Design and implementation of 4 bit binary weighted current steering DAC

Abstract

A compact current-mode Digital-to-Analog converter (DAC) suitable for biomedical application is repesented in this paper .The designed DAC is binary weighted in 180nm CMOS technology with 1.8V supply voltage. In this implementation, authors have focused on calculaton of Non linearity error say INL and DNL for 4 bit DAC having various type of switches: NMOS, PMOS and Transmission Gate. The implemented DAC uses lower area and power compared to unary architecture due to absence of digital decoders. The desired value of Integrated non linearity (INL) and Differential non linearity (DNL) for DAC for are within a range of +0.5LSB. Result obtained in this works for INL and DNL for the case DAC using Transmission Gate is +0.34LSB and +0.38 LSB respectively with 22mW power dissipation.

Full text

International Journal of Electrical and Computer Engineering (IJECE)
Vol. 10, No. 6, December 2020, pp. 5642~5649
ISSN: 2088-8708, DOI: 10.11591/ijece.v10i6.pp5642-5649  5642
Journal homepage: http://ijece.iaescore.com/index.php/IJECE
Design and implementation of 4-bit binary weighted current
steering DAC
Jayeshkumar J. Patel, Amisha P. Naik
Department of Electronics and Communication Engineering, Institute of Technology,
Nirma University Ahmedabad, India
Article Info
ABSTRACT
Article history:
Received Mar 20, 2019
Revised May 9, 2020
Accepted May 27, 2020
A compact current-mode Digital-to-Analog converter (DAC) suitable for
biomedical application is repesented in this paper. The designed DAC is
binary weighted in 180nm CMOS technology with 1.8V supply voltage.
In this implementation, authors have focused on calculaton of Non linearity
error say INL and DNL for 4-bit DAC having various type of switches:
NMOS, PMOS and transmission gate. The implemented DAC uses lower
area and power compared to unary architecture due to absence of digital
decoders. The desired value of Integrated non linearity (INL) and Differential
non linearity (DNL) for DAC for are within a range of +0.5LSB. Result
obtained in this works for INL and DNL for the case DAC using
transmission gate is +0.34LSB and +0.38 LSB respectively with 22mW
power dissipation.
Keywords:
CMOS current-steering DAC
DAC
DNL
INL
Transmission gate
Copyright © 2020 Institute of Advanced Engineering and Science.
All rights reserved.
Corresponding Author:
Jayeshkumar J. Patel,
Department of Electronics and Communication Engineering,
Institute of Technology, Nirma University,
S G Highway, Ahmedabad, Gujarat, India.
Email: jayesh.patel@nirmauni.ac.in
1. INTRODUCTION
The Digital to Analog converter (DAC) is a circuit which converts digital signal into analog one.
It is widely used in digital signal processors. DACs are often used to convert finite- precision time series to
a varying physical data. These are mainly used in different applications like data distribution and acquisition
systems, amplifier, Electronics display [1-7].
The Current steering DACs are the more commonly used architecture because of their small size and
simplicity, high resolution and high speed. Based on the binary principle, current sources are scaled. Here for
ith current source, output current is equal to the 2i*I, Where I = Least significant bit (LSB) current.
For the design of DAC, various switches like NMOS, PMOS and transmission gate are explored.
Characteristics of switching elements are one of the prominent factors for dynamic non linearity of DAC [8, 9].
In the proposed 4-bit DAC, four binary weighted current sources are used, those are represented
as: Io, 2Io, 4Io and 8Io. The main advantage of this architecture is number of current cells required will be
same as no. of bits. Hence, this architecture is most suitable for higher resolution implementations.
The disadvantage with this architecture: it produces number of glitches (unwanted signal) on the contrary
the unary architecture offers higher accuracy with greater linearity at the cost of chip area and power
overhead [10, 11].
2. CURRENT STEERING DAC
DAC are vailable in form of various architectures: Decoder based DAC, Weighted R DAC, R-2R
Ladder DAC, charged DAC, Cuurent steering DAC. Compared to other Architecturrs, Current steering DAC

