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| Piezoelectric hydrophone low noise amplifier design questions |
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| Sparker:
Hello guys! I always feel a beginner when it comes to analog things, so I need help with this sensitive topic. :) So, I need to design an LNA for a piezoelectric hydrophone to later digitize the signal with a single-ended ADC (the ADC of my STM32 for now). Hydrophone works both in TX and RX modes (it's for a sonar). Antenna properties: operating at ~240 kHz, nothing else is known about it now. Specs for the amplifier are: input voltage is expected to be in the order of 10's of uVolts, so I need ~10^5 voltage gain. Double-pole bandpass frequency response centered at 240 kHz with Q~10 is probably OK as it will be later processed with DSP. Also I want to digitally(from an MCU) control its gain in, at least, 20 dB range or so. Also this thing is going to be installed not far from noisy things like a lot of DC-DC converters for powerful LEDs. I intend to make it with high frequency opamps. For the gain-control part I will probably use some digital potentiometer between gain stages(or is there a better choise?). Also i can design the active filter part myself. When it comes to the first stage of the amplifier, I am not sure what to do. I am considering two options: 1. I am not experienced with noise-sensitive things, but I guess that the common mode rejection ratio mentioned everywhere is an important part in this device to suppress common noise at inputs. I could make the first stage with a transformer(to serve as unbalanced->balanced converter) followed by a low noise high speed opamp, like one of these: AD8646, AD8651, AD8655 or OP37. But I am not sure about CMRR of a transformer, especially if it will be hand-wound on some ferrite ring. How high can it be? 2. Another option is to eliminate the transformer and take a nice instrumentation amplifier like INA128 or INA2128. One more cool instr. amp. I found is AD8231, more noisy but hey, it has digitally-programmable gain. One more piece of advice I have seen in the Internet is to go fully differential from input to ADC (Once I had a look at a professional Imagenex sonar board, and they did exactly that with fully-differential opamps :)). But I am not sure how much sense it makes in my case if I am using a single-ended ADC. Also one more general thing I would like to clarify about opamp circuit design is this: if I only care about high frequency part of spectrum, should I care much about opamp specs like offset voltage and input current? As I understand it makes huge difference only if I were to amplify DC signals. Thanks to anyone who has made it through this wall of text! |
| David Hess:
I am just going to ramble. --- Quote from: Sparker on April 21, 2018, 01:24:33 pm ---input voltage is expected to be in the order of 10's of uVolts, so I need ~10^5 voltage gain. --- End quote --- With that much gain over the same frequency range, you will have to be careful about coupling between stages causing oscillation. Superheterodyne radio receivers avoid this problem by mixing so they can distribute the gain over different frequency ranges limiting the game at any intermediate frequency. --- Quote ---I intend to make it with high frequency opamps. For the gain-control part I will probably use some digital potentiometer between gain stages(or is there a better choise?). --- End quote --- Operational amplifiers are probably the easiest as far as design. For the stages after the input stage, I would probably use current feedback operational amplifiers but operational transconductance amplifiers can combine gain control and bandpass filtering in one stage so might be worth considering. Someone might still make an integrated intermediate frequency amplifier intended for AM receivers which could be used in an application like this. 240kHz isn't that far from 455kHz. --- Quote ---1. I am not experienced with noise-sensitive things, but I guess that the common mode rejection ratio mentioned everywhere is an important part in this device to suppress common noise at inputs. --- End quote --- See below about input stage differential inputs. Power supply rejection ratio matters for limiting input noise as well. Consider using a separate low noise voltage regulator for the input stage. --- Quote ---I could make the first stage with a transformer(to serve as unbalanced->balanced converter) followed by a low noise high speed opamp, like one of these: Another option is to eliminate the transformer and take a nice instrumentation amplifier --- End quote --- If you have a differential input, then noise is doubled but this does not matter for later stages. A transformer can be used for impedance matching to minimize input noise and drive a single ended input from a differential source avoiding the increased noise of a differential input. The impedance matching from the transformer gives a lot more leeway in selecting an input amplifier. I think you can get lower input noise with a discrete input stage but the improvement may not be significant. --- Quote ---One more piece of advice I have seen in the Internet is to go fully differential from input to ADC (Once I had a look at a professional Imagenex sonar board, and they did exactly that with fully-differential opamps :)). But I am not sure how much sense it makes in my case if I am using a single-ended ADC. --- End quote --- There is a lot to recommend using a differential signal path except for the transducer signal input as mentioned above. Power supply noise including interference from other stages of the amplifier gets rejected as common mode noise so this can help prevent oscillation. --- Quote ---Also one more general thing I would like to clarify about opamp circuit design is this: if I only care about high frequency part of spectrum, should I care much about opamp specs like offset voltage and input current? As I understand it makes huge difference only if I were to amplify DC signals. --- End quote --- Since you can use AC coupling, input offset voltage does not matter. Input bias current can still be an issue because it comes with input current noise which combined with input impedance adds to the input voltage noise but this is unimportant after the low noise input stages. |
| Sparker:
Hello, David! Thanks for your advice on the design of this circuit! I don't have any experience with current feedback or transconductance amps yet, but I will see what I can do with them in a simulator. As for the transformer, as I understand I must use it to match source resistance to input resistance for optimal noise. But I'm not sure if I will ever know it, unfortunately. It is only not clear for me why there is noise increase in case of differential inputs. I thought that differential inputs are meant for decrease of noise. |
| Zero999:
What's the bandwidth? I presume it's not necessary to go from DC to 240kHz? I imagine the bandwidth is very narrow. An AM radio-style receiver sounds like a good idea. The signal could be heterodyned down to much lower frequency, thus lowering the noise, the sample rate and processing requirements. |
| Sparker:
--- Quote ---What's the bandwidth? I presume it's not necessary to go from DC to 240kHz? I imagine the bandwidth is very narrow. --- End quote --- You are right, BW is narrow, that's what I meant by "Double-pole bandpass frequency response centered at 240 kHz with Q~10" in my initial message. I am using undersampling to digitize it, the 240kHz carrier aliases to fs/4, then I use DSP techniques to demodulate it. I agree that an AM receiver IC might suitable for the task, in fact I've seen some old soviet IC like this used in one of our other sonars. |
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