Circuit for ultrasonic threshold detection

Hello Ajit,

Your circuit is straightforward; however, a few changes may help you better achieve your performance goals:

• The TH4503 does provides the necessary minimum gain-bandwidth (GBW) required to support your filter requirements. The effect of using the 33k resistor will increase the gain slightly and move the center frequency down by a small amount. You can see the affects if you simulate the filter using TINA-TI. I can't help but wonder why you chose to use 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 suggest dividing 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 will need to place an effective bypass capacitor path at each of the points where Vcc and the 1/2 Vcc voltage connects to the operational amplifiers. These need to appear as a very low impedance at the frequency of interest. See the data-sheet for each amplifier you have selected for power supply bypass capacitor details.
• The TLV2460 operational amplifiers that drive the 1/2 Vcc lins may become unstable (oscillate) if the capacitive load on their output becomes too large. A device that is designed to provide the 1/2 Vcc function, eliminate the need for two matched resistors, and will drive a wide range of capacitance is the TLE2426. There is a small possibility of oscillation under a certain load condition, but for your circuit doesn't appear to be an issue (Fig 17 in data-sheet).
• I don't know what filter synthesis program you used for the filter, but I do note a discrepancy in one component value when I check the circuit using TI's FilterPro. The input resistors (R1) are shown in your schematic as 6.9 k. FilterPro produces a value of 12.28 k. Possibly, one of my assumptions about your filter response is incorrect.

I hope this helps.

Regards, Thomas

PA - Linear Applications Engineering