OTA ile sayısal/analog ve analog/sayısal dönüştürücü tasarımı

Abstract

OTA ( İşlemsel iletkenlik kuvvetlendiricisi) giriş gerilimlerinin fark ile kontrol edilen bağımlı bir akım kaynağından oluşan devre elemanıdır. Günümüzde OTA, özellikle yüksek frekanslarda temel aktif eleman olarak, işlemsel kuvvetlendiricinin (OP- AMP) yerine almaya başlamıştır. Yüksek frekans performansının iyiliği, dışardan uygulanan kontrol işareti ile iletkenlik (eğim) değerinin ayarlanabilmesi, CMOS teknolojisi ile uyumluluğu ve entegre devre üretimi kolaylığı vb. OTA'ya olan bu ilgi artışının nedenleri arasındadır. Bu tezin amacı, Sayısal/ Analog (D/A) ve Analog/Sayısal (A/D) dönüştürücülerde çalışma frekansını OTA kullanarak yükseltmektir. Nitekim, bunda başarılı olunmuştur. Sayısal/ Analog dönüştürücüler için, OTA'nın toplama devresinden yararlanılarak iki farklı yöntem incelenmiştir. Analog/Sayısal dönüştürücüler için ise, OTA ile gerçekleştirilen integral alıcı devre ve Sayısal/Analog dönüştürücüler kullanılarak dört farklı yöntem incelenmiştir. Yöntemler örnek üzerinde gerçekleştirilerek, sonuçlar karşılaştırılmışıtır. Ek'te "Micro cap" devre analizi programı yardımıyla, 4-6-8-10-12-14 ve 16 bitlik OTA ile gerçekleştirilen DAC'lar için frekans cevabı ve analog çıkışı (4-6-8-10 bitlik durumunda) verilmektedir.

Most of the real-world physical quantities such as voltages, current, temperature, pressure and time etc are available in analog form. Even though an analog signal represents a real physical parameter with accuracy, it is difficult, store or transmit the analog signal without introducing considerable error because of the superimposition of noise as in the case of amplitude modulation. Therefore, for processing, transmission and storage purposes, it is often convenient to express these variables in digital form. It gives better accuracy and reduces noise. The operation of any digital communication system is based upon analog to digital (A/D) and digital to analog (D/A) conversion. Analog / digital converters, which range from monolithic ICs to high performance hybrid circuits, modules, and even boxes convert analog data - usually voltage into an equivalent digital form. Characteristics of A/D converters include absolute and relative accuracy, linearithy, no missing codes, resolution, conversion speed, stability and price. The A/D converter (ADC) is operated at the rate of fcik samples per second. For proper operation, most A/D converters require that the input be held constant while conversion is taking place. Thus, the ADC must be preceded by a S/H amplifier to freeze the band - limited signal just prior to each conversion. Also Digital / Analog convertion reconstitute the original data after processing, storage or even simple digital transmission from one location to another. The basic converter usually consist of an arrangment of weighted resistance value, each controlled by particular level or 'significance' of digital input data that is switched to develop varying output voltages, currents, or gains by selective summation in accordance with the digital input code. The output of a D/A converter is IX proportional to the reference source used. The D/A converter (DAC) is usually operated at the same frequency as the ADC and if the application demands, is equipped with appropriate deglitching circuitry at the output. Finally the staircase like output is passed through a smoothing filter to ease the effects of quantization noise. Although most converters for data-handling applications are used with essentially fixed references, there is a special class of converter, capable of handling variable and even bipolar ac reference source. Fig 1. depicts the most general context within which A/D and D/A conversion is used. Analog signal Vj is converted to be processed, or perhaps just transmitted or recorded, in digital form. Once processed, received, or retrieved, the signal is D/A converted to be reused in analog form. Analog in Antialiasing filter S/H ADC DSP DAC smoothing filter Analog out Fig 1. Typical A/D and D/A converter application In this study, the aim is realization Analog/Digital converters using Operational Transcoductance Amplifiers (OTAs) that provide linear electronic tunability of its transfer gain (gm). Section 2 is about OTA element and internal diagram of Bipolar and CMOS OTAsand limitation on input signal amplitude is described. The OTA is a differential voltage controlled-current source whose transconductance can be controlled by an external current. The high frequency performance of the OTA is noteworthy and quit advantageous according to OP- AMP. While an OTA can operate at frequencies in MHz's, for a typical OP- AMP this is limited in some hundred KHz's. The circuit sambol for an ideal OTA is shown in Fig 2.a. As shown in Fig 2.b it is modeled by an ideal VCIS characterized by the relation l. = gm(V*-V) (D with infinite input impedance and infinite output impedance. Tipically, gm is a very small number. Fig2.a. OTA symbol V*. Jo ^gm(V--V) Fig 2.b. Equivalent circuit of ideal OTA The unique feature of an OTA is that it is possible to vary gm over a wide range by means of an external control current Icnt. For Bipolar OTA' s there is a linear dependence of gm on Icnt that can be written as gm= K Icnt. The linearity of the dependence of gm on Iont will ordinarily be valid over a several decade range of Icnt for example 0. 1 to 400 uA. The gain bandwidth limiting parasitics, which may be of even greater significance than the intrinsic frequency dependence of gm, are the input and output impedances. These effects are shown in the more complex model of Fig 3. Representative values of the resistors are R;=1MQ and Ro=50Mfi. The capacitors of a few picofarads. V* V~ V Ç.HR. t3*,t> g^sXVW) :cJ Ro v0 Fig 3. An OTA model with input and output impedances XI An important circuit of OTA is implementation of a summer. This requires one OTA for each input to be summed. The output currents of these OTAs are conveniently summed with another OTA. The configuration for summing two input V, and V2 to produce an output V0 is shown in Fig 4. Fig 4. Summer The output voltage V0 = -Ios / gms = (Id + Io2> / gms = (Vi gml + V2 gra2) / gn V0 = gml / gms V! + g^ /gms V2 (2) Thus V0 is the scaled sum of Vi and V2. In the physical design of an OTA circuit, due consideration must also be given to the intrinsic properties of the device. One of these is the limitation on the magnitude of the input signal for linear operation. In section 3, Digital/ Analog converters are defined and two method for Digital/analog conversion using OTA is described. The schematic of a DAC is shown in Fig 5. VR Fig V. Schematic of a DAC XII The input is an n-bit binary word D and is combined with a reference voltage Vr to give an analog output signal. For a voltage DAC, the D/A converter is mathematically described as V"=K(b12-1+b22-2+....+b"2-n) (3) where V0is the output voltage and bib2...bn is the n-bit binary fractional word. There are various ways to implement Eq.(3). In this thesis, Eq(3) is implemented by using of the summer circuit of OTA as shown in Fig 4. The resulting circuits which are completely suitable for the integrated circuit technology, have a good high frequency behavior. In section 4, Analog/Digital converters are defined and some methods of the ADCs are realized by using of the OTA. In the Appendix, analog output of 4-6-8-10 bit OTA-DACs for a counter digital input and frequency response of 4-6-8-10-12-14-16 bit OTA-DACs are presented.