Noise in electrostatic transducer receiver circuit

[yeoshiki]

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.

 

[mneary]

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.

 

[yeoshiki]

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?

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.