ISSN 1063-7397, Russian Microelectronics, 2017, Vol. 46, No. 1, pp. 45–54. © Pleiades Publishing, Ltd., 2017.
Original Russian Text © M.M. Pilipko, D.V. Morozov, D.O. Budanov, 2017, published in Mikroelektronika, 2017, Vol. 46, No. 1, pp. 50–60.
Comparative Analysis of CMOS Circuits of a Thermometer-to-
Binary Encoder for Integrated Flash Analog-to-Digital Converters
M. M. Pilipko*, D. V. Morozov, and D. O. Budanov
Peter the Great St. Petersburg Polytechnic University, St. Petersburg, 195251 Russia
Received February 7, 2016
Abstract⎯The speed of a flash analog-to-digital converter (ADC) is limited by both the comparator response
time in the input analog part of the circuit and the delay time of the encoder that converts the thermometer
code on the comparator outputs into the output straight binary code. In this paper, we consider the problems
of the synthesis of CMOS circuits of encoders for integrated flash ADCs. New encoder circuit designs with a
reduced delay are proposed. The comparative analysis of the main characteristics of CMOS circuits of encod-
ers based on the data of circuit simulation for the 180 nm MOSFET technology is presented.
Flash ADCs are characterized by a low bit depth
(up to 8 bits) and significant power consumption (up
to units of watts). However, in applications that
require the input signal processing with a frequency
greater than 1 GHz, this ADC type has no alternative.
The flash ADC structure consists of two parts (Fig. 1).
The first part is an array of comparators that compare
the input signal level with the set of reference voltages,
which are, for example, generated by a chain of resis-
tors. Depending on the comparison result, each com-
parator outputs a logical zero or a logical one. The
thermometer code is generated at the output of the
array of comparators. The position in the code below
which there are ones and above which there are zeros
is the digital equivalent of the analog input signal. The
second part of the ADC is a digital circuit, the ther-
There are several types of encoders for integrated
flash ADCs. They differ in circuit complexity, power
consumption, and speed [1–9]. In this paper, we con-
sidered the problem of the synthesis of CMOS circuits
of encoders and proposed new circuit designs for
encoders that combine reduced current consumption
with a reduced delay. The results of circuit simulation
using 180 nm MOS technology parameters are given.
The comparative analysis of the main characteristics
of the considered encoder circuits was carried out.
1. KNOWN TYPES OF ENCODERS
1.1. Encoder Counting Ones in the Input Code
This encoder is a tree of full adders. It counts the
number of ones in the thermometer code. The sum-
mation order of the input signals can differ. However,
in order to reduce the delay time of the generation of
output bits, the Wallace tree structure is used .
Thus, the encoder is usually called the Wallace tree
encoder  (Fig. 2). This circuit can correct errors.
For the thermometer code containing k errors, the
Fig. 1. Three-bit flash ADC structure.