Noise in electrostatic transducer receiver circuit

I am building a ultrasound recevier circuit based on a electrostatic transducer. The transducer requires a 200V DC bias in order to operate.

At the output of the transducer, I used 2 simple RC filter stage. The first stage is a high pass filter that removes the 200V DC bias voltage. The second stage is a low pass filter that filters away the high frequency noise from the DC-DC converter.

However, it seems like I am still getting a 200 mVpp 146 kHz noise component. I can only see my received signal which is at 50 kHz 20 mVpp when I use a digital filter on my oscilloscope.

Anyone can help suggest how to improve the signal to noise ratio? I am using a DC-DC converter from Hitek (Model: GMA 12-200PE). I have also attached my schematic in the post.

Put an aggressive low pass filter between the DC-DC converter and the transducer (before the 500M resistor). Put a 10K series resistor in both of the output lines (yes, one of them between the (-) output and Ground), and 100nF capacitors across the 200V, both before and after the 10k's.

Also bypass the 12V input to the DC-DC. Like 0.1uf ceramic in parallel with 10uF electrolytic.

Thanks for you reply. Just to further clarify, actually the DC-DC convert has only 3 pins. Input, Output and Common Ground. In this case, where would you suggest I place the 10k series resistors? The 10k resistor should be place immediately after the 200V output, even before the low pass filter?

mneary said:

Put an aggressive low pass filter between the DC-DC converter and the transducer (before the 500M resistor). Put a 10K series resistor in both of the output lines (yes, one of them between the (-) output and Ground), and 100nF capacitors across the 200V, both before and after the 10k's.

Also bypass the 12V input to the DC-DC. Like 0.1uf ceramic in parallel with 10uF electrolytic.

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