aramosfet
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Hi
I need to measure DC voltage 0-30V on a microcontroller and display the same. Since we need to measure a large voltage I made a differential amplifier to measure input voltage and added a NFET switched voltage divider at input to measure higher voltages. the switch will be off at lower voltage to get better resolution.
However , the problem with above circuit is the effective impedance at non-inverting input of opamp when the FET switch is off is 200K, Hence the differential amp does not work properly.
So I tried to isolate V+ input by inserting an unity gain opamp that takes Vdiv and feeds the differential amplifier. Here, if the input voltage can exceed opamp supply voltage (4.096V) if the switch is off. To protect opamp input I added a Schottky diode (1N5818) from Vdiv to VCC, But the the problem with this was, the reverse leakage from diode increases the voltage at Vdiv point.
What is the best way to design a differential amplifier wot measure inputs greater than supply range?
Thanks
I need to measure DC voltage 0-30V on a microcontroller and display the same. Since we need to measure a large voltage I made a differential amplifier to measure input voltage and added a NFET switched voltage divider at input to measure higher voltages. the switch will be off at lower voltage to get better resolution.
However , the problem with above circuit is the effective impedance at non-inverting input of opamp when the FET switch is off is 200K, Hence the differential amp does not work properly.
So I tried to isolate V+ input by inserting an unity gain opamp that takes Vdiv and feeds the differential amplifier. Here, if the input voltage can exceed opamp supply voltage (4.096V) if the switch is off. To protect opamp input I added a Schottky diode (1N5818) from Vdiv to VCC, But the the problem with this was, the reverse leakage from diode increases the voltage at Vdiv point.
What is the best way to design a differential amplifier wot measure inputs greater than supply range?
Thanks