Chapter 2

Field Programmable Analog Array (FPAA)

2.1 Introductio n

f I' AA is an integrated circuit, which can be configured to implement various

analog function!i using n set of configurable analog blocks (CAB) and a programmable

interconnection network, and is programmed using on-chip memories. Programming of

an FPAA is done both in terms of the topology of the circuit to be im.itantiated, and in

terms of its cin;uil parameters r29].

A generic FPJ\A containing several C/\Bs connected together through the use: of

an interconnection network fl 3). The confi811n11ion oil s tring is ston::d in u shift register.

Some bits in the bit string are used to configure the connectivity of the interconnection

network. Other bits are U!ied to program the functionality of the CAD:s.

A CAB is programmed to implement one of i:everal analog functions such as

addition, multiplicntion, logarithm, amplification, integration, and so on. Finally some

bits are used to fine-tune the parameters defining the various functions reaJiled in the

CABs. For example, parameters could include the gain of an amplifier or the comer

frequency of a l os~y integrator. Interconnection networks can take the form of a tree,

crossbar, or data path r 13].

function parameters. such a.c: gain and comer freq uency, ore usunlly prosrarnmcd

using a voltage in a continuous range between the power supplies. These continuous

variable parameters are discretized and then loaded into the shift register in digital fonn,

12

and can be converted to an analog value using signal converters. The analog value is then

w;;cd to control transconductors and variablt: capacitors. The CABs on an FP AA may be

homogeneous or heterogeneous [ 13). For example, an FPAA could contain specialized

CABs, which realize only a few different functions, or the FPAA could contain CABs

which arc homogeneous but can be configured as many different functions.

2.2 F P AA D esign B nckgro u nd

This section is an overview of the circuit techniques used in the various research

and commercial FPAA designs. As an t!Xllmple, Sivilotti implemented a conceptual

FPAA design called Proto-chip [ 171. The Proto-chip's CAils nrc designed at Lhc

lransisto r level, and the interconnection network is based on a tree structure. Its target

application was for the prototyping of analog neural networks. A successful physical

design was the one by L ee and Gulak [12j. It was based on sub-threshold techniques and

operates below lOOkHz frequencies. Its target application was for the hardware

implementation of neural networks. The advantage of using the sub-threshold technique

was that the currents used are extremely low, leading to a very low power design.

Different types of FPAA were studied & they arc summarized in the Table 2.1.

The different Available PPAAs are Commercial FPAAs and University FPAAs, in which

Anadigm is the most popular FP AA.

2.3 FP AA D esign Technologies

The structure of the CAB's components depends on the design technologies used.

FPAAs are designed in both continuous-time nnJ Jiscrete-timc domai11:>. A discrclc;-lime

13

FPAA is designed with switched-capacitor or switched-current technology. The idea is to

obtain a variable resistance using a different frequency for commutation of the

interrupter. The advantages of this technology can be appreciated in terms of

programmability and insensitivity to resistance in programming switches [15].

Table 2.1: Different comm ercial and research FPAAs

FPAA Technology Technique n o u tin g

Bandwidth Source Elcment11

AN120E04 Anadigm CMOS Switched Data-path 2MHz

AN121E04 Canacitor

AN131E04

MPAA020 Motorola Discrete Di:st:rete Array 200KHz time time

IMP lnc. CMOS Switched Array 125kHz

EPAC50E30 Capacitor

FAS- Fast Analog Bipolar Continuous Switchable cells 4MJiz

TRAC020 Solutions time

Lee & Gulak University CMOS Continuous Transconductor lOOkHz

of Toronto time

Gaudet & P .G. Univ. CMOS Current Transmission IOMHz

Gulak of Toronto conveyor Gate

A continuous-time FPAA is usually designed using t:ransconductor or Op-Amp

technology. The busic cell consists of an Op-Amp or OTA and programmable capacitors

linked by a transconductor-based array. These devices have advantages in terms of

bandwidth, but have narrow programming range for their parameters [29].

14

2.3.l Discrete Time (Switched Capacitor) FPAA

Switch Capacitor circuits are popularly used for realizing analog signal processing

blocks like:: filtering functions, gain-stages, voltage-controlled oscillators, modulators etc.

in MOS integrated circuits. As a filtering technique, switched-capacitor are popular due

to their accurate frequency as well as good linearity and dynamic range. Accurate

discret~time frequency are obtained since filter coefficients are determined by

capacitance ratio which cim be set quite precisely in an integrated circuits and is better

than that which occurs for integrated RC time constants. Once lhe coefficients of

switched-capacitor discrete-time filter are accurately determined, its overall frequency

response remains u function of the:: clock (or sampling) frequency. The clock frequencies

can also be set very precisely through the use of a crystal oscillator [30].

