Electrical Engineering Dept, Ecole Polytechnique de Montreal

LOGARITHMIC PROGRAMMABLE PREAMPLIFIER DEDICATED TO ULTRASONIC RECEIVERS R. Chebli, A. Kassem, M. Sawan PolySTIM Neurotechnology Laboratory, Electrical Engineering Dept., Ecole Polytechnique de Montreal Email: robertchebli I abdallah.kassem I mohamad.sawan0,polvmtl.ca

ABSTRACT- This paper concerns the design, the implementation and the validation of a fully integrated preamplifier dedicated to ultrasonic receivers. The preamplification technique is based on two amplification stages: a logarithmic stage called True Logarithmic Amplifier (TLA) and a programmable-gain module built around a Timing Gain Compensator (TGC). The TLA largely amplifies small amplitude signals, and moderately the large amplitude ones. However, the TGC is used to compensate signal attenuation caused by its traveling several human body tissues. Those main building blocks of an ultrasonic receiver are realized using CMOS 0.35pm technology. Spectre simulations of both the TLA and TGC show unity gain bandwidths of 100 MHz and 127 MHz respectively when driving a load of 1pF. Measurements of the fabricated chip are done in our laboratory using an external digital controller programmed in FPGA. The total chip area is 7.2 mm2 including the digital part needed to program the TGC. Index Terms- Logarithmic Ampliyer, Preamplifier, Ultrasonic

the proposed preamplifier is the subject of section 111. Section IV presents both the simulation and experimental results obtained for the prototype, and the paper is concluded in section V.

11. BRIEF DESCRIPTION OF THE GLOBAL ULTRASONIC SYSTEM The block diagram of the global ultrasonic system, including a microprocessor to synchronize the movement of sweeping, transmission, reception, scan converter and displaying, is shown in Figure 1. The entire operation of the system is performed in real-time process.

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Systems, Echography, TGC, Mixed-signal IC.

DISPLAY

I. INTRODUCTION Today ultrasound technique is one of the most popular and efficient non-invasive methods in medical diagnostics. The term ultrasound refers to high-frequency sound waves, above the limits of sensitivity of the human ear (-20 KHz). The ability of the ultrasonic wave, in non-invasive medical diagnostics, is to obtain information about the structure and nature of tissues and organs of the body. Process in microelectronics have significant impact to build handheld ultrasound devices. The miniaturization of such systems provides low-power dissipation, low-noise, good bulk, facility displacements and low weight [ 13. Available receivers are made by discrete components on PCBs [2], and/or combined with software drivers [3] which are used in different occasions to get the needed functions. We proposed recently an integrated version of the front-end preamplifier which is dedicated to hand-held ultrasound devices. The implemented receiver contains the two main building blocks, a True Logarithmic Amplifier (TLA), and a Timing Gain Compensator (TGC) [I]. In this paper, we report the measurement of the previously fabricated device, which groups the basic above presented building blocks of the ultrasonic receiver. Section I1 presents a brief description of the global ultrasonic system. The topology of

0-7803-7448-7/02/%17.0002002 IEEE

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Fig.1. Block diagram of the global ultrasonic system. The emitter of an ultrasonic system generates an ultrasonic beam in front of a body. Then a receiver is used to receive the returned small amplitudes ( 5 pV to 500 mV) echoes. Amplification and compensation techniques are next employed to focus on the needed signal. Following the logarithmic amplification, the received amplified signal is processed by a variable gain amplifier to compensate the attenuation of the received ultrasonic wave. This compensation allows the amplitude of all ultrasonic waves to be in the same dynamic range from 0 dB to 80 dB. The Scan Converter resamples the echo’s envelope to transform it into luminous brightness on the screen [4]. A postprocessing is then performed to select zones which must be privileged by a number of levels of gray (amplitude). A decompression step based on a D/A conversion reproduces a video signal which comes from the digital data. The video signal processing block (DSP) is used as an interpolator, demodulator and a band-pass filter in order to form the envelope of the resulting signals and to eliminate their side lobes. Finally, an LCD displays the images which are controlled by the microprocessor through the Graphical Programmed InpuUOutput (GPIO).

111. TOPOLOGY OF THE PREAMPLIFIER The preamplifier groups two main stages: a logarithmic amplification circuit followed by a stage of compensation circuit.

and their corresponding gains. Indeed, the dynamic range is equal to (A + where A is the gain per stage, and its value is the following:

A= A. The LogarithmicAmplifier The logarithmic amplifier (logamp) converts input signals with a wjde dynamic range into a defined range output voltages. The topology of the implemented logamp allows to maintain the frequency and the phase input signals within a dynamic range of 80 dB [5]. The logamp (also called a =A) is implemented of six dual-gain stages connected in cascade. Each stage consists of two parallel modules: limiting and unity gains and the outputs of these two amplifiers are tied to an analog adder. The schematic of Figure 2 shows the differential topology of one dual gain stage. The gain is 16.6 dB per stage which gives a TLA with total gain of 100 dB. This gives the logarithmic transfer function as illustrated in Figure 3. Vdd

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where, KI and K3 are the transconductance constants of

transistors MI and M3.

