Optical receivers with dc cancellation bias circuit and embedded offset cancellation
In optical receivers, cancelling the DC component of the incoming current is a key to increasing the receiver's effectiveness, and therefore increase the channel capacity. Ideally, the receiver includes a DC cancellation circuit for removing the DC component; however, in differential receivers an offset may be created between the output voltage components caused by the various amplifiers. Accordingly, an offset cancellation circuit is required to determine the offset and to modify the DC cancellation circuit accordingly.
1. An optical receiver comprising:
at least one photodetector capable of generating a differential input current including first and second input current components in response to an optical signal;
a trans-impedance amplifier (TIA) capable of converting the first and second input current components into first and second input voltage components;
a DC cancellation circuit (DCCC) for cancelling a DC portion of the first and second input current components or the first and second input voltage components by comparing the first and second input current components or the first and second input voltage components to respective first and second reference voltages or currents, and sinking the DC portion of the first and second input current components based on the comparisons; and
an offset cancellation circuit (OCC) capable of obtaining first and second voltage samples after the TIA and of cancelling offset between the first and second input voltage components by modifying at least one of the first or second reference voltages.
2. The optical receiver according to claim 1 , wherein the offset cancellation circuit comprises:
a voltage comparator for determining the offset by comparing the first and second voltage samples; and
a voltage summer for adding or subtracting the offset to or from one of the first or second reference voltages.
3. The optical receiver according to claim 1 , wherein the offset cancellation circuit comprises:
a voltage comparator for determining the offset by comparing the first and second voltage samples; and
a voltage summer for adding a portion of the offset to one of the first or second reference voltages and subtracting another portion of the offset from the other of the first or second reference voltages.
4. The optical receiver according to claim 1 , further comprising a variable gain amplifier (VGA) capable of amplifying the first and second input voltage components to a desired output voltage forming first and second output voltage components.
5. The optical receiver according to claim 4 , further comprising first and second OCC sensing points positioned between the TIA and VGA enabling the offset cancellation circuit to sample the first and second input voltage components, respectively, for the first and second sample voltages.
6. The optical receiver according to claim 1 , further comprising first and second OCC sensing points positioned after the VGA enabling the offset cancellation circuit to sample the first and second output voltage components, respectively, for the first and second sample voltages.
7. The optical receiver according to claim 4 , further comprising:
a driver stage after the VGA for driving subsequent stages of the optical receiver; and
first and second OCC sensing points positioned after the driver stage enabling the offset cancellation circuit to sample the first and second output voltage components, respectively, for the first and second sample voltages.
8. The optical receiver according to claim 1 , wherein the DC cancellation circuit comprises:
a voltage comparator for comparing a sample of each of the first and second input current components or the first and second input voltage components to the first and second reference voltages, respectively, generating first and second comparisons; and
a current sink for sinking the DC portions of the first and second input current components based on the first and second comparisons.
9. The optical receiver according to claim 8 , further comprising first and second DCCC sensing points positioned before the TIA enabling the DC cancellation circuit to sample the first and second input current components, respectively.
10. The optical receiver according to claim 8 , further comprising first and second DCCC sensing points positioned after the TIA enabling the DC cancellation circuit to sample the first and second input voltage components, respectively.
11. The optical receiver according to claim 10 , wherein the offset cancellation circuit comprises:
a voltage comparator for determining an offset by comparing the samples of the first and second input voltage components; and
a voltage summer for adding or subtracting the offset to or from one of the first or second reference voltages.
12. The optical receiver according to claim 8 , wherein the offset cancellation circuit comprises a weighted summer capable of adding or subtracting a difference between the first and second voltage samples, and modifying the first and second reference voltages used by the current sink based on the difference.