Switched Capacitor circuits arc readily suited for programming and

reconfiguration because of the existing switche;:s, which can bt: used for programming and

reconfiguration. Switches used in SC networks must be distinguished from the switches

used for progrnmming. Programming switches either co1rnecl or disconnect additional

elements to the element to be programmed. Switches in SC circuits form an integral part

of the circuit and they act as sampling elements o.clivated by non-overlapping clock

phases. They transfer charges on a particular set of capacitors to other capacitors. The

programming and reconfiguration of capacitors switched by non-overlapping clock

phases; can be achieved without using any additional switches.

In order to program the values of un-switched capacitors, we use programming

switches such as the programmable capacitor array (PCA). By altering the clock

frequency, effective capacitor values can be changed without changing their physical

15

d;mcnsion3. SC cin;uits can be designed very accurately by adjusting the ratio ot

capacitors, which can be achieved in a CMOS process. The voltage-mode operation and

the u:;c of Op-Amps provide nearly ideal summing of different signals. More importantly,

no complicated on-chip tunins circuil i::s neces:mry as is the case in current-mode anc.I

other continuous time applications. However, the sampled-data nature of the SC circuits

requires pre and post-processing filters for anti-aliasing and reconstniction (smoothing)

l26,30-31 ].

Advantages of ~witched Capacitor Circuits are

i) C'ompatihility with CMOS technology

ii) Good accuracy of time constants

iii) C.ood voltage linearity and

iv) Good temperature chaructcristics.

Disadvantages of Switched Capacitor Circuits are

i) Experience clock feed-through

ii) Require a non-overlapping clock and

iii) Band'l'.ridth of the signal must be less than the clock frequency.

2.3.2 Continuous Time FPAA

In this technology continuous-time signals are routed among programmable

analog blocks to implemem the circuit. The configurable connections are realized by

CMOS switches and are either buffered or nulled to cancel parasitic error. The function

blocks are Op-Amps combined with passive networks, which allow programmable

16

transfer function<: with nccurucy insensitive;; 10 vurialions in process parameters and

environment f26,31-32].

Advantages of continuous time circuit

i) Doesn't required non-overlapping clock

ii)Anti-aliasing fi lter at receiver not requi red

iii)Wide bandwidth

iv)Good voltage linearity

Disadvuntages of continuous time circuit

i)Limiled tuning range

ii)Required Multi-Valued Memories (MVM) in each connection cell

2.3.2.1 Op-Amp bused FPAA

In designing FPAA, the transconductor can be used as a connection cell in a

Crossbar network for interconnecting CAns lhot consist of Op-Amp with leedback

capacitors that can be connected into the circuit with switches controlled by registers.

Since the transconductor is insensitive to distributed parusitic capaciumcc the PP AA will

also be parasitic insensitive. Transconductance of the transconduclors. can be chan~ed by

adjusting the control voltages, und has a nming ningc limited to approximately one

decade only (without introducing excess noise). The architecture based solely on four­

transistor transconductors consumes large area especially for low frequency applications

<luc to the use of long transistors and a Multi-Valued Memories (MVM) in each

connection cell [33).

17

2.3.2.2 Differential Op-Amps with feedback capacitors-based FPAA

The CABs can be configured as integrators (differential Op-Amps with feedback

capacitors). Controlling the gate voltages of the transconductors through the multivalued

memories can program different transfer functions. The frequency response of the filter

will not be affected by the parasitic capacitance of the transconductors and the co1mection

wires because the parasitic capacitance will be cancelled in a first order approximation

due to differential signaling [33].

2.3.2.3 OTA-based,FPAA

In recent years, OTA (Operational Transconductance Amplifier) based FPAA

(field programmable analog arrays) have received great interest, as they can achieve

benefits in analog circuit and system design as field programmable gate arrays (FPGA)

have in the digital counterpart. Having an operational transconductance amplifier (OT A)

of programmable transconductance and a programmable capacitor, it is possible to build

til ters for a wide frequency range. For most CMOS processes, it is not possible to create a

capacitor with a wide range of programmability, which can operate at high frequencies.

Thus it is very important to have a widely programmable transconductance. Further, OT A

is an excellent current mode device to reali:i::e high frequency resistor less analog designs

[14,18-19, 32].

A survey of the literature dealing with OTA-based FPAA was done. Academic

research has reported the following FPAAs.

• The OTA-based FPAA using Cross-Bar Interco1mection from Bengal

University [ 18).

18

• OT A-based FP AA based on Floating-Gate Devices from Georgia Institute

of Technology [ 1 S.19].

OTA-based FPAA using CABs from Technical Univcrsily of Gdansk,

Polctnd [14).

FPAAs mentioned above, are University FPAAs in which FPAA of Technical

University of Gdansk, Poland is found to be the most suitable. ll uses fewer switches

compared to the others because of its regular structure vf interconn~ction network.

