I have a concept of using a one channel ADC for three differents analog inputs sources.
My question is: should I worry if the the analog inputs have differents inputs impedance, one is very high internal impedance, the second is very low internal impedance and the third is zero internal impedance? ( OpAmp buffer between the ADC and the source analog inputs)
If I only wanted to select one analog inputs at once, I thought of having a mosfest switch between the analog inputs and the OpAmp?
Below is the circuit with voltage controller switch used as Analog swich in LTspice.
Electronic – Multiple Analog input for one channel ADC with Amplifier drivers
adcanalogcircuit-designoperational-amplifier
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Best Answer
The right thing to do for any application is to make sure that you buffer the signal sufficiently before you feed it to the ADC.
How much buffering you need depends on the impedance of the source and the impedance of the ADC – which in turn will be given in the ADC's datasheet. Usually, we use ADC buffers (opamps in voltage follower configuration) to drive ADCs.
So, really, make all three inputs buffered. A quad-opamp IC costs literal cents, and the voltage follower configuration is really nothing but the input signal connected to the non-inverting input of the opamp, and the inverting input connected to the opamps output, and use the latter to drive the ADC. So, that's the cost of one connection plus the cost of one quad opamp to give every single input a reliable buffer. Do it!
Switching: You can switch buffered signals using an analog switch, but you shouldn't do that for high-impedance signals, typically. So, voltage buffer for each input, switch after that, ADC last.