Differential analog memory cell and method for adjusting same

An operational amplifier having a differential analog memory cell adjustable through a pair of voltage inputs for correcting the offset of the amplifier output voltage, the operational amplifier comprising:

• a differential input stage having a current input, first and second current outputs connected to the input and output, respectively, of a current mirror, and inverting and non-inverting inputs, for distributing the current from the current input between the first and second current outputs according to the voltage difference between the inverting and non-inverting inputs;
• an output buffer having a current input, a signal input connected to one of the current outputs of the differential input stage, and an output forming the output of the operational amplifier, for coupling a signal from the differential input stage to the amplifier output;
• a differential analog memory cell having first and second floating gate MOSFETs, the first and second floating gate MOSFETs having their drains connected to the input and output, respectively, of the current mirror in parallel with the first and second current outputs of the differential input stage, and their sources connected to a common current source, coupling capacitors to connect the floating gates of the first and second MOSFETs to a reference voltage, and means for injecting charge onto each floating gate, for adjusting the current at the input and output of the current mirror by injecting charges onto the floating gates of the first and second MOSFETs;
• and
• a control circuit having a first input for receiving an initiation signal, a pair of outputs connected to the inverting and non-inverting inputs of the operational amplifier, and a pair of outputs connected to the charge injection means of the differential analog memory cell, the control circuit selectively connecting the inverting and non-inverting inputs of the differential stage together on receipt of an initiation signal, and repeatedly sensing the voltage at the output of the operational amplifier to activate one of the charge injecting means depending on the difference between the output voltage and a reference voltage, to correct the offset in the output voltage.

A method for precisely adjusting the charge on a differential analog cell comprising differentially connected floating gate MOSFETs, the method comprising the steps of:

• sensing the voltage at the output of the differential analog memory cell;
• generating a series of voltage pulses for application to the floating gate of a selected MOSFETs in response to the difference between the voltage at the output of the differential analog memory cell and a reference voltage, until the difference between the
• voltage at the output of the differential analog memory cell and the reference voltage reverses polarity;
• and
• applying a voltage pulse to floating gate of the other MOSFET, to adjust the output of the differential analog memory cell back toward the reference voltage.