Electronics > Beginners
Piezoelectric hydrophone low noise amplifier design questions
David Hess:
--- Quote from: Sparker on May 16, 2018, 10:21:45 pm ---With the input of the first opamp completely shorted I am getting about 8 nV/sqrt(Hz) which totally satisfies me for now, until I shield the whole circuit completely.
--- End quote ---
I just happened to run across this yesterday and it applies to your application. Check out figure 52 on page 17 of the AD797 datasheet which shows the use of a low input voltage noise high input current noise bipolar operational amplifier with a large area photodiode. It makes my point about low noise with a high capacitance transducer at medium frequencies.
--- Quote ---But then I have a problem: I use the LM2594 DC-DC converter running at 140 kHz to generate the negative -2.5V rail for the op amps. When I connect the transformer to the input of the opamp (and remove the short between opamp's input and GND of course), somehow the 140kHz and 280kHz from the converter get coupled in so much that the output of the circuit gets saturated. :palm: I've provided much filtering for the -2.5V rail with an additional LC filter, which should result to about 2uV of ripple at the -2.5V rail. From all this I conclude that most likely the transformer is responsible for this. What do you think?
I should have probably used the pot core for the transformer because it's better shielded, but honestly I thought that toroids don't leak electromagnetic field and thus don't pick it up.
I've tried putting a tiny tin box over the toroid but it didn't change anything much.
--- End quote ---
Bobbin type inductors like you used are not self shielding like toroids and pot cores so they spray magnetic flux everywhere which gets into everything.
Operating the switching regulator at the frequency of interest is just asking for trouble whether magnetic flux gets into everything or not. Filtering will need some serious attention.
Sparker:
This thing is so weird. I've replaced the converter's main inductor (the one which gets pumped with energy) with a toroid of the same type I used for the amplifier input. Generally I was getting almost same level of coupling, bug by tuning its rotation very fine, I could find the position which produced almost no 140kHz(and its harmonics) leakage. It's of course impractical because the tiniest movement will result to 140kHz noise increase.
Pot cores are very hard to find here, but I think I've found a few, going to try them next week.
As for converter's frequency, would you suggest getting a converter with higher or lower frequency of operation? The majority of converters operate in this frequency band. My guess is, the higher it is the better, because it will be easier to filter it.
David Hess:
Toroidal and e-core inductors do not leak as much flux. Distance between the power supply and sensitive circuits helps; magnetic flux falls with the cube of the distance.
Some switching regulators allow their oscillator to be phase locked to an external reference; this would allow a non-interfering frequency to be used.
Another alternative is to disable the switching regulator during the receive gate.
Higher frequencies can be filtered with smaller components but electrostatic coupling increases at higher frequencies so parasitic capacitance becomes more important.
Keeping interference from the switching regulator out of your 4nV/sqrtHz noise level will be tricky.
Sparker:
I've learned that there is no such thing as a perfectly shielded transformer.
So, I've tried the pot core transformer for the input of the circuit, but I was still getting a lot of coupling from the switching power supply. So I tried the last option: a different frequency DC-DC converter. ST1S10 runs at 900 kHz and I am getting no troubles with it. :-+
Sparker:
I think I need some help with this device again...
I can't decide which of the protection schemes to use. In the attachment there are two schematics.
First question is, which way to connect the diodes. In the starfish sonar they used the top variant, with two double-diode assemblies connected to the ground. In other designs I can see only one pair of diodes, like in the bottom picture. The top variant produces more voltage when the input is saturated but It's not a problem for me.
Second question I can't resolve is: do I need to connect the central tap of the primary of the transformer to the ground? I see no reason to do that here, but I might be mistaken.
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