Differential measurements with a large common mode input voltage

1. An apparatus comprising:

a differential amplifier circuit configured to receive a voltage at a non-inverting input, wherein the differential amplifier circuit generates an output voltage having a magnitude proportional to the received voltage over a voltage range to be measured at a specified output common mode voltage; and

a current source electrically connected to an inverting input of the differential amplifier circuit configured to subtract a midpoint of a voltage range to be measured at the input of the differential amplifier, wherein a circuit supply voltage provided to the differential amplifier circuit and the current source is less than the voltage at the input.

2. The apparatus of claim 1 , including a common mode biasing circuit configured to adjust an input common mode voltage of the differential amplifier circuit.

3. The apparatus of claim 1 , including a resistor connected to an inverting input of the differential amplifier, wherein the current source applies a value of current to the resistor to generate a voltage equal to the midpoint of the voltage range of the received voltage.

4. The apparatus of claim 1 , wherein the current source includes a plurality of unit-sized current mirrors configured to be enabled and disabled to generate the value of current.

5. The apparatus of claim 4 , including a state machine circuit configured to enable and disable the current mirrors according to a successive approximation algorithm.

6. The apparatus of claim 1 , wherein the voltage received at the input of the differential amplifier circuit is a battery voltage.

7. The apparatus of claim 1 , including a stacked battery voltage source and the voltage received at the input of the differential amplifier circuit is a battery voltage generated by the stacked battery voltage source.

8. The apparatus of claim 1 , including a sensor circuit having a bias voltage greater than the circuit supply voltage and the voltage received at the input to the differential amplifier circuit is generated by the sensor circuit.

9. The apparatus of claim 1 , wherein the apparatus is a cellular telephone.

10. A method comprising:

providing a circuit supply voltage to a differential measuring circuit;

receiving a voltage at a non-inverting input to a differential measuring circuit, wherein the received voltage is greater than the circuit supply voltage;

applying current at an inverting input to the differential measuring circuit to shift the received voltage by a midpoint of a range of voltage values to be measured; and

generating, at an output of the differential measuring circuit, a voltage shifted from the received voltage by the midpoint and having a magnitude proportional to the received voltage over the range of voltage values to be measured at a specified output common mode voltage.

11. The method of claim 10 , including adjusting an input common mode voltage of the measurement circuit to be lower than the circuit supply voltage.

12. The method of claim 10 , wherein receiving a voltage at an input to a differential measuring circuit includes receiving a battery voltage at the input to the differential measuring circuit.

13. The method of claim 10 , wherein receiving a voltage at an input to a differential measuring circuit includes receiving a voltage from a sensor circuit at the input to the differential measuring circuit.

14. The method of claim 13 , wherein applying current at an input to the differential measuring circuit includes applying current from a plurality of unit-sized current mirrors, and wherein the method includes trimming the applied current by automatically selectively enabling and disabling the current mirrors.

15. The method of claim 14 , wherein trimming the applied current includes trimming the applied current by selectively enabling and disabling the current mirrors according to a successive approximation algorithm.

16. The method of claim 13 , wherein applying current at an input to the differential measuring circuit includes applying a value of current to a resistor electrically coupled to the input to generate a voltage equal to a midpoint of a range of voltage values to be measured.

17. The method of claim 16 , including automatically trimming the applied current to minimize a difference between a voltage of the resistor and the midpoint of the range of voltage values to be measured.

18. An electronic system including:

a battery source having a battery voltage;

a differential amplifier circuit having a non-inverting input electrically coupled to the battery source, wherein a circuit supply voltage provided to the differential amplifier circuit is less than the battery voltage;

a current source electrically connected to an inverting input of the differential amplifier circuit and configured to subtract a midpoint of a voltage range of the battery source to be measured; and

a common mode biasing circuit configured to adjust an input common mode voltage of the differential amplifier circuit, wherein the differential amplifier circuit generates an output having a magnitude proportional to the battery voltage over the voltage range to be measured at a specified output common mode voltage.

19. The electronic system of claim 18 , wherein the battery source is a stacked battery voltage source.

20. The electronic system of claim 18 , including a resistor connected to the inverting input of the differential amplifier, wherein the current source applies a value of current to the resistor to generate a voltage equal to the midpoint of the voltage range of the received voltage.