1. A presentation on
Design of OPAMP Based R-2R Ladder Type 4-bit Digital to
Analog Converter (DAC) Using 90nm CMOS Technology
Submitted by Under the guidance of
Subhajit Shaw Mr. Soumen Pal
M.TECH in Micro Electronics & VLSI Designs
NARULA INSTITUTE OF TECHNOLOGY
UNIVERSITY Roll No: 12710414002.
UNIVERSITY Registration No: 141270410002 of 2014-2015.
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2. Contents
INTRODUCTION
TWO STAGE CMOS OP-AMP
GIVEN SPECIFICATIONS OF OP-AMP
DESIGN PARAMETERS OF OP-AMP
SPICE CIRCUIT DIAGRAM OF CMOS OP-AMP(Using T-SPICE Tool)
SIMULATED RESULTS OF CMOS OP-AMP
R-2R LADDER DAC
SPECIFICATIONS OF R-2R LADDER DAC
SPICE CIRCUIT DIAGRAM OF R-2R LADDER DAC (Using T-SPICE Tool)
SIMULATED RESULTS OF R-2R LADDER DAC
CONCLUSION
FURTHER ENHANCEMENT
REFFERENCES
2
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3. Introduction
High resolution digital to analog converters (DACs) are highly
demanded in today’s wireless communication applications.
The basic theory of the R-2R ladder network is that current
flowing through any input resistor (2R) encounters two possible
paths at the far end.
The total resistances of both paths are the same (also 2R), so the
incoming current splits equally along both paths.
The R-2R resistor ladder network directly converts a parallel
digital symbol/word into an analog voltage.
The goal of this thesis is to design the R-2R Ladder type 4-bit
digital to analog converter (DAC) using 90nm CMOS technology
which is based on two stage CMOS Op-Amp.
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4. Two Stage CMOS Op-Amp
The two stage CMOS Op-amp is widely used because of its simple structure
,robustness and very high Gain.
Current mirror are used extensively in CMOS Op-amp circuits both as
active load elements and biasing circuits to get a high AC voltage gain.
Designing of an Op-amp requires some predefined electrical specifications
such as gain ,band width, slew rate, input common mode range and
maximum output swing .
Op-amp are designed to be operated with negative feedback connection to
ensure the stability of the system.
The basic structure of two stage CMOS op-amp is shown in the following
diagram Which includes differential gain stage(First stage), output
stage(Second stage), compensation circuit and biasing circuit.
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5. Basic circuit diagram of two stage op-amp
5
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6. Given design specifications
Electrical Parameters Expected values
Supply voltage ±1V
Load Capacitance 1 pF
Unity gain frequency 100MHz
Slew rate ±10 volt/µsec
Input Common mode range ±0.4volt
Output swing ±0.9 volt
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7. Design procedure of two stage op-amp
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8. Design procedure of two stage op-amp
cont’d
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9. Design parameters of op-amp
Parameters Value Unit
Cc 200 fF
CL 1 pF
(W/L)1,2 9500/100 nm/ nm
(W/L)3,4
2170/100 nm/ nm
(W/L)5,8
120/100 nm / nm
(W/L)6
21400/100 nm/ nm
(W/L)7
480/100 nm / nm
(W/L)9
120/100 nm/ nm
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Narula Institute of Technology
10. Circuit diagram of two stage op-
amp(Using T-SPICE Tool)
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11. DC response of designed op-amp in non
inverting mode
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12. Transient response in non inverting mode
V(IN)
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13. Transient response in non inverting mode
V(OUT)
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16. What is a DAC?
DAC
100101…
A digital to analog
converter (DAC) is a
device that converts
digital numbers (binary)
into an analog voltage or
current output.
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17. What is a DAC?
Each sample is converted from binary to analog, between 0 and Vref for
Unipolar, or Vref and –Vref for Bipolar
10111001 10100111 10000110010101000011001000010000
Digital Input Signal
AnalogOutputSignal
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18. R-2R Ladder D/A Converter
The 4-bit R-2R ladder type DAC is the most popular DAC.
It uses a ladder network containing series-parallel combinations
of values R and 2R.
It is easily scalable to any desired number of bits.
It’s uses only two values of resistors which make for easy and
accurate fabrication and integration.
Output impedance is equal to R, regardless of the number of
bits, simplifying filtering and further analog signal processing
circuit design.
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19. R-2R Ladder D/A Converter
0
4 bit converter
0 0 0
Each bit corresponds to a switch:
• If the bit is high, the
corresponding switch is
connected to the inverting
input of the op-amp.
• If the bit is low, the
corresponding switch is
connected to ground.