Int J Elec & Comp Eng ISSN: 2088-8708 
Design and implementation of 4-bit binary weighted current stering DAC (Jayeshkumar J. Patel)
5643
is faster and low power consuming. There is no need of extra buffer to drive a load. A current steering DAC
uses a reference current source. The source is replicated in each branch of DAC. The current sources belong
to the branch is switched on or off according to digital inputs. In case of binary weighed current steering
DAC, Current source having the value of 2N*Io. Where Io is reference current. No. of switches are same as
no. of current sources and same as N. Based on ON/OFF of current sources, total current is added and it will
be the output current. Switches are MOS switches- NMOS, PMOS or transmission gate and sane are
controlled directly by digital inputs. Output current is as per input code [12-18]. N-bit DAC is represented as
low shown in Figure 1.
Figure 1. N-bit DAC representation
For N-bit DAC, output is expressed as follow:
 
REF
N
N
OUT
N
REF
N
NOUT
VV
V
DDV





2
2
2
22
1
max,
0
0
1
1
A 4-bit binary weighted current steering DAC is designed and implemented with various switching
approaches suitable for biomedical application. Though this architecture occupies lesser digital area and
power, but suffers from glitches specifically when have more numbers of transitions in input. The authors
have calculated INL, DNL of 4-bit Binary Current Steering DAC having various type of switches: NMOS,
PMOS and transmission gate [9, 12]. DAC are evaluated based on the various parameters like Resolution,
poer concumption, setteling time, dynamic range, non-linearity error (INLand DNL). In this paper, focus is
given on INL and DNL. Differential nonlinearity (acronym DNL) represents a deviation of actual step size
with reference to ideal step size, where step size is a difference of analog outputs for adjacent input
values [6, 10]. Mathematically DNL for DAC is represented as follow:
󰇛󰇜󰇛  󰇜 󰇛󰇜
  
Integral nonlinearity (acronym INL) represents a deviation of actual analog output of DAC with reference to
expected ideal value for given digital input value. It is also expressed in terms of DNL as follow [6, 10].
INL (n) 󰇛󰇜


The characteristics of Switch play an important role for high speed, low power and high-resolution
DAC. Which decides the Non linearity say DNL and INL of DAC. There are other architectures in
implementation say unary current steering DAC. Said architecture is complex in terms of number of current
sources. Here each current source has the same value of Io (reference current) but number of current sources
are (2N-1) and same no. of switches as well. It offers advantage in form of less glitches but required more

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5644
area. It is also required to have additional hardware to convert binary code into thermometer
code [19-22].
Looking to implementation, area efficiency and free from additional hardware for binary to
thermometer code conversion, Binary current steering DAC structure is the simplest one. N bit configuration
which needs only N current sources. They are straight-forwardly worked by the linear binary input codes.
However, due to the inadequate synchronization of the switches and dynamic behavior of the circuit,
large glitches in form of impulses are observed at the output terminal. This problem is addressed using better
switch. This structure also offers a merit in form of less no. of transistors as well. The 4-bit binary weighted
current steering DAC is as shown in Figure 2 [17, 20, 23-25].
Figure 2. 4-bit binary weighted current steering DAC
The present work is focused to design and analyse the effect of various types of switches on non
linearity eroor say DNL and INL. Based on understanding from the literature survey, three types of switches
say NMOS, PMOS and transmission gate have been tried and simulation of each one has been carried out.
For the proposed design and simulation, cadence tool is used with 180 nm CMOS technology. The proposed
current steering DAC offers desired INL and DNL with rated power consumption.
3. VARIOUS SWITCHING APPROACHES
A main source of nonlinearity originates because of glitches in the current cell. More no. of
transitions results more no. of changes the states of switches say on to off or vice versa. In case of 4-bit
binary weighted DAC, when input changes from 0011 to 0100, big glitch is observed because of 3
transitions. Similarly, when input changes from 0111 to 1000, even big glitch will be there as there are 4
transitions. In case of unary weighted DAC, there is only 1-bit transition so there is no glitch but it needs
more no. of current sources; for 4-bit unary current steering DAC, 15 current sources of having same value
are required. Thermometer code is used to control the switches. Additional hardware is required to convert
binary code into thermometer codes [26-29].
Characteristics of switch also play an important role. There are various options for the same:
NMOS, PMOS, Transmission gate. NMOS and PMOS are used as a single device and controlled by sinlge
input. While transmission gate is parallel combination of NMOS and PMOS and complementary control
inputs are desired in this case [22].
4. SIMULATION RESULTS AND DISCUSSIONS
4-bit binary weighted current steering DAC have been implemented using cadence virtuoso in
CMOS 180 nm technology. This converter developed and simulated in a 180 nm CMOS technology with
supply voltage of 1.8 V. Based on three different type of switches, three possible combinations have been
simulated. DNL and INL were calculated for all three DACs. It has been observed that the DNL and INL in
case of DAC having Transmission gate are as ±0.42 LSB and ±0.4LSB, respectively. With the operating
frequency of 200 Mhz, simulated power consumption was 22 mW.

