Offset Correction Techniques for Voltage Sense Amplifiers (2006)

This report deals with offset correction techniques for voltage sense amplifiers and is divided
into two different parts: 1) mismatch and 2) offset correction techniques. First a literature
study is done on the subject mismatch with specially focus on the future. Mismatch of a
transistor is determined by the mismatch factors σVT and σβ which are depend strongly on the
inverse square root of the effective gate area. For these mismatch factors a future expectation
will be given. The outcome is that the mismatch factors σVT and σβ will increase significantly if
the transistor downscaling process continues. Also a deeper insight will be given about the
amount of contribution of these mismatch parameters to the offset current of a transistor.
Because the matching accuracy of the currents in the voltage sense amplifier determines
mainly the sensitivity of this special amplifier.
Voltage sense amplifiers are used extensively in memories. For this reason a short paragraph
about the memory architecture is written for more background information. Basically what the
sense amplifier does is detecting the information which is located in the memory cells. This
information is read out and results in a small voltage difference between two bitlines. The
sense amplifier evaluates this small voltage difference and converts it into a logic level output
signal.
A basic voltage sense amplifier is the starting point in the further research to offset correction
techniques. Different offset correction techniques are investigated and applied to this voltage
sense amplifier. First all the techniques are tuned to get the best performance in sensitivity.
Then in the second step the different offset correction techniques are optimized in speed.
Finally a comparison is made based on all the results. In this way the best offset correction
techniques for the voltage sense amplifier are found.
The following offset correction techniques are investigated: threshold cancellation, β offset
reduction, metastable offset correction and calibration. Threshold cancellation is an already
known technique which is used often by voltage sense amplifiers. β offset reduction is also not
new, but the technique is not really elaborated in practice realizations in the past. The
metastable offset correction technique is a new designed technique which has some
similarities with the threshold cancellation technique. Calibration is a general offline technique
which adjusts the voltage sense amplifier to a very sensitive system.
Threshold cancellation cancels the threshold voltages of the two most important transistors in
the voltage sense amplifier. Short circuit connections are applied to these transistors to find
the real threshold voltages. These threshold voltages are measured and stored in capacitors.
The capacitors are put in serial with the gates of the two transistors to cancel the threshold
voltages. The technique operates quite well but is not really suitable for the voltage sense
amplifier if a high speed is required.
β offset reduction uses a technique which turns on lightly the correct transistor during the
decision of the voltage sense amplifier. Other transistors which could have β mismatch are
turned on later when the decision is already made by the voltage sense amplifier. In this way
the offset current due to only β offset can be reduced significantly. β offset reduction is a very
simple technique which has a reasonable sensitivity. Furthermore the technique operates at
high speed and does not have large power consumption.
The metastable offset correction technique uses the metastable property of the two inverters
in the sense amplifier. The two inverters in the sense amplifier are set into metastable states
before the sense amplifier makes a decision. In this way the inverters are compensated for the
offset. One of the inverters goes out the metastable state when a memory cell is read out and
a decision is enforced by the sense amplifier. The technique is very suitable for the voltage
sense amplifier and gives good results in sensitivity and speed.