Temperature sensor using digital double sampling

1. An on-chip temperature sensor for generating a digital output signal representing a temperature value of a Complementary Metal Oxide Semiconductor (CMOS), comprising:

a proportional to absolute temperature (PTAT) buffer for alternately generating a first voltage signal representing a voltage across a first node of the PTAT buffer and a second voltage signal representing a voltage across a second node of the PTAT buffer;

an analog to digital (A/D) converter coupled to the PTAT buffer for converting the first voltage signal to a first digital voltage signal and for converting the second voltage signal to a second digital voltage signal; and

a digital output generating block for receiving the first digital voltage signal and the second digital voltage signal, and comparing a difference between the first digital voltage signal and the second digital voltage signal with a digital voltage reference signal to generate the digital output signal;

wherein the PTAT buffer and the A/D converter are both single-ended circuits.

2. The on-chip temperature sensor of claim 1 , wherein the digital voltage reference signal is generated using a one-time calibration mode.

3. The on-chip temperature sensor of claim 1 , further comprising a gain stage, coupled between the PTAT buffer and the A/D converter, for alternately receiving the first voltage signal and the second voltage signal, and amplifying the first voltage signal and the second voltage signal before they are input to the A/D converter.

4. The on-chip temperature sensor of claim 3 , wherein the gain stage is a single ended circuit.

5. The on-chip temperature sensor of claim 1 , wherein the PTAT buffer comprises:

a first bipolar junction transistor (BJT) coupled between ground and a reference current generated by a band gap circuit;

a second BJT coupled between the reference current and ground;

a first switch coupled between the reference current and the first BJT, wherein when the first switch is closed, the PTAT buffer outputs the first voltage signal; and

a second switch coupled between the reference current and the second BJT, wherein when the second switch is closed, the PTAT buffer outputs the second voltage signal.

6. The on-chip temperature sensor of claim 5 , wherein the digital output generating block receives the first digital voltage signal at a first time, receives the second digital voltage signal at a second time, determines a difference between the first digital voltage signal and the second digital voltage signal to generate a resultant digital signal, and compares the resultant digital signal with the digital voltage reference signal to generate the digital output signal.

7. The on-chip temperature sensor of claim 6 , wherein the first BJT and the second BJT are PNP BJTs, and the digital output generating block comprises:

a first adder for receiving the first digital voltage signal and the second digital voltage signal and generating the resultant digital signal; and

a second adder for adding the resultant digital signal to the digital voltage reference signal to generate the digital output signal.

8. A method for generating a digital output signal representing a temperature value of a Complementary Metal Oxide Semiconductor (CMOS), comprising:

alternately generating a first voltage signal representing a voltage across a first node and a second voltage signal representing a voltage across a second node;

converting the first voltage signal to a first digital voltage signal and converting the second voltage signal to a second digital voltage signal; and

comparing a difference between the first digital voltage signal and the second digital voltage signal with a digital voltage reference signal to generate the digital output signal;

wherein the first voltage signal and the second voltage signal are both single-ended signals.

9. The method of claim 8 , wherein the digital voltage reference signal is generated using a one-time calibration mode.

10. The method of claim 8 , further comprising:

amplifying the first voltage signal and the second voltage signal before they are respectively converted to the first digital voltage signal and the second digital voltage signal.

11. The method of claim 8 , wherein the step of alternately generating the first voltage signal and the second voltage signal comprises:

providing a first switch coupled to a first bipolar junction transistor (BJT);

providing a second switch coupled to a second BJT;

closing the first switch and opening the second switch to generate the first voltage signal corresponding to a voltage across the first BJT; and

closing the second switch and opening the first switch to generate the second voltage signal corresponding to a voltage across the second BJT.

12. The method of claim 11 , wherein the step of comparing a difference between the first digital voltage signal and the second digital voltage signal with a digital voltage reference signal comprises:

receiving the first digital voltage signal at a first time;

receiving the second digital voltage signal at a second time;

determining a difference between the first digital voltage signal and the second digital voltage signal to generate a resultant digital signal; and

comparing the resultant digital signal with the digital voltage reference signal to generate the digital output signal.

13. The method of claim 12 , wherein the step of determining a difference between the first digital voltage signal and the second digital voltage signal comprises:

adding the first digital voltage signal and the second digital voltage signal to generate the resultant digital signal;

and the step of comparing the resultant digital signal with the digital voltage reference signal comprises:

adding the resultant digital signal to the digital voltage reference signal to generate the digital output signal.