Offset cancellation for sampled-data circuits
A comparator based circuit with effective offset cancellation includes first and second amplifiers and an offset capacitor operatively connected to the first and second amplifiers. An offset voltage source generates an offset voltage. A first switch connects the offset voltage source to ground during a first time period. The first amplifier generates an output voltage in response to the first switch connecting the offset voltage source to ground during the first time period. A second switch connects the offset capacitor to ground during a second time period. The first switch disconnects the offset voltage source from ground during a third time period, and the second switch disconnects the offset capacitor from ground during the third time period.
1. A comparator with effective offset cancellation, comprising:
a first amplifier having an input terminal;
an offset capacitor operatively connected to said first amplifier;
a second amplifier operatively connected to said offset capacitor;
a first switch operatively connected to said input terminal;
a second switch operatively connected between said offset capacitor and a first predetermined voltage; and
a control circuit to control operations of said first and second switches;
said first switch, in response to said control circuit, connecting said input terminal to a second predetermined voltage for a period of time;
said second switch, in response to said control circuit, connecting said offset capacitor to the first predetermined voltage for said period of time;
said period of time being substantially shorter than a time constant associated with said offset capacitor to reduce a noise bandwidth of a sample signal from said offset capacitor.
2. The comparator as claimed in claim 1 , wherein said first predetermined voltage is a system common-mode voltage.
3. The comparator as claimed in claim 1 , wherein said second predetermined voltage is the system common-mode voltage.
4. The comparator as claimed in claim 2 , wherein said system common-mode voltage is ground.
5. The comparator as claimed in claim 3 , wherein said system common-mode voltage is ground.
6. A comparator with effective offset cancellation, comprising:
a first amplifier having an input terminal;
an offset capacitor operatively connected to said first amplifier;
a second amplifier operatively connected to said offset capacitor;
a first switch operatively connected to said input terminal;
a second switch operatively connected between said offset capacitor and a first predetermined voltage;
a control circuit to control operations of said first and second switches; and
a noise averaging circuit operatively connected to said offset capacitor;
said first switch, in response to said control circuit, connecting said input terminal to a second predetermined voltage for a period of time;
said second switch, in response to said control circuit, connecting said offset capacitor to the first predetermined voltage for said period of time;
said period of time being substantially shorter than a time constant associated with said offset capacitor to reduce a noise bandwidth of a sample signal from said offset capacitor.
7. The comparator as claimed in claim 6 , wherein said first predetermined voltage is a system common-mode voltage.
8. The comparator as claimed in claim 6 , wherein said second predetermined voltage is the system common-mode voltage.
9. The comparator as claimed in claim 7 , wherein said system common-mode voltage is ground.
10. The comparator as claimed in claim 8 , wherein said system common-mode voltage is ground.
11. The comparator as claimed in claim 6 , wherein said noise averaging circuit includes a second offset capacitor.