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I am imagining a boat, open water, sunshine and sun block. Must be fun... Can I assume you are monitoring biological sources? Do you do any spatial processing as well? What sort of hydrophone are you using? Awesome... at

What is your ultrasound application? at

A very simplistic way to look at it... With the input open circuit, there are several noise sources at the input that are being amplified. These include R2's Johnson noise and IC1A's input noise sources (post 503 schematic). The mic's output resistance is ca 400R. If we place an _ideal_...

To reduce unwanted LF signals, I would start with reducing only the feedback shunt capacitor (C3 in post 503) to somewhere in the .47uF to .22uF range. You can further reduce C3 to .1uF to .05uF if necessary. All that is really important is to ensure LF sounds do not clip the preamp output...

CorBee, With only one mic input, I would have thought it logical for it to feed both L & R ADC inputs ( as well as both sides of the line/headphone output when fed thru). at

It seems that a cap between mic in and mic gnd is a possible solution. I would think setting mic gain to minimum might also be helpful. Also, it appears that it is not so much noise (a non repetitive waveform/signal) as it is a fixed frequency clock, etc, leaking or folding and appears to be...

'Twas good for a chuckle... Thanks for your test info. at

CorBee, Just a bit of FYI: When referring to distortion components, the second harmonic is at a frequency that is twice the frequency of the fundamental. The third harmonic is at a frequency that is three times the fundamental, the 4th=4X, the 5th=5X, etc. As an example, using a sine wave...

WMXZ, All your recent posts make valid points. I have utilized and driven both SAR and sigma/delta ADC's. However, we do not have access to the ADC in SGTL5000, the ability to drive it, or any "real" data regarding it. Looking at the data sheets for a "proper" ADC and trying to compare any...

Absolutely not! The mic and preamp are going to produce noise (or stray signal, RF, etc) at frequencies above the Nyquist. Any noise or signal coming out of the preamp above the Nyquist will fold back into the passband. Folded back noise will add to the noise already in the desired passband...

Edwin, Don't forget to consider using an ADA4841-2 for this circuit. aethertech

Come on you guys, you're dealing with an old fart who has little time on his hands. Please give me a break... Edwin, Everything is correct except R2 should be 10K. Your gonna' lose about 6dB from the mic with that 470R (and the added loading will increase the mic's THD). Other comments...

The problem I see is that when the reed relay is open, so is the input to the circuit. With no low impedance source connected, the noise floor is going to be very high. Connect a low value resistor (10R to 51R as mentioned) across the detector's input. That way when the relay is open, the...

Edwin, The problem with using the smaller value caps is that it increase the impedances seen throughout the circuit and will produce more noise. Same for reducing the values of the feedback and shunt resistor. Resistor noise increase with higher resistance. I would try the circuit as I...

I could not find the specs for the SY6600 but a similar model (FY6600) states -45dB THD for sine waves below 1MHz. I suspect it is a bit better than that at the frequencies being used, but being a 14 bit DDS there will be a fair amount of distortion produced. I do not quite understand the reed...

What are you using as a signal source?

The only way to determine any performance change with the bias setting (or use of internal gain) would be to feed a clean sine (<-100THD) into the line in and look at the spectrum produced. If using the mic and existing preamp while driven from a not so clean acoustic source, I suspect their...

Also, The 5000 has a register for setting internal bias. Typically this can be used to set the bias below the default setting to reduce power consumption. I suspect that the best THD performance can be had at the default setting, or even moreso at the one setting available higher than the...

CorBee. Thanks for the welcome! Better than simulating the circuit, it has been built and measured. At 35dB of gain and at 40kHz, the circuit approaches -100dB THD (.001%). It works well out to 180kHz. Driven by this circuit, the ADC's THD performance and the mic's self noise will dominate...

Hello again, Regarding my hastily added comment regarding a resistor and decoupling cap between VCC 3V3 and the mics power input, use a resistor between 499R to 1K. I would also use a .1uF ceramic in parallel with a larger 10uF or greater cap for the decoupling cap(s). Placing the .1uF on the...

Greetings to everyone, I have been following this thread as time permits with great interest. I have a fair degree of experience with analog and analog to digital conversion at the frequencies of interest. Being an "analog guy", I can only tip my hat in envy to the digital gurus once the...