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FFT and 60 Notch filter software

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WatchfulEye:
I've extracted the input conditioning circuit and done a noise simulation using some estimates from some datasheets for the LF412.

The sim shows approx 750 nV rms input referred noise over a 1 kHz bandwidth - but note that there is a separate buffer for both In- and In+ - so the sum is approx 1 uV rms. Using the typical estimate of Vnoise p-p as 6x Vnoise rms - this gives a broadband noise of approx 6 uV p-p - which looks approximately consistent with the supplied files (although I haven't reviewed one with the gains set correctly).

It might be possible to replace the LF412 with a more modern and lower noise op-amp. I haven't checked catalogs. You might be constrained by the DIP package, but you might be able to use an SMD adapter to convert a SOIC dual op amp to DIP.

loop123:

--- Quote from: WatchfulEye on March 15, 2024, 06:41:14 pm ---I've extracted the input conditioning circuit and done a noise simulation using some estimates from some datasheets for the LF412.

The sim shows approx 750 nV rms input referred noise over a 1 kHz bandwidth - but note that there is a separate buffer for both In- and In+ - so the sum is approx 1 uV rms. Using the typical estimate of Vnoise p-p as 6x Vnoise rms - this gives a broadband noise of approx 6 uV p-p - which looks approximately consistent with the supplied files (although I haven't reviewed one with the gains set correctly).

It might be possible to replace the LF412 with a more modern and lower noise op-amp. I haven't checked catalogs. You might be constrained by the DIP package, but you might be able to use an SMD adapter to convert a SOIC dual op amp to DIP.

--- End quote ---

Deja Vu. We have focused on the Iso-Z isolation head in the thread "Can changing amplifier in Sullen-Key filter affect the frequency response much?" last Feb 4. https://www.eevblog.com/forum/beginners/can-changing-amplifier-in-sullen-key-filter-affect-the-frequency-response-much/  my initial message was altering the values used in the LF411CP in the ISO-Z Isolation head box.  Is the LF412 in the BMA another Sallen-key filter?   Near the end of the thread. The solution there was to increase the gain to 100X before going to the ISO122. I tried it, it worked. But I didn't continue using the ISO-Z because I'm using pure batteries in the BMA and people convinced me no galvanic isolation needed. Also, I didn't use it because at microvolts. I'm afraid the dc converter may contribute some noises as well as retained noises even after 100x gain. At that time my target was 1mV. But lowered it to 10uV now. The message of RFDX was the first time he gave figures of the chip (there I learnt to compute noises) where he said.

"The noise spectral density of the ISO122 is 4uV/SQR(Hz) ("Noise" on p.5 of the datasheet). To get the noise voltage (RMS) multiply with SQR(bandwidth-in-Hz). With the bandwidth switched to the max. of 50kHz, the noise voltage at the output would be 0.894mV RMS. That is alot for an input signal of only 1mV (RMS?). With a bandwidth of only 100Hz, the noise voltage is only 40uV RMS. The main amplifier also contributes some noise to the output but it is probably very little in comparison to the ISO-amp and can be ignored.

Fig. 12 in the datasheet shows the unfiltered 20mVpp ripple from the 500kHz clock feedthrough plus the broadband noise from the ISO-amp. Fig. 13 shows only the broadband noise at the output after being filtered by the additional 50kHz Sallen-Key lowpass filter. The 500kHz ripple is gone (buried in noise). The broadband noise has at least 6...7mVpp. Divide this by 6.6 to get the RMS value."

My solution for it was to eliminate the ISO-Z entirely that was why in my 2nd to last message. I showed you picture of the inside of the ISO-Z without the pcb. Going now to the main BMA-200. What if I will just eliminate the LF412C stage. You said "LF412C, which conditions the signal before going to the AMP01". What is the purpose of conditioning it? Why happens if it is not conditioned and the input goes directly to the +IN and -IN of the AMP01? Thanks.

WatchfulEye:
The BF412 in the BMA-200 acts as a pre-amplifier with gain of 2 and very high input impedance. It also provides trimming for CMRR and DC offset.  Due to its near infinite input impedance and very low bias current, it has near zero current noise, so is ideal for buffering very high impedance signal sources.