Int J Elec & Comp Eng ISSN: 2088-8708 
Design and implementation of 4-bit binary weighted current stering DAC (Jayeshkumar J. Patel)
5645
Authors represented and compared three architectures and their outputs in form of currents.
Figure 3 to Figure 8 shows the simulated results and output of proposed segmented DAC using various kinds
of switches say NMOS, PMOS and transmission gate. Each architecture having two parts: one is current
mirror and second is switching elements, current mirror part is common in all three architectures.
In case of NMOS switch-based architecture, big glitches have observed and same results poor non
linearity. Glitches are available when there are a greater number of transitions in digital inputs e.g when input
change from 0111 to 1000, prominent glitch is there. The step size should be equal but it is observed that
even in some cases of input changes, it is reduced rather than to be increased. Same will have adverse impact
on non-linearity error in terms of INL as well as DNL. Here in case of PMOS switch, same kind of
observations are there as observed in case of NMOS switches. As digital input increase, output should
increase. It is not always observed in case of NMOS and PMOS kinds of switches.
Transmission gate is one good option as a switch. Architecture having transmission gate offers a big
advantage in form of reduction of glitches as well as continuous rise of current as desired which makes lesser
value of INL and DNL. Graphs for INL and DNL of proposed current steering DAC using transmission gate
switches are represented in Figure 9 and Figure 10 respectively. The simulated result of binary weighted
DAC using transmission gate as a switch are as shown in Table 1.
Figure 3. Architecture of binary weighted DAC with NMOS switches
Figure 4. Current output of binary weighted DAC with NMOS switches

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Figure 5. Architecture of binary weighted DAC with PMOS switches
Figure 6. Current output of binary weighted DAC with PMOS switches
Figure 7. Architecture of binary weighted DAC with transmission gate switches

Int J Elec & Comp Eng ISSN: 2088-8708 
Design and implementation of 4-bit binary weighted current stering DAC (Jayeshkumar J. Patel)
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Figure 8. Current output of binary weighted DAC with transmission gate switches
Figure 9. DNL graph of 4 bit binary weighted DAC having transmission gate
Figure 10. INL graph of 4 bit binary weighted DAC having transmission gate

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Table 1. Simulated result of DAC
Parameters
Value
Parameters
Value
Technology
180
INL (Max)
0.34 LSB
Resolution
4-bit
DNL (Max)
0.38 LSB
Approach
Binary weighted
Power (Max)
22mW
Supply voltage
1.8 V
Frequency
200 Mhz
5. CONCLUSION
A binary weighted 4 bit current-mode digital to Analog converter (DAC) useful in the field of
biomedical application designed and simulated using 180nm CMOS Process.In this implementation
the authors have calculated INL and DNL of DAC having NMOS, PMOS and transmission gate as
a switch. It is desired to have INL and DNL in the range of +0.5 LSB. Based on comparison. It has been
observed that Digital to Analog convertor with transmission gate as a switch, DNL and INL are 0.38 LSB
and 0.34 LSB respectively. Power consumption is observed as 22mW.
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BIOGRAPHIES OF AUTHORS
Jayeshkumar J. Patel is pursuing his Ph. d from Nirma University, Ahmedabad in the field of
mixed signal circuit design. He obtained his M. Tech in EC (VLSI Design) from Institute of
Technology, Nirma University, Ahmedabad in 2007. He did his B.E from BVM College of
Engineering, Vallabh Vidyanagar. He has published many technical papers in national and
international journals. He has more than 18 years of experience in teaching at UG & PG levl.
He also worked for industry for about 5 years. His research interest includes Analog and Mixed
signal VLSI
Dr. Amisha Naik is working as Associate professor at EC department, Nirma University,
Ahmedabad since 2001. She teaches both at UG and PG level. She did B. E in Electronics from
Regional Engineering College, Surat in the year 1997 and obtained M. Tech and Ph. D in VLSI
Design area in the year 2006 and 2011 respectively from Nirma University, Ahmedabad. She has
published many papers in national and international journals. She is guiding several Ph.D
students in the area of analog and mixed signal circuit design.