For direct 10uV mapping into the ADC like in the case of the g.USBamp which doesnt have amplifier. It may be true but when the output is 0.5V (after 10uV x 50000 gain in main amp). What would be wrong for the 0.5V at audio Adc? would it make it like 0.4V or 0.6V?? How about the chip shown in spec in my last 2 messages used by the E1DA? does it keep stable reference voltage?
It would be really helpful if you clarify for us -- and for yourself! -- what your requirements are.
- What frequency range do you need to capture (lower and upper limit)?
- Do you need to capture only low-level signals, say in the 10 µV range -- in which case you could and should use an analog pre-amplifier, and then can work with an ADC with low resolution?
- Or do you need to pick out a 10 µV signal sitting on a 1 V background, as implied by your original questions in this thread -- in which case you need an ADC with correspondingly high resolution, if you want to use digital signal processing techniques to pick out the weak signal?
- Why do you think precise, absolute voltage levels are so critical for you to control or measure? I understand that you intend to study some EEG-like brain/computer interface -- won't the physiological differences and drifts, day-to-day and run-to-run variations of contacting the electrodes etc. be dominant by far?
Given your limited budget, there is no room for an approach like "I'd rather get something that covers all the bases and all potential future needs". If there is any chance to establish a solution for $200, it will need to be tailored to your specific needs. Hence, until you have a clear and consistent picture of what your signals are and how you need to control and measure them, there is not much point in equipment discussions.
I already have a 50,000 gain amplifier to work with 10uV (100Hz to 2400Hz) signal. The CWE BMA-200. Since the output of the amplifier is 0.5V to 2 V. Then even audio ADC can work with it, right? When you talk about stable reference voltage. Can it make the output vary between say 0.5V to become 0..4V or does it only occur in micro or millivolt signal?
I already have the E1DA ADC as well as a 16 channel amp that doesn't have any amplifier inside. The 16 channel g.USBAmp has this description I shared earlier. My question concerns its last sentence.
https://www.gtec.at/product/gusbamp-research/"g.USBamp uses wide-range DC-coupled amplifier technology in combination with 24-bit sampling. The result is an input voltage range of +/- 250 mV with a resolution of < 85,7 nV! This means that any physiological signal can be recorded directly, without additional hardware. Neither high electrode offset voltage nor large artifacts resulting from electrical or magnetic stimulation will saturate the amplifier inputs. This feature is an important requisite for various artifact treatment and correction techniques"
"Each of the 16 analog to digital converters operates at 2.4576 MHz. Oversampling 64 times yields the internal sampling rate of 38,400 Hz (per channel and for all channels!). In addition, a powerful floating point Digital Signal Processor performs oversampling and real-time filtering of the biosignal data (between 0 Hz – 2,400 Hz). Therefore, a typical sampling frequency of 256 Hz yields an oversampling rate of 9,600. This results in a very high signal to noise ratio, which is especially critical when recording evoked potentials (EP) in the EEG or identifying small amplitude changes in high-resolution ECG recordings. You are measuring far below the noise-range of conventional amplifiers.""
Today due to JeremyC comments that I only knew about nV/Sqrt(Hz) noise formulas. I researched for hours about bits, how to compute for dynamic range in dB and even to relate to nV/Sqrt(Hz). Now I know. But I still can't totally comprehend the above description "You are measuring far below the noise-range of conventional amplifiers."? How so? What is typical noise-range of conventional amplifiers and how could it be measuring far below? Please give actual figure as examples. Thank you.