# Circuit for ultrasonic threshold detection

Posted on Feb 6, 2014

I have designed the attached circuit for detecting how long an ultrasonic pulse need to to travel a distance. the input signal is going to be from a 40 k ultrasonic receiver. First stage is 40k Band pass - 1k pass band - differential MFB filter with a gain of 2. 25dB at Center frequency. In feedback path of differential amplifier I have used 33k in

stead of ideal value of 31. 6k. I will appreciate some useful comments on how this circuit can be improved. Also I am going to design a PCB using smt components. I need suggestion to improve the circuit like adding capacitor to supply rail and VOCM which will reduce power supply noise etc. The TH4503doesprovidesthe necessary minimum gain-bandwidth (GBW) requiredto support your filter requirements. The effect of using the 33k resistor willincreasethe gain slightly and move thecenter frequencydownby asmall amount. You can see the affects if you simulate the filter using TINA-TI. I can`t help but wonder why youchose touse the 33 k 1%, when 31. 6 k 1 % is a standard value too. I am concerned with attempting to get a gain of 1000 v/V at 40 kHz from a single stage. The OPA320 does not have sufficient bandwidth to supply that gain at 40 kHz. I suggestdividing the gain up equally between two operational amplifiers in a dual amplifier ( OPA2320 ). The OPA320 appears to have about 55 dB of open-loop gain at 40 kHz and each stage would need 29. 5 dB of closed-loop gain -so there isn`t much loop gain left. The OPA2350 has about 5 dB more gain at 40 kHz so two stages may come closer in developing the 1000 V/V (60 dB) closed-loop gain you seek. A higher gain-bandwidth operational amplifier may be necessary. You willneed to place an effective bypasscapacitor path at each of the points where Vcc and the 1/2 Vcc voltage connects to the...

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