should be chosen equal or larger than three times the shaping time (τs), otherwise the signal-to-noise ratio of the system would worsen significantly. If τs is of the order of 10 μs, a recovery time as long as 30 μs must be used in the baseline restorer. A so-long time constant cannot be realized with a passive network in an integrated circuit, because large-value capacitors and resistors require a too large chip area. We have addressed this issue by using a switchable current-conveyor structure that behaves like a large-value resistor. A switch permits to exclude the current-conveyor equivalent resistor when signals occur. The gate signal is provided by an amplitude discriminator, placed at the baseline-restorer output.