Induction Balance Stuff - Single/Multi Frequency Response, GB, Disc, Measurements, Ideas, Fun, etc.

[pito]

Amplitude of the noise peak to peak

[Aziz]

Amplitude of the noise peak to peak

We can ask the AI and calcutate the noise voltage contribution for different bandwidth demodulation and the limit of signal resolution (nV) next time.
I dont have much time at the moment.

[Aziz]

Hi all,

here is, what Claude Code AI says about the noise analysis.
I have adapted the prompt for my application parameters.

AI-Prompt:

I have a sensor coil of a metal detector with series resistance of Rs=2 Ohm.
I have an ultra-low-noise amplifier with gain of G=200 with input referred
noise density en=0.4 nV/sqrt(Hz). Noise includes the sensor coil resistance.
I have an 24-bit, 192 kHz ADC system with -120 dB noise floor level above 10 kHz.
My ADC's dynamic input voltage range is +/- 2.5V.
My signal operating frequency is 45 kHz.
Amplifier output is fed into the ADC system (AC voltage). I have a block size of 1920 samples.
I have a block processing of 100 blocks per second.
I demodulate my tiny signals in the sensor coil with digital lock-in amplifier (I/Q) with
the internal digital synthetisized clock reference (45 kHz) in each block size.

What is the lowest voltage at the sensor coil, I can detect and process for the block?
With the ultra-low-noise amplifier G=200 and without the amplifier (sensor connected directly
to the ADC system)?
How much is the noise level (Vrms, Vpp) for both configurations?
Make me a comprehensive table for comparison.

Add to the comparison EMA (exponential moving average) to the processed blocks with
Alpha A=0.05. Make me a comprehensive PDF document with formulas, tables of the answers.

See the attached PDF for the AI answers.

[Aziz]

Hi all,

Quote: "Amplifier noise, not gain, is what really matters here..."

See the full AI chat in the zipped html file. Very very interesting.
Sweet spot for amplifier gain G is at 80 - 100 times.
But with figure-8 RX coils (or anti-interference RX coils), we need more gain.
Aziz

[JoyJo]

And when will all the words run out and the first prototype will appear in reality? This topic has been very long, but nothing has changed.

[Aziz]

And when will all the words run out and the first prototype will appear in reality? This topic has been very long, but nothing has changed.

Due to lack of time, I did not solve all the issues yet: Running the op-amp(s) out of specs, less parts, simple design.
The amplifier will be the quietest front-end ever however.

[Aziz]

Hi all,

how to address an amplifier input noise e_n <= 0.3 nV/rt Hz? For a gain of 50 - 200 times?
I am right now at 0.33 nV/rt Hz with ZTX851 (1x NPN).
I am sure, I can go below 0.3 nV this time. With lots of tricks.
I have seen, that the PNP type FZT 951 TA (SMD) is even cheaper than the ZTX851.

Now this goal is easily available now. Vbe matching of two PNP transistors out of 50 pieces makes it possible.
But I have to check it out with a simple noise calculation prediction Excel table.

[Aziz]

Hi all,

yep, it is possible to go below e_n = 0.3 nV/rt Hz. The AI says it.

The best option (the lowest noise) is to use two Vbe matched FZT951TA (SOT-223 housing) PNP transistors.
The FZT951TA cost only 0.53 EUR/piece. But this transistor has the lowest noise easily available today.
Vbe match <= 1 mV at 5 - 6 mA collector current at the same ambient temperature. The matched transistors must be thermally coupled together. So they must have the same temperature.

The trick #1:
Using two matched transistors with 5 mA collector current each is way better than using a single transistor with double the collector current (10 mA).
When Vbe matched, no emitter degeneration resistors needed (to avoid thermal runaway and unbalanced collector currents).
Resistors are evil. They generate huge amount of noise. You can almost hear them.

I have to modify the circuit to operate with two PNP transistors.

There are more tricks, which will help us arriving the final goal.
Anyway, the total power consumption will rise.
Aziz

[Aziz]

Hi all,

I have made an Excel table for predicting the input referred noise e_n for the preamp.
You can see, where the big improvement potential is. And very remarkable fact is, that the op-amp noise contribution of the NE5532 is really very very low.



If we change to two parallel transistors, we are in game for the 0.3 nV/rt Hz challenge.
The zipped Excel table in the attachment is for playing and dreaming of ultra low noise silence.
Cheers