Output circuit for SPAD
An output circuit for a single-photon avalanche diode. In some embodiments, a system includes: a first sensing element; a bus line; a first pull-down circuit; a first pull-up element; and a second pull-up element, the first pull-up element, the second pull-up element, and the first pull-down circuit being connected to the bus line, the first pull-down circuit being connected to the first sensing element and configured to be activated based on a signal from the first sensing element, the first pull-up element having a lower drive strength than the first pull-down circuit.
1 . A system, comprising:
a first sensing element;
a bus line;
a first pull-down circuit;
a first pull-up element; and
a second pull-up element,
the first pull-up element, the second pull-up element, and the first pull-down circuit being connected to the bus line,
the first pull-down circuit being connected to the first sensing element and configured to be activated based on a signal from the first sensing element,
the first pull-up element having a lower drive strength than the first pull-down circuit.
2 . The system of claim 1 , wherein the first sensing element is a single-photon avalanche diode.
3 . The system of claim 1 , comprising:
a plurality of sensing elements including the first sensing element; and
a plurality of pull-down circuits including the first pull-down circuit,
each of the pull-down circuits being connected to the bus line,
each of the pull-down circuits being connected to, and configured to be activated based on a signal from, a respective sensing element of the plurality of sensing elements,
wherein the sensing elements and the pull-down circuits share the bus line and the first pull-up element.
4 . The system of claim 1 , wherein first the first pull-down circuit comprises a pull-down transistor.
5 . The system of claim 4 , wherein the first pull-down circuit further comprises a row-select transistor.
6 . The system of claim 5 , wherein the row-select transistor is connected between the pull-down transistor and the bus line.
7 . The system of claim 1 , wherein the first pull-up element and the second pull-up element together have a lower drive strength than the first pull-down circuit.
8 . The system of claim 7 , wherein:
the first pull-up element has a drive strength less than one-half of a drive strength of the first pull-down circuit, and
the second pull-up element has a drive strength less than one-half of a drive strength of the first pull-down circuit.
9 . The system of claim 1 , further comprising a buffer amplifier connected to the bus line.
10 . The system of claim 9 , wherein the buffer amplifier is connected to and end of the bus line.
11 . The system of claim 9 , wherein the buffer amplifier comprises a complementary metal oxide semiconductor (CMOS) inverter.
12 . The system of claim 11 , wherein the buffer amplifier comprises three CMOS inverters.
13 . The system of claim 1 , further comprising a bias circuit for applying a bias voltage to the first sensing element, the magnitude of the bias voltage being greater than 2.0 V.
14 . The system of claim 13 , wherein the magnitude of a voltage drop across the first pull-up element, when the bus line is pulled down by the first pull-down circuit, is less than 1.5 V.
15 . The system of claim 1 , wherein the first pull-up element comprises a transistor having a gate connected to a fixed voltage.
16 . The system of claim 1 , comprising:
a plurality of pull-up elements including the first pull-up element and the second pull-up element, and
a processing circuit, configured to turn on or off each of the pull-up elements according to a location of a pixel being read out.
17 . The system of claim 1 , comprising:
a plurality of pull-up elements including the first pull-up element and the second pull-up element; and
a pixel surveying circuit configured to detect faulty pixels, and to turn off the pull-up elements when a faulty pixel is being read out.
18 . A system, comprising:
a means for sensing;
a bus line;
a first pull-down circuit;
a first pull-up element; and
a second pull-up element,
the first pull-up element, the second pull-up element, and the first pull-down circuit being connected to the bus line,
the first pull-down circuit being connected to the means for sensing and configured to be activated based on a signal from the means for sensing,
the first pull-up element having a lower drive strength than the first pull-down circuit.