Autotracking feedback circuit and high speed A/D converter using same

DETAILED DESCRIPTION A detailed description of preferred embodiments of the present invention is provided with respect to FIG.
1 through FIG.
9.
FIG.
1 shows a simplified block diagram of a pipelined analog to digital converter which includes the common mode feedback circuit and the active common mode feedback bias circuit according to the present invention.
The pipelined analog to digital converter receives an analog input voltage on line 10 at the input of a sample and hold amplifier 11.
The output of the sample and hold amplifier 11 is provided to an analog to digital converter 12 which produces an output bit on line 13.
The output bit on line 13 is provided to one-bit digital to analog converter 14, the output of which is supplied to the negative terminal of a summing node 15.
The positive terminal of the summing node 15 receives the output of the sample and hold amplifier 11.
Thus the difference between the input voltage provided by the sample and hold amplifier 11 and the analog value of the digital signal on line 13 is provided as input to an interstage amplifier 16.
The output of interstage amplifier 16 is provided to a subsequent stage in the analog to digital converter, which includes stage 2 (not shown) through stage M.
The digital values provided by the stages 1 through M, are provided to error correction logic 20 which provides the output of the pipelined analog to digital controller on line 21.
According to the present invention, the sample and hold amplifier 11 includes a common mode bias terminal which receives a common mode voltage VCM.
A common mode feedback circuit 22 is coupled to the output of the sample and hold amplifier 11, and provides the common mode voltage VCM to the amplifier 11.
In addition, an active common mode feedback bias circuit 23 produces a common mode feedback bias voltage VCMB, is included which causes automatic tracking of the common mode voltage with variations in temperature, process, operating speed and other factors.
In a similar manner, the interstage amplifier 16 includes a common mode feedback circuit 24, with an active common mode feedback bias circuit 25 which provides auto tracking