Methods and systems for low noise operation of analog in memory compute MAC arrays
A matrix multiplication and addition (MAC) operating system is described where different cell currents flowing through different cells in a matrix for different PWM time intervals can be integrated and converted into a numeric value for the cumulative charge. The numeric value of the cumulative charge computed by the ADC is equivalent to MAC operations over multiplicity of cells. The operation is inherently error prone due to parasitic coupling effects from the switching of the memory cells in the array. The presented system minimizes the errors due to parasitic coupling through memory array.
1 . An ADC system is described which reduces analog to digital conversion errors and is comprised of:
a PWM (pulse width modulator) circuit providing various pulse-widths to a multiplicity of flash cells to charge or discharge different amounts of electric charge depending on a Control Gate (CG) bias and a pulse-width of a respective PWM pulse on a cell.
2 . The system of claim 1 , wherein a Control Gate (CG) line value appears as a static analog signal with a voltage level defined by the Control Gate (CG) ref voltage.
3 . The system of claim 2 , wherein a CG_line voltage level defines and produces a charging or discharging cell currents (“I”) over different PWM pulse widths.
4 . The system of claim 3 , wherein a Select Gate (SG)_line value controls the time (“t”) over which the cell produces the cell current.
5 . The system of claim 4 , wherein a flash cell produces a total charge q=i*t.
6 . The system of claim 5 , wherein a Analog Multiplier-and-Accumulator (MAC) system is built with a Flash Cell array, and wherein a plurality of flash cells in the Flash Cell Array produce cumulative charge which is summation of all the charge produced by each cell in a bit line of the array of cells.
7 . The system of claim 6 , wherein a plurality of select lines of the Analog MAC system are driven by a select gate driver system and a plurality of control gate lines of the Analog MAC system are driven by a control gate driver system.
8 . The system of claim 7 , wherein a CG_line and an SG_line connects to a flash cell control gate and a flash cell select gate.
9 . The system of claim 8 , wherein the plurality of flash cells, when activated by the CG_lines and SG_lines, generate cell current with a total charge which is summation of charge produced by each cell in a bit line and which is sensed by the ADC in analog domain via bit-lines and converted to a numeric value for digital manipulation.
10 . The system of claim 9 , wherein the errors in ADC conversion is minimized by reducing the glitches from PWM switching to couple into a plurality of memory cells.
11 . The system of claim 10 , wherein various spurious errors due to system non-linearities are corrected by trimming circuits.
12 . The system of claim 10 , wherein the ADC conversion errors are minimized by keeping the select gate voltage static (always on) during charge accumulation and using the control gate voltage as well as a control gate voltage turn on time to control both a current and the duration of current.
13 . The system of claim 12 where the control gate voltage is minimized so as to reduce a parasitic capacitive coupling to the memory cells which can accumulate to cause errors in the ADC count.
14 . The system of claim 12 where the memory cells are programmed to lower threshold voltages in line with a lowered control gate voltage so as to still provide a targeted current levels.