Method and apparatus for sensing flash memory using delta sigma modulation
A simple method and device for accurately measuring flash memory cell current. The sensing scheme comprises an integrator, an analog to digital converter, and a digital to analog converter. The method comprises the acts of applying an input current and a feedback output current to a summer, integrating the resulting summer output over time, passing the integrated output to a clocked comparator, outputting a comparator output which controls a feedback circuit that keeps the integrator's voltage at the same level as a reference voltage, and outputting a digital average current to a counter. Delta sigma modulation (averaging) is employed to cancel out noise that would otherwise affect the cell current measurement.
1. A sense amplifier circuit comprising:
a summer for summing a current from a memory cell and a feedback current to form a summed current;
a capacitance element connected to receive the summed current and to integrate the summed current over a first time, the capacitance element being charged and discharged during the first time;
an analog-to-digital converter connected to receive the integrated summed current from the capacitance element, the analog-to-digital converter comprising a plurality of inverter circuits and at least two switches, each switch being connected between an output of a first associated inverter circuit and an input of a second associated inverter circuit, an output of a last inverter circuit of the plurality of inverter circuits being a digital output corresponding to an average current and a count of a number of times the capacitance element is discharged during the first time; and
a digital-to-analog converter connected to receive the digital average current and to output the feedback current to the summer.
2. The sense amplifier circuit of claim 1 , wherein the capacitance element is a capacitor.
3. The sense amplifier circuit of claim 1 , wherein the capacitance element is bitline capacitance.
4. The sense amplifier circuit of claim 1 , wherein the digital-to-analog converter comprises a current source and a switch, the switch being controlled to switch in the current source in response to the digital average current.
5. The sense amplifier circuit of claim 1 , wherein the digital-to-analog converter comprises a flash memory cell serving as a current source and a switch, the switch being controlled to switch in the flash memory current source in response to the digital average current.
6. The sense amplifier circuit of claim 1 , further comprising a counter for generating a digital code based on the digital output.
7. A memory device comprising:
a flash memory cell; and
a sense amplifier circuit connected to the cell, the sense amplifier circuit comprising:
a summer for summing a current from a memory cell and a feedback current to form a summed current;
an integrator connected to receive the summed current and to integrate the summed current over a first time, the integrator being charged and discharged during the first time;
a clocked comparator circuit connected to receive the integrated summed current from the integrator, the clocked comparator circuit generating a digital output corresponding to an average current and a count of a number of times the integrator is discharged during the first time; and
a digital-to-analog converter connected to receive the digital average current and to output the feedback current to the summer.
8. The memory device of claim 7 , wherein the clocked comparator circuit comprises:
a plurality of inverter circuits; and
at least two switches, each switch being connected between an output of a first associated inverter circuit and an input of a second associated inverter circuit, the switches being controlled by clock signals,
wherein an output of a last inverter circuit of the plurality of inverter circuit comprises the digital output.
9. The memory device of claim 7 , wherein the integrator is bitline capacitance.
10. The memory device of claim 7 , wherein the digital-to-analog converter comprises a current source and a switch, the switch being controlled to switch in the current source in response to the digital average current.
11. The memory device of claim 7 , wherein the digital-to-analog converter comprises a flash memory cell serving as a current source and a switch, the switch being controlled to switch in the flash memory current source in response to the digital average current.
12. The memory device of claim 7 , wherein the sense amplifier circuit further comprises a counter for generating a digital code based on the digital output.
13. A method of sensing a state of a memory cell, the method comprising:
inputting a memory cell current;
discharging a voltage stored at an integrator;
determining when the voltage stored at the integrator decreases below a reference level;
combining a feedback average current to the memory cell current to charge the integrator to a voltage above the reference level;
repeating the discharging through combining steps for a sensing time; and
counting a number of times the integrator is discharged during the sensing time,
wherein the number of times the integrator is discharged during the sensing time corresponds to a value stored in the memory cell.
14. The method of claim 13 further comprising outputting a digital code corresponding to the number of times the integrator is discharged during the sensing time.
15. The method of claim 13 , wherein the act of discharging the voltage stored at the integrator comprises disconnecting the feedback average current from the memory cell current when the voltage stored at the integrator is determined to be above the reference level.
16. The method of claim 13 , wherein the memory cell current is flash cell current and the feedback average current is an average cell current.