Input voltage-induced spurious current canceler for autozero type comparator in analog-to-digital converters

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT FIG.
6 shows the basic scheme of this invention.
A sampling switch SWi is connected to an analog input signal Vi and another sampling switch SWr is connected to a reference voltage Vref.
The output side of these sampling switches are connected together to sample Vi during the autozero mode and to sample Vref during the campare mode.
The common connection of these two switches is sampled with a voltage Vx and connected to a T-network.
The T-network has two capacitors C1 and C2 connected in series to the input of an inverter INV with a voltage Vy.
The joint between the two capacitors is shunted to ground through a control switch SW2, and has a voltage V*.
The inverter is short-circuited between the input Vy and the output Vz through the reset switch SW1 during the autozero mode.
There is no connection between Vy and Vz during the compare mode.
The T-network constitutes a current canceler.
During the autozero mode, the spurious current induced by Vi flows through SW2 and is bypassed by the current canceler.
There is no current flowing through C2 and SW1.
The current canceler can be constructed with MOS transistors (MOSFET) as shown in FIGS.
7(a), (b), and (c).
In FIG.
7(a), an MOSFET M1 is connected between V*, and ground as the shunt branch of the T-network.
In FIG.
7(b), an MOSFET M2 is connected between V*, and the positive supply V.
sub.
DD as the shunt branch of the T-network.
In FIG.
7(c), two complementary MOSFETs Mp and Mn are connected respectively to V.
sub.
DD and ground to serve as the shunt branch of the T-network.
All three circuits provide low impedance path to ground at point V*.
During the autozero mode of the ADC, the operation is shown in FIG.
8(a).
SWi is on, SWr is off, SW1 is on and SW2 is on.
The left-hand electrode of C1 is impressed with a voltage Vi.
The right-hand electrode of C1 is at a potential V*, which is approximately equal to zero.
Any spurious current induced by Vi flows from V* to ground through SW2, and no longer flows toward C2 to the right-hand side of V*