Download Cascode Current Sense Amplifier

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
Conventional Current Sense Amplifier
35
30
# of occurences
25
Series1
20
15
10
5
0
-5
-4
-3
-2
-1
0
1
Input-Offset Current (uA)
2
3
4
5
Cascode Current Sense Amplifier
Cascode Sense Amplifier
30
25
20
Series1
15
10
5
Input Offset Current (nA)
10
00
80
0
60
0
40
0
20
0
0
-2
00
0
-4
00
# of occurences
35
Improved Wilson Current Mirror Circuit.
Improved Wilson Sense Amplifier
30
25
15
Series1
10
5
Input-Offset (nA)
80
0
70
0
60
0
50
0
40
0
30
0
20
0
10
0
0
0
-8
00
-7
00
-6
00
-5
00
-4
00
-3
00
-2
00
-1
00
# of occurences
20
0.3
0.33
0.34
0.35
0.35
0.36
0.37
0.38
0.38
0.38
0.39
0.39
0.39
0.4
0.4
0.41
0.41
0.41
0.42
0.42
0.43
0.45
0.45
0.46
0.5
Input Offs et C urrent (uA)
Offset Variation with Threshold
Voltage
Conventional Sense Amplifier
3.5
3
2.5
2
1.5
1
0.5
0
T hre shold V ola tg e (V )
offs et
Cascode Current Sense Amplifier
1200
1000
800
600
O ffs et
400
200
0
0.3
0.34
0.35
0.36
0.37
0.38
0.39
0.4
0.41
0.41
0.42
0.42
0.43
0.44
0.45
0.46
0.49
Input Offs et C urrent (nA)
Cascode Sense Amplifier
T hre shold Volta g e (V)
Improved Wilson Current Sense Amplifier
600
400
200
0
-200
0.29
0.34
0.37
0.37
0.38
0.38
0.39
0.4
0.4
0.4
0.41
0.41
0.41
0.42
0.42
0.43
0.43
0.44
0.46
Input O ffs et C urrent (nA)
Improved Wilson Sense Amplifier
-400
-600
T hre shold Volta g e (V)
offs et
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!