Systems and methods for measuring picoampere current levels

1. A circuit for measuring picoampere current levels, comprising:

an operational-amplifier that has differential inputs and differential outputs;

a switching structure that switchably couples an input line to one of said differential inputs in response to signals from a timing circuit;

a first integrating capacitor coupled to one of said differential inputs and to one of said differential outputs;

a second integrating capacitor coupled to the other of said differential inputs and to the other of said differential outputs;

a charge injection compensation structure that alternately injects charge into said input line and removes charge from said input line in response to signals from said timing circuit;

wherein said charge injection compensation structure comprises:

a first charge injection device that has a first width; and

a second charge injection device that has a second width that is different than said first width, wherein said second charge injection device is driven by a waveform from said timing circuit that is an inverse of a waveform that drives said first charge injection device.

2. The circuit of claim 1 wherein said operational-amplifier, said switching structure, said first and second integrating capacitors, and said charge injection compensation structure are integrated on a single integrated circuit.

3. The circuit of claim 1 wherein said switching structure switchably couples an analog ground to the other of said differential inputs of said operational-amplifier in response to a timing circuit.

4. The circuit of claim 1 wherein said switching structure switchably couples a bias level to the other of said differential inputs of said operational-amplifier in response to a timing circuit.

5. The circuit of claim 1 wherein said operational-amplifier has an adjustable direct-current (DC) offset for said differential inputs, wherein said DC offset is a non-zero offset adjusted to minimize charge injection caused by switching of said switching structure.

6. The circuit of claim 1 further comprising:

a gain stage that is operable to amplify said differential outputs of said operational-amplifier.

7. The circuit of claim 1 further comprising:

a differentiator for differentiating said differential outputs of said operational-amplifier.

8. A method for integrating a signal to facilitate measurement of picoampere current levels, comprising:

receiving said signal on an input line;

switchably providing said signal to differential inputs of an operational-amplifier in response to a timing circuit;

integrating said signal utilizing a first integrating capacitor coupled to a first input of said differential inputs and to a first output of differential outputs of said operational-amplifier and a second integrating capacitor coupled to a second input of said differential inputs and a second output of said differential outputs;

alternately injecting charge into and removing charge from said input line in response to said timing circuit;

wherein said alternately injecting and removing charge includes:

driving a first charge injection device that has a first width according to a first waveform; and

driving a second charge injection device that has a second width that is different than said first width, wherein said second charge injection device is driven by a second waveform that is an inverse of said first waveform.

9. The method of claim 8 further comprising:

differentiating said differential outputs of said operational-amplifier.

10. The method of claim 8 wherein said receiving, switchably providing, integrating, and said selectively injecting and removing charge are performed on a single integrated circuit.

11. The method of claim 8 further comprising:

switchably providing an analog ground to an input of said different inputs opposite to said input that receives said signal in response to a timing circuit.

12. The method of claim 8 further comprising:

switchably providing a bias level to an input of said different inputs opposite to said input that receives said signal in response to a timing circuit.

13. The method of claim 8 wherein said operational-amplifier has an adjustable direct-current (DC) offset for said differential inputs, wherein said DC offset is a non-zero offset adjusted to minimize charge injection caused by switching of said switching structure.

14. The method of claim 8 further comprising:

amplifying said differential outputs of said operational-amplifier.

15. A circuit for measuring picoampere current levels, comprising:

operational-amplifier means for amplifying differential inputs;

switching means for switchably coupling an input line to one of said differential inputs in response to signals from a timing circuit;

first capacitor means, for integrating a signal from said input line, coupled to one of said differential inputs and to one of differential outputs of said operational-amplifier means;

second capacitor means, for integrating said signal from said input line, coupled to the other of said differential inputs and to the other of differential outputs of said operational-amplifier means;

charge injection compensation means for alternately injecting charge into said input line and removing charge from said input line in response to signals from said timing circuit;

wherein said charge injection compensation means comprises:

a first charge injection means having a first width; and

a second charge injection means having a second width that is different than said first width, wherein said second charge injection means is driven by a waveform from said timing circuit that is an inverse of a waveform that drives said first charge injection means.

16. The circuit of claim 15 further comprising:

gain stage means for amplifying said differential outputs of said operational-amplifier.

17. The circuit of claim 15 wherein said operational-amplifier means, said switching means, said first and second capacitor means, and said charge injection compensation means are integrated on a single integrated circuit.