The AMP01 is bipolar input, rather than FET input like the BF412. As such it has higher bias currents and higher current noise, which means that if the impedance of your signal is more than about 200 kOhms, it will likely give more noise than the BF412.

I've downloaded your last wav file, and done some measurements with it. Unfortunately, the netech simulator does not appear to be properly specified, and none of the documentation states what "10 uV" means (rms, VPeak, or Vpeak-peak). In order to do calculations I've just had to assume that the netech is calibated, and by what is meant by amplitude. In my previous calculations I'd assumed rms, but after some consideration, I wonder if it is supposed to be Vp-p.

Anyhow, assuming that it is Vp-p, then I estimate around 60 nV/sqrtHz broadband noise in the 50-100 Hz range, rising below about 50 Hz - which is pretty close to what I simulated for the BF412 input stage.

Some physiological amplifiers specify noise over a band, which can make comparison difficult. For example, one amplifier specifies 0.4 uV rms between 1-30 Hz. This is a fairly average for FET amplifier, and basically equivalent to the BF412 amplifier in the BMA-200 (i.e. this amplifier would provide similar noise to your current system).

loop123:

--- Quote from: WatchfulEye on March 15, 2024, 09:40:16 pm ---The BF412 in the BMA-200 acts as a pre-amplifier with gain of 2 and very high input impedance. It also provides trimming for CMRR and DC offset.  Due to its near infinite input impedance and very low bias current, it has near zero current noise, so is ideal for buffering very high impedance signal sources.

The AMP01 is bipolar input, rather than FET input like the BF412. As such it has higher bias currents and higher current noise, which means that if the impedance of your signal is more than about 200 kOhms, it will likely give more noise than the BF412.

I've downloaded your last wav file, and done some measurements with it. Unfortunately, the netech simulator does not appear to be properly specified, and none of the documentation states what "10 uV" means (rms, VPeak, or Vpeak-peak). In order to do calculations I've just had to assume that the netech is calibated, and by what is meant by amplitude. In my previous calculations I'd assumed rms, but after some consideration, I wonder if it is supposed to be Vp-p.

Anyhow, assuming that it is Vp-p, then I estimate around 60 nV/sqrtHz broadband noise in the 50-100 Hz range, rising below about 50 Hz - which is pretty close to what I simulated for the BF412 input stage.

Some physiological amplifiers specify noise over a band, which can make comparison difficult. For example, one amplifier specifies 0.4 uV rms between 1-30 Hz. This is a fairly average for FET amplifier, and basically equivalent to the BF412 amplifier in the BMA-200 (i.e. this amplifier would provide similar noise to your current system).

--- End quote ---

https://www.eevblog.com/forum/projects/instrumentation-amplifier-modification-or-replacement/

If I can't do away without the BF412, then I may need to replace the AMP01 itself. In this above 4 page thread I asked people what is the best replacement. They said the AMP01 5nV/Sqrt(Hz) is difficult to beat. They didn't know it requires the BF412 which can add a lot of noise. Do you know what FET or other amplifiers that can be used that can replace both BF412, AMP01 and doesn't have that broadband noise at 10uV at 1000Hz and higher? It is ok if the pins are not the same as the AMP01.. I can rewire it in the socket. I have 2 pcs of BMA and one can be used as experimental board for the new amp replacement. You can reply in that thread so people who have thought about the replacement can comment on it too. Many thanks.

WatchfulEye:
I've had a quickj look for some alternatives to the LF412.

You could try something like an OPA2132P, which should be a direct plug in replacement. It has a much lower noise of 8nV/sqrt Hz.

There are lower noise amplifiers but they are things like SOIC packages which will need adapters to be soldered in. You are also limited by the 4.99 kOhm resistors which each contribute 9 nV/sqrt Hz of noise - so once the OPA2132 is in, the resistors will be the biggest noise source.

If you really fancy getting some SOIC-8 to DIP-8 adaptors, then something like an OPA827 could be tried, but it is diminishing returns because of the resistors.

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