Sense amplifier with adaptive reference generation

1. In a digital memory system including a memory cell arranged to store charge, apparatus providing an adaptive reference voltage comprising:

a bit line coupled to the cell and arranged to conduct a current based on the charge stored in the cell;

a transfer gate;

a reference voltage generator arranged to generate a reference voltage;

a sense amplifier; and

a control circuit operative during a first mode of operation to precharge the bit line to generate a bit line precharge voltage and to cause the transfer gate to sample and store the precharge voltage, operative during a second mode of operation to cause the transfer gate to isolate the bit line from the sampled and stored precharge voltage, to cause the reference voltage generator to generate the reference voltage in response to the sampled and stored precharge voltage and to couple the bit line and the reference voltage to the sense amplifier.

2. The apparatus of claim 1 further comprising a supply voltage, wherein the reference voltage generator comprises a first capacitor and a second capacitor and wherein the reference voltage comprises the sampled and stored precharge voltage modified by a fraction of the supply voltage determined at least in part by the relative values of the first capacitor and the second capacitor.

3. The apparatus of claim 2 wherein the control circuit is operative to couple at least one of the first and second capacitors to the reference voltage generator after the bit line has been isolated from the sampled and stored precharge voltage.

4. The apparatus of claim 1 wherein the sense amplifier comprises a differential sense amplifier.

5. The apparatus of claim 1 wherein the control circuit comprises:

a precharge transistor arranged to precharge the bit line during the first mode of operation;

a first switch transistor arranged to couple the sense amplifier to the bit line during the second mode of operation;

a second switch transistor arranged to couple the reference voltage to the sense amplifier during the second mode of operation; and

a third switch transistor arranged to isolate the sense amplifier from the supply voltage during at least the second mode of operation.

6. In a digital memory system including a memory cell arranged to store charge and a bit line, a method of providing an adaptive reference voltage to a sense amplifier comprising:

precharging the bit line to generate a bit line precharge voltage;

sampling and storing the bit line precharge voltage;

isolating the bit line from the sampled and stored bit line precharge voltage;

generating a reference voltage in response to the sampled and stored precharge voltage; and

coupling the bit line and the reference voltage to the sense amplifier so that the sense amplifier receives a voltage based on the charge stored in the memory cell.

7. The method of claim 6 further comprising a supply voltage, wherein said generating a reference voltage comprises modifying the sampled and stored precharge voltage to a fraction of the supply voltage by capacitance dividing.

8. The method of claim 6 wherein said generating a reference voltage occurs after said isolating the bit line.

9. The method of claim 6 wherein the sense amplifier comprises a differential sense amplifier.

10. The method of claim 9 wherein said isolating the sense amplifier occurs before said energizing the sense amplifier.

11. The method of claim 6 wherein the steps are performed in the order stated in claim 6 .