ADC, a temperature sensor, a non-contact transponder, and a method of converting analog signals to digital signals
An ADC is disclosed which has, as a first stage, a successive approximation converter, or other compensated, direct comparison converter, followed by a sigma delta modulation converter as a second stage. The sigma delta converter may beneficially be a first order modulator. The resulting ADC combines accuracy with low power consumption per conversion, and thus is particularly suited for use in temperature sensors for applications such as RFID transponders. Such a temperature sensor and an RFID transponder are also disclosed. There is also disclosed a method of analog-to-digital conversion, comprising a first successive approximation register or other compensated, direct comparison conversion stage followed by a sigma delta modulation stage, which, further, may be combined with providing a proportional-to-absolute-temperature (PTAT) signal, for low-power, accurate temperature sensing.
1. An analog-to-digital converter, comprising:
an analog input,
a direct comparison converter,
a sigma delta converter, and
a digital output,
the direct comparison converter being configured to determine a first approximation to the digital output, and the sigma delta converter being configured to determine the digital output from the first approximation,
wherein the direct comparison converter includes digital logic, an integrator and a comparator, and the integrator and the comparator form a part of the sigma delta converter.
2. An analog-to-digital converter as claimed in claim 1 , wherein the direct comparison converter is a successive approximation register converter.
3. An analog-to-digital converter as claimed in claim 1 wherein the sigma delta converter comprises a first order sigma delta modulator.
4. An analog-to-digital converter as claimed in claim 1 , wherein the digital output has at least 10-bit resolution.
5. An analog-to-digital converter as claimed in claim 1 , wherein the digital output has at least 15-bit resolution.
6. An analog-to-digital converter as claimed in claim 1 , wherein first approximation has a resolution of at least 4 bits.
7. An analog-to-digital converter as claimed in claim 1 , wherein the sigma delta converter is configured to operate without a guard-band capacitor.
8. An analog-to-digital converter, comprising:
an analog input,
a direct comparison converter,
a sigma delta converter, and
a digital output, the direct comparison converter being configured to determine a first approximation to the digital output, the sigma delta converter being configured to determine the digital output from the first approximation, and the sigma delta converter being configured to use at least one of dynamic error correction and dynamic element matching.
9. A temperature sensor comprising: an analog-to-digital converter, including
an analog input,
a direct comparison converter,
a sigma delta converter,
a digital output, the direct comparison converter being configured to determine a first approximation to the digital output, and the sigma delta converter being configured to determine the digital output from the first approximation; and
a bipolar core, which bipolar core comprises two transistors each having a bias-emitter voltage and configured such that in operation the difference between the respective bias-emitter voltages is proportional to absolute temperature.
10. A non-contact transponder comprising a temperature sensor as claimed in claim 9 and a power circuit for providing power to the temperature sensor.
11. A non-contact transponder as claimed in claim 10 , wherein the power circuit is configured to, in use, extract power from an electro-magnetic field.
12. A method of converting an analog signal to a digital signal comprising:
a successive approximation register stage of successive approximating, followed by
a sigma delta converting stage; wherein the successive approximation stage is carried out by a successive approximation register comprising an integrator and a comparator, the sigma delta stage is carried out by a sigma delta converter, and prior to the sigma delta stage the integrator and the comparator are reconfigured to comprise a part of the sigma delta converter.
13. A method as claimed in claim 12 , wherein the sigma delta converting stage is based on a first order modulator.
14. A method of sensing temperature, comprising:
determining an analog input which is proportional to absolute temperature, and
converting the analog input to a digital output by the method of claim 12 .