Method and apparatus for minimizing noise pickup in image sensors
View Patent ↗A method and apparatus for minimizing noise pickup in iSoC sensors using an improved Analog Capacitor Memory (ACM) design and optimized timing methods that together prevent direct connection of the electrical grounds between the low-speed signal processing circuit and high-speed signal processing circuit of an imaging System-on-Chip sensor. The ACM includes a two-terminal capacitor and two pairs of terminals. Each pair of terminals is connected via switches to separate circuits. The switches are controlled to isolate one side of the ACM from the other, thereby reducing the noise pickup between the circuits.
1. An Analog Capacitor Memory (ACM) cell comprising:
a capacitor having a first node and a second node;
a first switch connected to the first node;
a second switch connected to the second node;
a third switch connected to the first node;
a fourth switch connected to the second node;
the first and second switches are connected to a first circuit; and
the third and fourth switches are connected to a second circuit;
wherein the first and second switches are closed and the third and fourth switches are open during a first period, and wherein the first and second switches are open, and the third and fourth switches are closed during a second period, such that a ground domain of the first circuit is decoupled from a ground domain of the second circuit; and
wherein the second switch is opened before the first switch to remove any voltage dependent charge injection, and wherein the fourth switch is opened before the third switch to remove any voltage dependent charge injection.
2. The ACM cell of claim 1 , wherein the first and second switches are connected to a first circuit operating at a first frequency.
3. The ACM cell of claim 2 , wherein the third and fourth switches are connected to a second circuit operating at a second frequency.
4. The ACM cell of claim 3 , wherein the first circuit is a column buffer circuit, and the second circuit is a line driver amplifier circuit, and wherein the first frequency is lower than the second frequency.
5. The ACM cell of claim 1 , wherein the first period is a write operation, and the second period is a read operation.
6. The ACM cell of claim 3 , wherein the capacitor comprises a Metal-Insulator-Metal capacitor.
7. An image sensor comprising:
a first circuit operating at a first frequency;
a second circuit operating at a second frequency;
a plurality of Analog Capacitor Memory (ACM) cells connecting the first and second circuits, each ACM cell comprising:
a capacitor having a first node and a second node;
a first switch connected to the first node;
a second switch connected to the second node;
a third switch connected to the first node; and
a fourth switch connected to the second node;
wherein the first and second switches are connected to the first circuit, and the third and fourth switches are connected to the second circuit;
wherein the first and second switches are closed and the third and fourth switches are open during a first period, and wherein the first and second switches are open, and the third and fourth switches are closed during a second period, such that a ground domain of the first circuit is decoupled from a ground domain of the second circuit; and
wherein the second switch is opened before the first switch to remove any voltage dependent charge injection, and wherein the fourth switch is opened before the third switch to remove any voltage dependent charge injection.
8. The ACM cell of claim 7 , wherein the first circuit is a column buffer circuit, and the second circuit is a line driver amplifier circuit, and wherein the first frequency is lower than the second frequency.
9. The ACM cell of claim 7 , wherein the first period is a write operation, and the second period is a read operation.
10. The ACM cell of claim 7 , wherein the capacitor comprises a Metal-Insulator-Metal capacitor.
11. A method of isolating ground noise between a first circuit and a second circuit in an image sensor, the method comprising:
connecting the first circuit to the second circuit with at least one Analog Capacitor Memory (ACM) cell, wherein the ACM cell comprises:
a capacitor having a first node and a second node;
a first switch connected to the first node;
a second switch connected to the second node;
a third switch connected to the first node; and
a fourth switch connected to the second node;
wherein the first and second switches are connected to the first circuit, and the third and fourth switches are connected to the second circuit;
closing the first and second switches and opening the third and fourth switches during a first period; and
opening the first and second switches and closing the third and fourth switches during a second period;
wherein the at least one ACM cell isolates the first circuit from the second circuit such that a ground domain of the first circuit is decoupled from a ground domain of the second circuit; and
wherein the second switch is opened before the first switch to remove any voltage dependent charge injection, and wherein the fourth switch is opened before the third switch to remove any voltage dependent charge injection.