Requires only two precision resistance value (R and 2R)
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20. R-2R Ladder Example
Convert 0001 to analog
1
1/ 2 1/ 2
eqR R
R R
0 1 1
1
2
R
V V V
R R
1 2 2
1
2
R
V V V
R R
2 3 3
1
2
R
V V V
R R
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21. R-2R Ladder Example
Convert 0001 to analog
R
2R
0
1
8
refV V
out 0
R 1
V
2R 16
refV V
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22. R-2R DAC Summary
Conversion results for each bit
Digital bit Analog Conversion
0001
0010
0100
1000
,0 /16out refV V
,1 /8out refV V
,2 / 4out refV V
,3 / 2out refV V
3 ,3 2 ,2
1 ,1 0 ,0
out out out
out out
V b V b V
bV b V
for
3 2 1 0 ( 0 or 1)ib b b b b
Conversion equation for N-bit DAC
( )
1 2
N
ref
out N i i
i
V
V b
Resolution
2
ref
N
V
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23. Advantages
Only two resistor values
Does not need as precision resistors as Binary weighted DACs
Cheap & Easy to manufacture
Faster response time
Disadvantages
Slower conversion rate
More confusing analysis
R-2R DAC Summary
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25. Specification - Resolution
25
Resolution of a DAC is the change in output voltage for a
change in the least significant bit (LSB) of the digital input.
Resolution is specified in “bits”.
Most DACs have a resolution of 8 to 16 bits
Example: A DAC with 10 bits has a resolution of
Higher resolution (more bits) = smoother output.
ref10
ref
1024
1
2
Resolution V
V
bitsofnumberN
V
V N
ref
LSB
where
2
Resolution
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26. Also called the conversion rate or sampling rate
- rate at which the register value is updated
Rate of conversion of a single digital input to its
analog equivalent
Conversion Rate depends on
clock speed of input signal
settling time of converter
When the input changes rapidly, the DAC
conversion speed must be high.
Specification - Speed
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27. The time required for the input signal voltage to settle to the expected
output voltage (within +/- ½ of VLSB).
Ideally, an instantaneous change in analog voltage would occur when a
new binary word enters into DAC
Fast converters reduce slew time, but usually result in longer ring time.
Specification – Settling Time
tdelay
tslew tring
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28. Specification – Linearity
Linearity(Ideal Case)
Digital Input
Perfect Agreement
Desired/Approximate Output
AnalogOutput
Voltage
NON-Linearity(Real World)
AnalogOutput
Voltage
Digital Input
Desired Output
Miss-alignment
Approximate
output
The difference between the desired analog output and the actual
output over the full range of expected values.
Ideally, a DAC should produce a linear relationship between a digital
input and the analog output, this is not always the case.
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29. A specified voltage used to determine how each digital input will
be assigned to each voltage division.
Types:
Non-multiplier DAC: Vref is fixed (specified by the
manufacturer)
Multiplier DAC: Vref is provided via an external source
Specification – Reference Voltage
Full Scale Voltage
Defined as the output when digital input is all 1’s.
1
1
0
2 1
1
2 2
N N
ref
fs refi N
i
V
V V
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30. Circuit diagram of R-2R Ladder D/A
Converter(Using T-SPICE Tool)
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31. Output Waveform of D /A Converter
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34. Comparison between
Expected Output and Simulated Output
Table 2:-Deviation among successive outputs
Sl.No. Expected Result
(mV)
Simulated Output
(mV)
1 62.5 56.99
2 62.5 54.45
3 62.5 75.20
4 62.5 52.35
5 62.5 71.89
6 62.5 52.99
7 62.5 69.49
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35. Comparison between
Expected Output and Simulated Output
Sl.No. Expected Result
(mV)
Simulated Output
(mV)
8 62.5 58.57
9 62.5 54.45
10 62.5 70.16
11 62.5 58.32
12 62.5 40.21
13 62.5 31.36
14 62.5 19.55
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36. Conclusion
In this work, a two stage op-amp has been designed using
90nm CMOS technology and a 4-bit R-2R ladder type
digital to analog converter is also realised using designed
Op-Amp. The simulation results confirm that the design
procedure is suitable for op-amp based DAC design in
90nm CMOS technology. The designed DAC is simulated
using TANNER Tool using 90nm CMOS technology. The
simulation results shows that the deviations among
successive outputs become more non-linear for higher order
digital input bits as depicted in Table 2.
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37. Future plan
In this work 90nm CMOS technology has been used. Further
reduction in MOSFET channel length can be done to ensure
more integration.
The design of DAC can be enhanced further, considering the
specifications like resolution, offset error, Differential Non-
Linearity (DNL) and Integral Non-Linearity (INL).
In Communication System and signal processing purpose,
A/D converter and D/A converter are inseparably used as a
front end and rare end blocks respectively. The designed D/A
converter can also be utilised to design A/D converter.
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38. References
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39. 6. Amana Yadav , “Design of Two-Stage CMOS Op-Amp and
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References
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40. 11. “ world’s fastest Digital to Analog Converter (DAC)” from
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41. 16. Scott E. Thompson et al:“A 90-nm Logic Technology
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