A) OTA-based FPAA using Cross-Bar Interconnection

OT A-based FP AA using Cross-Bar interconnection structure enables realization

of any arbitrary network of OT As [ 181. A simple CMOS linear OTA has heen used. The

PPAA is a regular squan: array of OTAs inlcrco1mected, a:; shown in Fig. 2 .1 , with a

crossbar structure having horizontal and vertical interconnection lines. The horizontal

interconnection lines enter as input to the OT A cells (inverting and non inverting terminal

of OTA), while the cell outputs arc connected to the vertical lines. In this crossbar

structure, input of any cell can be connected to the output of any other cell of the FPAA

via switches placed on the intersection of vertical and horizontal Jines. Also the input and

the output of the same cell can be connected. Since the signals are in current mode, these

are summed on the interconnection links. Further, if more than one cell receives the

signal from a cell , the s ignal will be divided among the receiving cells.

Connectivity between the OTA blocks in the array plays a significant role on the

perfonnance of the circuit realized on the FPAA. For long signal lines cross talk, parasitic

effects, and electromagnetic interference are the main sources o f noise. This noise affects

19

the stability and performance of the analog and mixed sigual dn.:uil, particularly at high

frequencies.

·:

0

~- EB Q)~t s + 4 ,. rS

~- ~ ~ 8 •_•.c

:~Q) Q t b1-

~ @"l

:JL-6 ~ :::t .,

t"' 109876 1 34 OTA cont

""' :ol

Figure 2.1 Generalized OT A based FP AA structure with interconnection

In view of this situation, alternative topologies like Local Interconnection and

Global Interconnection have been rropm:ecl . l-lowevcr, ;" order to achi"'v" hii;hcr

flexibiliiy in realizing any linear I nonlinear design, they stick to crossbar interconnection

topology.

B) OTA-based FPAA based on Floating-Gate Devices

The computational logic in the FPAA was organized in a compact computational

analog block (CAB) that consist~ of four-by-four matrix multiplier, three wide-range

OTAs, three fixed value capacitors, a capacitively-coupled current conveyor (C4), a

signal-by-signal multiplier, one pFET and one nFET [ 15, 19].

20

~=~-...

(a) (h)

Figure 2.2 (a) Computational Analog Block (CAB) for an FPAA based on flMting­r;atc devices, (b) Overnll block diagram for a large-scale FP AA. The switching interconnects are fu lly coonectablc c ross bur networks built using floating-gate transistors

Crossbar Network of size 40 X 46 was required for each CAB for connecting

circuit components within it. CABs were positioned in two columns and two rows; an

illustration is shown in Fig. 2.2. CABs wen: tiled across lhe chip in regular mesh-type

architecture with busses and local interconnects in-between. /\. huiie amount of switches

were required, results in large parasitic effects on FPAA chips. These were due to

parasitic resistance and capacitance on FPAA interconnects [ 151.

C) OT A-based FP AA using CMOS transistors

A general configurable analog block (CAB) WO!! used, which consists of the

programmable OTA, programmable capacitor and MOSFET switches [141. Using the

CABs, the universal tunable and field program-able analog arruy (FPA/\.) were

constructed. The OT A is based on two cross-coupled differential MOS pairs and digitally

programmable current mirrors are shown in Figure 2.3 . The overall transconductance.

"gm", of the OT A is given by equation (2.1 ).

21

dlvu~ !Im - -- = J( \ <i.IL

dVi.! (2. 1)

Two methods cun be used to change the trunscomluctancc of OT A; one method is

to tune the floating bias voltage by an analog voltage, other is to make the gain of the

output current mirrors programmable in a digi tal way.

Trclilsistors Ml to M4 comprise the cross-coupled differential input pair. biased

by the current coming from the current source flowing through the floating voltage

source Vb. Both lhe current source and the voltage source are controlled by a control

voltage, so that simultaneous change of the bias current and voltage Vb is possible

..-~~~~~~~.--~~---.~~~D

1.,i.

1,,.,,, /..nl

I.A I · A Cunenl Cuntnl

MllTOIOI Mi11010l

Figure 2.3 Simplified schematic diagram of the CMOS programmable OTA

The relation between Vb and control voltage is linear. Voltage Vb can be set to

any value in the range from about 22 mV up to about 500 mV, which corresponds to the

range of control voltage from 1.9 to 3.1 V. This makes "gm" tunable by a factor of about

22. A current mirror of programmable gain can be n:ulized u::;ing the well-known high

compliance current mirror :itructure with 3 1 identical output stages. Thus OT A

transconductance can be adjusted by a factor of 700.

22

FPAA consists of 40 CABs positioned in eight columns and five rows,

additionally three OT As o t - o3 act as signal buffers. Input signals arc delivered through

lines i I. i2 ·and i3. Because of this, up Lo three different fillers can be realized I

simultaneously. The transconductance parameters of all the OT As are controlled by an

external control voltage and through digital switching of the output current mirrors.

While control voltage is common for all the nmplifiers i11 the array. it is still po<>sible to

set the transconduclu.ncc of every UT A scp:iratcly by :;ening the gt1in or the OT As current

mirror. lne FPAA was physically implemented in the 2-µm n-well CMOS process

through MOSIS. Programming of the FPAA is performed through serially ·hitting cliui tAI

words of 880 bits (14].

Fig. 2.4 Structure of FPAA

23