NDT.net • Nov 2005 • Vol. 10 No.11 
Design of a low noise preamplifier for ultrasonic transducerL. Svilainis*, V. DumbravaSignal processing department, Kaunas University of technology, Studentu str. 50, LT51368 Kaunas, Lithuania, tel. +370 37 300532, Email.:svilnis@ktu.lt *Corresponding Author Contact: Email: svilnis@ktu.lt, Internet: www.ktu.lt/ultra/ Published in ISSN 13922114 ULTRAGARSAS, Nr.2(55) 2005 AbstractThe low noise preamplifier, used for the ultrasonic transducer, design approach is presented. Optimization is based on noise model for both electrical and the acoustical part of receiver. Every noise component has been investigated separately, using various transducers and operational amplifiers models. Noise behavior has been presented in diagrams for analysis. Every noise component has been minimized separately. For signal propagation evaluation, narrowband signal case has been considered. In has been shown that different ultrasonic transducers amplifier circuitry and amplifier type has to be chosen individually in order to improve signaltonoise ratio.IntroductionDevelopment of aircoupled ultrasonics is raising demands for electronics used. Because of large difference of acoustic impedances of solid body under investigation and air, testing signal is attenuated. Attenuation can reach up to 100dB, so powerful transmitters and low noise receivers should be used. Since a conventional ultrasound equipment [1] is not suitable in this case, development of dedicated equipment is on its way. Among high power excitation techniques, users are trying to squeeze out better parameters from the transducer preamplifier [2, 3].
The ultrasonic transducer is excited by an electrical pulse and transmits the ultrasonic pulse into air and only then a signal is entering the medium under investigation. The ultrasonic pulse travels in the material and is emitting some energy into air. This energy is picked up by a receiving aircoupled transducer. In this paper we concentrate on the receiving transducer and the preamplifier in order to obtain the best signal to noise performance. Possible solutionsA few possible solutions for performance optimization are available:
Noise modelThe noise model can be developed either into analytical form [4] or be modelled using some circuit oriented software like PSPICE [6]. Despite that the PSPICE modelling seems to offer the easiest and closer to reality approach, we decided to use the analytical model, since in such a case noise contributors can be analyzed separately, cutting out the sources level of which can not be modified or controlled. For this purpose we have developed the analytical noise model, incorporating both transducer and electronics units [7].
The noise spectral density of the is input resistance R_{t}
where k=1.380658.10^{23} [J/C]  Bolzmann constant, T is the absolute ambient temperature.
The transducer noise spectral density is determined by the real part of the transducer impedance [4] The feedback circuit resistors contribute the noise densities With the amplifier noise gain we get the equation for the output noise density calculation:
Signal propagation modelTo simplify the development of the signal model, we subdivide the task into two possible applications:
where etr is the transducer signal internal voltage source. Even further, since we are interested on how amplifier circuit elements are influencing the signal level, we do not need an absolute value of the signal received. We just need to have some sort of indication how the signal level will change with circuit parameters. So, we will be investigating the signal level change, regarding the maximum signal level achievable. Since best propagation is achieved when Rt is infinity, Eq.6 becomes where SF denotes the normalized signal. Signal propagation analysisIn order to start analysis of signal propagation, we have investigated various air coupled transducers [2, 5, 9] in order to evaluate the range of transducer parameters. In order to restrict ourselves we have concentrated on (400 800) kHz frequency range transducers, because of popularity of this range. Results of investigation are presented in Table 1.
By analyzing the data presented in Table 1, one can see that R_{s} range is within 10OW to few kW. Therefore we have chosen such a range in the signal propagation analysis. Results of application of Eq.7 for resistance range mentioned are presented in Fig. 4.
It can be seen that the ratio of input resistance R_{t} and the transducer impedance R_{s} should be above 10 times in order to have lowest signal attenuation (the highest SF). Noise analysisIn order to evaluate the importance and influence of circuit parameters, we now investigate Eq.5 components. The operational amplifier internal noise voltage source en is of major interest. As mentioned above, the simplest approach would be to minimize this parameter, by choosing an appropriate amplifier. In addition to that, this member has the simplest expression. We have chosen three amplifiers as representatives of achievable limits. One (LMH6624) is presenting the lowest voltage noise, an other (OPA657) is exhibiting the lowest current noise source and the third one (CLC425) has both current and voltage noise in between the first two candidates. The parameters are presented in Table 2.We start analysis using those parameters which do not interfere with other components of noise generation. Since the feedback circuit is not influencing the input components (the transducer and the input resistor) noise, we analyse these components first of all. In order to be able to neglect the R_{1} and R_{2} noise, their voltage spectral density has to be 1/3 (less than 5%) of that of contributed by en. Let as solve Eq.5 components for this requirement. The maximum R_{1} value for R_{1} thermal noise exclusion is calculates as The maximum R_{1} value for R_{2} thermal noise exclusion is The maximum R_{1} value for exclusion of the amplifier current noise in generated voltage when loaded by R_{1} and R_{2} parallel connection is It can be proved that the reasonable gain value to use in calculations is about 100. The results obtained for this gain level will hold true for all gain values in the range from 2 to 200. The calculations results, basing all the assumptions and Eq.8, 9 and 10 are presented in Table 2. The last two columns summarize the values recommended for R_{1} and R_{2}. In order to suit all amplifiers, in subsequent calculations we will be using the lowest values R_{1}=10W, R_{2}=1kW.