B. The Time Gain Compensator amplifier The time gain compensation amplifier is a critical receiver component for the ultrasonic beams, since the magnitude of :the reflected ultrasound signal depends on the depth of penetration and it is much greater close to the receiver, the gain must be increased as time increases and compensation techniques must be used [6]. It allows to amplify per decibel each one centimeter of depth according to examined areas of the body. This technique consists of using an amplifier with programmable gain that pliiys the role of a TGC. A new approach of programming the gain is implemented, based on the variation of the widths of transistors, Figure 4 shows the block diagram of the whole TGC amplifier. The controlled gain with maximum dynamic range of 80dB is employed to precisely compensate each received echo from the abdomen human. The programmable gain of each stage is based on a transconductance (G,,,), a transimpedance (r,,,) and a buf’fer which is useful to separate the cascaded stages.

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IV. SIMULATION AND EXPERIMENTAL RESULTS

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The designed circuit, of the preamplifier were fabricated using CMOS 0.35 pm technology. The total area of the chip is 7.2 mm2 and the corresponding photomicrograph is shown in Figure 5.

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Fig.3. Simplified transfer hnction of a TLA. The TLA transfer function consists of a six lines segment with crack points indicating the different gain limits of the each limit stage [5]. Using differential topology circuits reduce the noise effects as well as the oscillations resulting from the retroactive gain of different stages through the substrate. Noting that the dynamic range and the precision are determined by the number of stages

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Tables 1 and 2 show the main characteristics of the implemented TLA and TGC respectively. The simulation was done with Spectre under CADENCE platform. For both TLA and TGC building blocks, the power consumption is 9.4 mW which is very low for ultrasonic applications. Figures 6 and 7 show the simulations of the TLA transfer function and the maximum gain of the TGC stages. The input noise is greatly reduced as shown in Figures 8 and 9 for the TLA and the TGC respectively. Also, the logarithmic error and stepping gain error show that this system has good precisions and finally both have large unity gain bandwidth [7].

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The logarithmic representation on input-output transfer characteristics of the TLA is shown in Figure 10. The dynamic input range of TLA is depicted in Figure 11. It shows the value of -32 dBm.

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Figure 12 a) and b) show the experimental testing results when combining the digital control part programmed in FPGA with the programmable gain amplifier. The output swing of the TGC varies between 1.96 V and 365 mV when the binary output words of the controller are addressed to get values between 5 and 20 dB respectively, and with input swing of 200 mVp-p.

V. CONCLUSION The realization of an integrated front-end preamplifier dedicated to ultrasonic receivers has been described using the CMOS 0.35pm technology. We proposed a new preamplification technique that uses a logarithmic amplifier followed by a programmable-gain amplifier. The layout simulation of this system shows several advantages: low noise, low consumption, high precision, wide bandwidth and low power-supply voltage adequate for the future portable ultrasonic devices.

ACKNOWLEDGMEMTS Authors would like to acknowledge the financial support from NSERC, Alliance Medical and Micronet, and the chips fabrication from Canadian Microelectronics Corporation. The authors address their special thanks to Dr. S . Tabikh for his input to this work.

REFERENCES [l] Chebli, R., Kassem, A., Sawan, M. cdntegrated front-end preamplifier dedicated to ultrasonic receivers)), IEEEICECSOI, Vol. 3, pp. 1103 -1 106, Sept. 2001.

[2] P. Harris, M. Andrews, G. Turner, cdJltrasonic Transmission and Reception from Bulk-Micromachined Transducers)),IEEE

(b) Fig.12. Experimental results: a) 20 dB gain, for input swing of 200 mVp-p, b) 5 dB gain, for input swing of 200 mVp-p.

Trans. on Ultrasonics, Ferroelectrics, and frequency control, Vol. 48, No. 1, pp 224-23 1, Jan. 2001. [3] G. Schmitz, H. Ermert, T. Senge, ((Tissue-Characterizationof the Prostate using Radio Frequency Ultrasonic Signals)), IEEE Trans. on Ultrasonics, Ferroelectrics, and frequency control, Vol. 46, No. 1, pp 126-135, Jan. 1999.

[4]J. Ophir, {Digital Scan Converters in Diagnostic Ultrasound Imaging)) Proceeding IEEE, Vol. 67, No. 4, pp 654-663, April 1979. [5] W. Barber et al, crA True Logarithmic Amplifier for Radar IF Applications)),IEEE- JSSC, vol. 15, pp 291-295, 1980. [6] F. Piazza, P. Orsalti, Q. Huang, H. Miyakawa, (4 2M3.V 71MHz IF Amplifier 0.4pm CMOS Programmable over 80dB Range)), IEEE ISSCC97, Digest of Technical Papers, pp78-7!>, Feb. 1997. [7] N. Scheinberg, R. Michels, (AMonolitic GaAs Low Power I,Band Successive Detection Logarithmic Amplifiem IEEE!JSSC, vol. 29, pp 151-154, Feb. 1994.

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