Column readout amplifier for image sensors
Examples include column readout amplifiers and image sensors including same. In one example, a column readout amplifier includes a signal amplifier having an amplifier output and first and second amplifier inputs, a filter capacitor having first and second terminals, the second terminal connected to a ground terminal, a buffer amplifier having a buffer amplifier input and a buffer amplifier output, a switching network configured to switchably connect the amplifier output to the buffer amplifier input and the buffer amplifier output to the first terminal of the filter capacitor during a first time period, and to switchably connect the amplifier output directly to the first terminal of the filter capacitor during a second time period, and a low-pass filter connected in a feedback path of the signal amplifier between the amplifier output and the first amplifier input, the low-pass filter including a series resistor and a capacitor.
1. A column readout amplifier comprising:
a signal amplifier having an amplifier output, a first amplifier input, and a second amplifier input, the second amplifier input being connected to a reference voltage terminal;
a filter capacitor having first and second terminals, the second terminal being connected to a ground terminal;
a buffer amplifier having a buffer amplifier input and a buffer amplifier output;
a switching network configured to switchably connect the amplifier output to the buffer amplifier input and the buffer amplifier output to the first terminal of the filter capacitor during a first time period, and to switchably connect the amplifier output directly to the first terminal of the filter capacitor during a second time period; and
a low-pass filter connected in a feedback path of the signal amplifier between the amplifier output and the first amplifier input, the low-pass filter including a series resistor and a capacitor.
2. The column readout amplifier of claim 1 , further comprising a capacitor connected between the amplifier output and the ground terminal.
3. The column readout amplifier of claim 1 , further comprising a feedback capacitor connected in series with the series resistor in the feedback path of the signal amplifier.
4. The column readout amplifier of claim 3 , wherein the signal amplifier comprises a capacitive transimpedance amplifier configured with a gain greater than one.
5. The column readout amplifier of claim 3 , wherein the capacitor is connected between a node between the series resistor and the feedback capacitor and the ground terminal.
6. The column readout amplifier of claim 1 , wherein a capacitance of the capacitor is smaller than a capacitance of the filter capacitor.
7. The column readout amplifier of claim 6 , wherein the capacitance of the filter capacitor is variable.
8. The column readout amplifier of claim 1 , wherein the buffer amplifier output is disconnected from the filter capacitor during the second time period.
9. The column readout amplifier of claim 1 , wherein the buffer amplifier has a gain substantially equal to one.
10. The column readout amplifier of claim 1 , further comprising an input capacitor connected to the first input of the signal amplifier.
11. An image sensor comprising:
a bit line conductor;
a pixel array having at least one column of addressable pixel sensors, each pixel sensor being coupled to the bit line conductor in response to a word select signal; and
a column amplifier connected to the bit line conductor, the column amplifier comprising
a signal amplifier having an amplifier output, a first amplifier input, and a second amplifier input, the second amplifier input being connected to a reference voltage terminal;
a filter capacitor having first and second terminals, the second terminal being connected to a ground terminal;
a buffer amplifier having a buffer amplifier input and a buffer amplifier output;
a switching network configured to switchably connect the amplifier output to the buffer amplifier input and the buffer amplifier output to the first terminal of the filter capacitor during a first time period, and to switchably connect the amplifier output directly to the first terminal of the filter capacitor during a second time period; and
a low-pass filter connected in a feedback path of the signal amplifier between the amplifier output and the first amplifier input, the low-pass filter including a series resistor and a capacitor.
12. The image sensor of claim 11 , further comprising a capacitor connected between the amplifier output and the ground terminal.
13. The image sensor of claim 11 , further comprising a feedback capacitor connected in series with the series resistor in the feedback path of the signal amplifier.
14. The image sensor of claim 13 , wherein the signal amplifier comprises a capacitive transimpedance amplifier configured with a gain greater than one.
15. The image sensor of claim 13 , wherein the capacitor is connected between a node between the series resistor and the feedback capacitor and the ground terminal.
16. The image sensor of claim 11 , wherein a capacitance of the capacitor is smaller than a capacitance of the filter capacitor.
17. The image sensor of claim 16 , wherein the capacitance of the filter capacitor is variable.
18. The image sensor of claim 11 , wherein the buffer amplifier output is disconnected from the filter capacitor during the second time period.
19. The image sensor of claim 11 , wherein the buffer amplifier has a gain substantially equal to one.
20. The image sensor of claim 11 , further comprising an input capacitor connected to the first input of the signal amplifier.