Noise cancellation in a single ended SAR converter
Noise cancellation in a single ended SAR converter. A single ended SAR converter front end is disclosed with common mode driver noise cancellation. The SAR converter front end includes a differential amplifier having positive and negative inputs and an output. A switched capacitor array is provided that is operable in a SAR data conversion operation to vary the voltage on one of the positive or negative inputs of the differential amplifier. A common mode driver drives a common mode node with a low impedance common mode voltage signal to a common mode node, and switching circuitry then switches the common mode voltage signal on the common mode node to the positive and negative inputs of the differential amplifier during a portion of a SAR data conversion cycle. A capacitor reference circuit is attached to the other of the positive or negative inputs of the differential, amplifier during at least the portion of the SAR data conversion cycle, the capacitor reference circuit and the capacitor array during at least the portion of the SAR data conversion cycle having relative capacitance values that increase the common mode rejection of any noise introduced to the common mode node at the input of the differential amplifier.
1. A single ended SAR converter front end with common mode driver noise cancellation, comprising:
a differential amplifier having positive and negative inputs and an output;
a switched capacitor array operable in a SAR data conversion operation to vary the voltage on one of said positive or negative inputs of said differential amplifier;
a common mode driver for delivering a low impedance common mode voltage signal to a common mode node;
switching circuitry for switching the common mode voltage signal on said common mode node to said positive and negative inputs of said differential amplifier during a portion of a SAR data conversion cycle; and
a capacitor reference circuit attached to the other of said positive or negative inputs of said differential amplifier during at least said portion of the SAR data conversion cycle, said capacitor reference circuit and said capacitor array during at least said portion of the SAR data conversion cycle having relative capacitance values that increase the common mode rejection of any noise introduced to said common mode node at the input of said differential amplifier.
2. The SAR converter front end of claim 1 , wherein said capacitor reference circuit has associated therewith reference voltage switching circuitry for switching a reference voltage to the other of said positive and negative inputs of said differential amplifier when not operating in said portion of the SAR data conversion cycle.
3. The SAR converter front end of claim 2 , wherein said reference voltage comprises ground.
4. The SAR converter front end of claim 3 , wherein the SAR converter front end is fabricated on an integrated circuit and said ground reference voltage is external to said integrated circuit.
5. The SAR converter front end of claim 2 , wherein said capacitor reference circuit comprises:
a first capacitor connected between the other of said positive and negative inputs of said differential amplifier and an internal reference voltage;
a switched capacitor connected between the other of said positive and negative inputs of said differential amplifier and a switched node; and
a switch for selectively switching the other end of said switch between said internal reference voltage and an external and offset ground reference voltage wherein said switch is operable to connect said switch to said external ground reference voltage during said portion of the SAR data conversion cycle and, otherwise to said internal reference voltage.
6. The SAR converter front end of claim 1 , wherein the relative capacitance values are substantially equal capacitance values.
7. The SAR converter front end of claim 1 , wherein said common mode rejection is maximized by said capacitor reference circuit.
8. The SAR converter front end of claim 1 , wherein said capacitor reference circuitry comprises a fixed capacitor.