Charge comparator with low input offset
A Direct Current (DC) charge comparator that provides low input offset by feeding complimentary plus and minus charge inputs to a single amplification path via an alternate input path switch. Multiple sample and hold circuits at the output of the amplification path permit comparison of the result when each of the charge inputs travels down each of the paths, to determine a correction.
1. A charge comparator comprising:
a switch connected to receive a pair of charge inputs as a plus charge input and a minus charge input, with the switch having a first state wherein the plus signal charge is coupled to a first output and the minus signal charge is coupled to a second output, and a second state wherein the plus charge input is coupled to the second output and the minus charge input is coupled to the first output;
a pair of charge integrators, connected to the first and second switch outputs, to provide a first integrated output signal and a second integrated output signal;
a pair of sample and hold circuits, connected to sample and hold respective ones of the first and second integrated output signals; and
a difference amplifier, coupled to the pair of sample and hold circuits, to determine a difference between the outputs of the sample and hold circuits.
2. A charge comparator comprising:
a switch connected to receive a pair of charge inputs as a plus charge input and a minus charge input, with the switch having a first state wherein the plus signal charge is coupled to a first output and the minus signal charge is coupled to a second output, and a second state wherein the plus charge input is coupled to the second output and the minus charge input is coupled to the first output;
a pair of charge integrators, connected to the first and second switch outputs, to provide a first integrated output signal and a second integrated output signal;
a pair of sample and hold circuits, connected to sample and hold respective ones of the first and second integrated output signals;
a difference amplifier, coupled to the pair of sample and hold circuits, to determine a difference between the outputs of the sample and hold circuits; and
a signal amplifier, connected to the output of the integrators, to provide an amplified output thereof.
3. A charge comparator as in claim 2 additionally comprising:
a second pair of sample and hold circuits, connected to cooperate with the first pair of sample and hold circuits, such that in a first mode, the first pair of sample and hold circuits stores respective integrated charge samples from the plus charge input and minus charge input, and such that in a second mode, the second pair of sample and hold circuits stores the plus signal charge and the minus signal charge, respectively.
4. A charge comparator as in claim 3 wherein the difference amplifier is connected to determine the difference between the output of the first pair of sample and hold circuits in a first mode and is connected to determine a difference in the outputs between the second pair of sample and hold circuit outputs in a second mode.
5. A charge comparator as in claim 2 wherein the charge integrators integrate the respective input charges over multiple clock periods prior to operating the sample and hold circuit.
6. A charge comparator as in claim 2 additionally wherein the difference amplifier compares a charge integrator result at a first time with a charge integrator result a second time, such that a plus charge input applied with the switch in one state is compared with the plus charge input with the switch compared in another state.
7. A charge comparator as in claim 2 wherein the amplifier output in a non-inverted switch state is compared to an amplifier output in an inverted switch state, such that when fed to the differential difference amplifier any input offset is amplified in the same way for both the inverted and non-inverted switch conditions, to cancel an input offset.
8. A charge comparator as in claim 1 additionally comprising:
a second pair of sample and hold circuits, connected to cooperate with the first pair of sample and hold circuits, such that in a first mode, the first pair of sample and hold circuits stores respective integrated charge samples from the plus charge input and minus charge input, and such that in a second mode, the second pair of sample and hold circuits stores the plus signal charge and the minus signal charge, respectively.
9. A charge comparator as in claim 8 wherein the difference amplifier is connected to determine the difference between the output of the first pair of sample and hold circuits in a first mode and is connected to determine a difference in the outputs between the second pair of sample and hold circuit outputs in a second mode.
10. A charge comparator as in claim 1 wherein the charge integrators integrate the respective input charges over multiple clock periods prior to operating the sample and hold circuit.
11. A charge comparator as in claim 1 additionally wherein the difference amplifier compares a charge integrator result at a first time with a charge integrator result a second time, such that a plus charge input applied with the switch in one state is compared with the plus charge input with the switch compared in another state.
12. A charge comparator as in claim 1 wherein the amplifier output in a non-inverted switch state is compared to an amplifier output in an inverted switch state, such that when fed to the differential difference amplifier any input offset is amplified in the same way for both the inverted and non-inverted switch conditions, to cancel an input offset.