JFET Design Spotlight: High-Speed Sonar Front End for Aquatic Imaging

This week’s Design Spotlight takes us into a sonar application proposed for multi-channel aquatic imaging. The design uses a dual matched JFET front end to meet the sensor’s differential loading requirements while providing the low-noise, high-speed performance needed for downstream A/D conversion.

“Just another JFET circuit that was composed and built for a Sonar application, proposed to be used in multiple for aquatic imaging.

One of the requirements was a loading impedance of 100 megohms for the sensor and an overall bandwidth of 5MHz. A couple of parameters to start with that aren’t typically specified together. This front-end using a dual matched JFET worked to satisfy the differential load requirement of this Piezo sensor. The input stage is sourced by a JFET current source and the input gates are biased at -6V. Both Drains would be feeding into ground referenced resistors so would need enough voltage offset to the gates to remain linear. From there a typical JFET differential current to voltage amplifier done in discrete parts is used for low noise gain. Only an 18pF capacitor was needed for amplifier compensation to eliminate the minor overshoot. Using a linear low gain discrete op-amp allows for high-speed performance with acute transient response. Compensation of an integrated op-amp is typically a compromise between speed and overshoot. Not much unusual here except that the amplifier cluster needed to be transient / EMI rugged and closely repeatable over a relatively wide temperature range. In addition, these were coated in a Parylene deposition to lock out moisture while maintaining a low dielectric influence on the circuit. Each single ended output was then matrixed to an A-D conversion bank and processed by FPGAs to resolve imaging with a little math. This, like many circuits was done by necessity and on the run. 

As usual ………… Kirkwood Rough”

Designing a high-impedance, low-noise front end for piezo, sonar, hydrophone, or other demanding sensor applications?

Explore Linear Systems ultra-low-noise JFETs, or contact us to discuss your application and request FREE engineering samples.

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