The next candidate for analysis should be the component which we can not be modified, e.g. transducer. Only transducer noise spectral density component e^{2}_{ntrtot} at the amplifier output with the gain 100 is presented in Fig.5. In order to reduce the complexity, the input resistance R_{t} is taken to be close to infinity.
The output noise can be calculated by integrating component e^{2}_{ntrtot} over a frequency range. One can see that noise is large at the transducer resonant frequency and is degrading by going apart. For the sake of simplicity, we take the integral over the wide frequency range 150 kHz to 2.5MHz, so there is no need for taking into account an individual transducer bandwidth. The value obtained represents the noise RMS value at the amplifier output: In such a way we have the ability to analyse noise RMS at the amplifier output behaviour versus the input resistance R_{t}. The diagrams obtained are presented in Fig.6. In order to be able to judge about the noise influence, the operational amplifier voltage noise source en contributed RMS level at the output is added. From the signal propagation analysis it can be seen that the input resistance R_{t} is desired to be over 10 times of the transducer R_{s}. Therefore R_{t}=10 R_{s} values for best signal performance are indicated for all three transducers.
One can see that using R_{t} which is optimal for signal propagation will cause an increase in the output noise level. For Tr1 this increase is insignificant with any amplifier, Tr2 noise will increase by about 2 times for the lowest en amplifier and will be insignificant with other two. Tr3 noise will increase about 3 times if the lowest en amplifier is used or increase 1.7 times if average en amplifier is used, but remains almost the same in the case of a relatively large e_{n}. In the same way we can calculate the output noise for the resistance R_{t} thermal noise component e^{2}_{nRttot} getting the E_{nRtRMS }at the amplifier output (Fig .7).
This time we see that it makes sense to increase the R_{t} value further above 10R_{s}, so the R_{t} thermal noise component can become negligibly low. This behaviour can be explained by damping of the thermal noise source voltage by a lower impedance of the transducer circuit. Taking the same integral over the frequency range we calculate the current noise source in the generated voltage contribution to the noise RMS. The E_{in+RMS} voltage at the amplifier output is presented in Fig. 8.
It is evident, that the LMH6624 amplifier is not suitable for the transducer Tr3 with high R_{s} and R_{oe} values. The E_{in+RMS }value for this combination is extending ten times above the e_{n} contributed noise level. The same comes to Tr2 and LMH6624 combination, only difference is not that large. Application of the OPA657 amplifier significantly reduces the E_{in+RMS} level it is lowest part of the drawing, so is not even notable. But it should be noted here that the OPA657 exhibits quite large e_{n} (5 times that of LMH6624), so this fact should be taken into account when shifting to the OPA657 amplifier. Finally, we calculate the total amplifier output noise E_{ntotRMS}. The results are presented in Fig. 9.
It is clearly visible that major noise contribution is by en. But for transducers with large R_{s} and R_{oe} (Tr2 and Tr3) current noise at the desired values of Rt starts prevailing over e_{n}. So, basing only noise analysis it can be predicted the that transducer Tr3 should be used with OPA657 or with CLC425 amplifiers with a lower current noise i_{n+}. SNR analysisTaking all noise sources into account and calculating the signal level at the output we normalize the value obtained to SNR value at R_{t} infinity and the lowest R_{s} transducer operating with the best en level amplifier LMH6624. Then the circuit with best noise performance SNR is 0dB. The SNR analysis results are presented in Fig.10.
Analysing the results obtained one can see that lowest en amplifier is suitable for a quite large range of transducer Rs values (10W to 400W the transducers Tr1 and Tr2). But such en does not leave much room for noise reduction the in case of large transducer Roe and Rs (the transducer Tr3). Even further bias currents of amplifiers investigated were not taken into account, because the resistance at the negative amplifier input is quite low (10W) and the positive input DC resistance is high (it is desired to have it above 10kW), the bias currents skew will cause output DC value out of limits. In literature [1] authors recommend using a low bias current amplifier in such a case (e.g.OPA657). Our solution is different we break amplifier feedback loop, so the amplifier DC gain gets equal 1. Therefore, the output DC shift due to bias currents skew is low. Taking into account that DC coupling is not needed in ultrasonic applications, the bias shift may be neglected. So, the high bias current amplifier can be used even with a high level of biasing mismatch. As a result of the analysis we present final schematics for the preamplifier (refer to Fig.11). The preamplifier will be used with relatively low impedance transducers [5, 9].
Some additional components have been added to schematics to keep the calculated noise performance. The capacitor C_{6} in series with R_{1} is introducing the high pass filter with the 3 dB cutoff frequency of 150 kHz. The capacitor C_{5} will add a low pass filter with the 2.5 MHz cutoff frequency. ConclusionsSeveral approaches have been used to improve noise performance of a transducer preamplifier for relatively narrowband signal case. By analysing every noise component it has been shown that en has major contribution to a noise level. The noise current source is of next importance. The approach have been demonstrated how i_{n} component can be reduced. Unfortunately, i_{n+} component influence is dependant an transducer parameters, so it can not be completely taken out. For some transducer types, with high R_{s} or R_{oe} this component starts prevailing over the e_{n} part. In order to have full noise performance evaluation, the SNR analysis has been used. The final design is best suited for relatively low source impedance transducers. If the amplifier for other type of transducer is needed, the circuit parameters should be adjusted in order to get the best noise performance.AcknowledgementsWe would like to thank the Dr. Darius Kybartas for his consultations and useful literature support.References

© NDT.net  Top 