Amplifier based on pure DC Coupled vs real chip-based amp

Thread Starter

Gpand

Joined Dec 11, 2023
105
Conventionally, amplifiers are chip based where the signal is really amplified. But some are based on the ADC itself. What are the disadvantages of pure DC amplifiers? Why is it called DC coupled? One of them may be poor common mode rejection ratio performance. How does the ADC based amplifier deal with it? What other disadvantages and advantages of ADC based DC coupled amplifiers?

Thanks.
 
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Thread Starter

Gpand

Joined Dec 11, 2023
105
But ADC by itself can be used as amplifier especially 24 bit, isn't it? Where is the amplifier in the diagram? How do you tell whether the amplifier uses the ADC only or extra chip?
 

Thread Starter

Gpand

Joined Dec 11, 2023
105
Btw.. I mentioned it may not have dedicated amplifier because it has no spec on the common mode rejection ratio or gain.
 

Ian0

Joined Aug 7, 2020
9,981
But ADC by itself can be used as amplifier especially 24 bit, isn't
No. The highest code corresponds to the voltage of the reference.
Use a lower voltage reference, and it will be more sensitive, but there are limitations.

How do you tell whether the amplifier uses the ADC only or extra chip?
Take the lid off and look inside.
 

Thread Starter

Gpand

Joined Dec 11, 2023
105
No. The highest code corresponds to the voltage of the reference.
Use a lower voltage reference, and it will be more sensitive, but there are limitations.
Can you give direct reference (pun unintended) for the above? Or illustrations what you meant the highest code corresponds to the voltage of the reference. What code? firmware code?

I mentioned it may not have dedicated amplifier because it has no spec on the common mode rejection ratio or gain.

Take the lid off and look inside.
I don't own it. It costs over $16,500. What could make it so expensive?
 
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Ian0

Joined Aug 7, 2020
9,981
Can you give direct reference (pun unintended) for the above? Or illustrations what you meant the highest code corresponds to the voltage of the reference. What code? firmware code?
The highest binary number output.
I mentioned it may not have dedicated amplifier because it has no spec on the common mode rejection ratio or gain.
Even if it were a differential input ADC (as sigma-delta types often are) it should have a CMRR.
What could make it so expensive?
Marketing?
 

Thread Starter

Gpand

Joined Dec 11, 2023
105
The highest binary number output.

Even if it were a differential input ADC (as sigma-delta types often are) it should have a CMRR.

Marketing?
Maybe it doesn't even have a differential input ADC? If none, then no CMRR and no gain?
 
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BobTPH

Joined Jun 5, 2013
9,106
What is your goal in asking all these questions about a biofeedback device? Are you trying to build an equivalent device?
 

Ian0

Joined Aug 7, 2020
9,981
What is your goal in asking all these questions about a biofeedback device? Are you trying to build an equivalent device?
If I were copying it, I would have had the lid off to look inside before now, even if it costs $16000.
 

Thread Starter

Gpand

Joined Dec 11, 2023
105
What is your goal in asking all these questions about a biofeedback device? Are you trying to build an equivalent device?
Someone is offering me a used one at good price. I just want to know if the specs are so advanced and unmatched anywhere in the world that it would be worth to spend on $1500 - $4000 software just to run it (see below). Or use others with free or cheaper software and hardware with similar performance. But it has no CMRR and gain data anywhere. It doesn't even have differential inputs?
 
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Thread Starter

Gpand

Joined Dec 11, 2023
105
See above for the prices of the software (right image). So unit $16700 + software $4000 + accessories cost $30000. And they don't have any competition. Is their technology a century beyond or so advanced none of you can duplicate it at even half the price?

I just want to know if there is no differential inputs. One must get another set of amplifiers to use it? So it's like an ADC only with some rough amplifier without differential inputs?
 

BobTPH

Joined Jun 5, 2013
9,106
Neurofeedback


Robert Thibault, a postdoctoral scholar at the Meta-Research Innovation Center at Stanford University, is also skeptical. He said neurofeedback advocates point to peer-reviewed research that have “impressive results,” but most are not rigorous double-blind, placebo-controlled trials. Of the dozen or so such trials, he said, “all but one concluded that fake neurofeedback works just as well as real neurofeedback.”
It seems a little ridiculous to me to have such a precision device (24-bit and oversampling) when studies have shown that a device that fakes the whole thing is just as effective.
 

Ian0

Joined Aug 7, 2020
9,981
But if it’s anything like “audiophile” audio products, isn‘t the effect proportional to the price paid?
 

Thread Starter

Gpand

Joined Dec 11, 2023
105
I know op-amps can reject common voltage in each input pin for noise reduction. Is it possible they put a very basic op-amp with CMRR that is not good (like only 70dB, that is why they didn't want to put any CMRR in the spec sheet). And they use the 24 Bit ADC oversampling to remove the noises? Can oversampling and averaging in ADCs remove any common mode noises that aren't removed by the op-amps?
 

MrChips

Joined Oct 2, 2009
30,920
This application is for recording biological signals such as EKG.

Hence you are measuring very high impedance signal sources over very long cables. Inevitably, you will encounter very high amounts of EMI. For this application, it is imperative that you use instrumentation amplifiers with high CMRR and with high input impedance. You don't use basic op amps in such an application.
 

Ian0

Joined Aug 7, 2020
9,981
I know op-amps can reject common voltage in each input pin for noise reduction. Is it possible they put a very basic op-amp with CMRR that is not good (like only 70dB, that is why they didn't want to put any CMRR in the spec sheet). And they use the 24 Bit ADC oversampling to remove the noises? Can oversampling and averaging in ADCs remove any common mode noises that aren't removed by the op-amps?
If it consists of single-ended inputs, then there is no "common mode" signal for it to reject.
 

Thread Starter

Gpand

Joined Dec 11, 2023
105
This application is for recording biological signals such as EKG.

Hence you are measuring very high impedance signal sources over very long cables. Inevitably, you will encounter very high amounts of EMI. For this application, it is imperative that you use instrumentation amplifiers with high CMRR and with high input impedance. You don't use basic op amps in such an application.
What will happen if you feed a subsystem with high CMRR of 100dB to a main system with CMRR of 50dB. Will the final output be 100dB or 50dB?

I am thinking whether the $16700 gUSBamp being only 50dB only needs another set of $2000 Preamplifier with high CMRR of 100dB to make it work.

gusbamp preamp.png
 
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