I need min. 20M ohm input impedance for differential analog input channel. Max input range is -10V~+10V DC.
To check, I tried with AD8221 at present with 10K series resistance and 10 Mohm for bias current return path to GND at both -, + inputs of AD8221. 1nF capacitor in parallel with 10 Mohm. Some how readings are stable and accurate but while connecting DSO at source, o/p of AD8221 becomes unstable and floating (less value than applied) though DSO reads source value correctly.
Not able to understand what is going wrong and how to achieve this..