The input_voltage or the supply_voltage for the OPAMP?The input voltage comes from a 3V linear regulator
Please try to input "as flat as possible" to your calculator..but I need it to be as flat and stable as possible: noise and drift are an issue. The output will be used as a ground reference for an amplifier.
Now you talk about the "other" amplifier, don´t you?The input signal of the amplifier has an amplitude of about 1/20mV,
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Things are getting clearer now.
* I may be wrong, but afaik EEG signals don´t need DC performance, so i see the voltage divider circuit uncritical
* the AD620 REF pin is the output_signal_reference pin. It is less critical against noise and fluctuation than the input pins. (Again, circuit is uncritical)
* The REF signal just lifts the output signal to your desired value. Most probably the AD620 output goes to an ADC with positive only input range.
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My recommendation:
if there is an ADC:
* use the ADC_V_Ref to generate the AD620_Ref. simple voltage divider folowed by a low noise buffer circuit.
* supply the low noise buffer with the same supply voltage than the ADC.
* use proper bulk and high frequency debouncing capacitors at each device´s supply pin.
* use proper filtering at the ADC_VRef pin.
* no special care needed for filtering of the AD620_VRef_voltage. (because it cancles out with the ADC_VRef signal)
total noise = sqrt(voltage_noise^2 + (current_noise x electrode_impedance)^2)
A good start for selection is when the amplifier is called "low noise" and the voltage_noise/current_noise is about equal to electrode_impedance.
I recommend to use excel to calculate and compare some amplifiers.
A 4.5V battery is probably three alkaline cells. It will quickly drop to about 3.6V then produce only about 3V when the battery needs replacing.
You show a 7805 which is a 5V regulator with a minimum input voltage rating of 7V and a recommended lowest input of 8V. From a battery that drops to only 3V??
Battery powered electronics use low dropout voltage regulators. I looked at what Texas instruments have. I entered a minimum input of 3.6V and a 3V/100mA output and they have 181 different ICs that will do it.
I was indeed trying to replace the AD620 with a lower power solution (I'd like my circuit to be battery powered).
I found the INA826 which has a RR Output. And V_ref range is -Vs +Vs on that one.
There is also the AD8237 which looks like a great choice (RR I/O as you suggested), low power, V_ref range of -Vs-0.3 ... +Vs+0.3
But since it has a different architecture I can't compare it's noise specs with the INA826. The AD8237 has lower current noise, but how to compare the total voltage noise? Is the Input Voltage Noise Density of the AD8237 equivalent to the RTI of the INA826?
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