Impact of Process Variation on Input Referred Current Offset in Current Sense Amplifiers Riya Shergill Meenakshi Sekhar

Current Sense Amplifiers Sense Amplifiers speed up the read process by amplifying the bit line differential. Current Sense Amplifiers (CSA) are similar to voltage sense amplifiers in function. Only they detect current differential instead of voltage. CSA’s have a low input impedance. Hence even when the voltage differential is small, current differential is significant. The differential current at the input is translated to a voltage differential at the output.

Why CSA? For a large memory arrays, bit line capacitance is very high. The time taken to obtain a specific voltage differential at the bitlines is longer for voltage sense amps than for current sense amps. This can be proved using the small signal models of the bitcells and sense amplifiers.

Input Referred Current Offset Process and Mismatch variations among transistors cause an inherent offset between the bit lines i.e. for any operational amplifier, even when the input is zero, the output need not be zero. Why do we care about offset? – It tells us minimum differential required before the sense amplifier can be triggered.

Project Goal It is obvious that the smaller the offset, the better. We also know that offset is caused due to variation. So, we tried to determine how the offset value varies with threshold voltage variation of the transistors.

Experimentation So what did we do? The sense amplifier has two inputs-from the BL and BLB. We used a reference current on one input of the sense amplifier and swept the current on the other input, to determine the minimum current differential before the right value is read. We repeated this for over 100 different threshold values using Monte Carlo in Cadence.

Processes involved in Simulation Parametric Analysis : Allows a parameter to be swept over a range of values and an analysis is performed for each swept value. Monte Carlo: Varies the threshold voltages (in our project) of all the transistors based on a distribution specified. Hence, we obtain an offset for each threshold voltage by determining when the parameter causes the output to flip and at which threshold value.

Circuits A Conventional Current Sense amplifier is a simple current mirror circuit.

Circuits Cascode Current Sense amplifier has a higher output impedance.

Circuits Improved Wilson Current Sense amplifier uses a negative feedback to equal the currents on the bit lines.

Simulation Results

Offset Current Evaluation Conventional Current Sense Amplifier

Cascode Current Sense Amplifier

Improved Wilson Current Mirror Circuit.

Offset Variation with Threshold Voltage

Cascode Current Sense Amplifier

Improved Wilson Current Sense Amplifier

Analysis Offset current varies as a Gaussian distribution for different threshold voltages. At low threshold voltages, the offset current is rather higher than when the threshold voltage is larger. The curves show a similarity in that, lower thresholds show a higher offset current and the offset current does not vary much at higher threshold voltages.

Conclusion Analysis is time consuming. Scripting is the best way out OCEAN scripts can be used to run Monte Carlo simulations as it offers more flexibility as far as evaluation is concerned. The GUI works, but scripting is easier.

Thank You!