adcanalogoperational-amplifier
I'm trying to connect a 0-10V analog input value to my MCU which is running on 3.3V. I used the below circuit but when the input is floating I read about 0.3V on the analog input. I need to eliminate the bias current from the analog input.
simulate this circuit – Schematic created using CircuitLab
Can anyone give me a hint what is the best practice to connect a 0-10V signal to an MCU?
The first schematic is terminating a 50 ohm impedance transmission line. The capacitor is likely there to provide some filtering of high frequency noise. Unless you're using a 50 ohm transmission line, this solution doesn't apply to you.
The second schematic is using a resistor to limit current through the MCU's protection diodes in the event that the signal is outside the rails. A parallel capacitor bypasses the resistor, improving the high frequency response.
What you use depends on your requirements. If you want to protect the microcontroller against signals outside the rails, a series resistor like R2 in the second schematic is useful, though it will limit its frequency response. The parallel capacitor is only likely to be important if you're measuring high frequency signals.
Yes, that would work fine, but you can make this even more simple:
simulate this circuit – Schematic created using CircuitLab
Zener diodes behave more or less as a "normal" diode when in forward mode so we can use that to clamp negative voltages.
When using a 12V, 400 mW zener diode the maximum current through the zener diode can be 400mW / 12 V = 33 mA. There will then be 12 V across R1, R1 needs to limit the current to 33 mA so it needs to be: 12 V / 33 mA = 364 ohm or higher. I added some margin and suggest 470 ohms then the actual current will be 12 V / 470 ohm = 26 mA and R1 will dissipate 12 * 26 mA = 0.312 W so you might want to use a 0.5 W resistor (or use two 1 kohm 0.25 W resistors in parallel).
I would make the capacitor C1 10 nF, that should suffice to remove most pulses. Only if you need to measure high frequency signals C1 might need to get a smaller value or must be removed. Then the zener diode might be an issue as well as zeners aren't very fast. But if you stay below 100 kHz or so there should be no issue with this circuit.
Best Answer
THe LM358 has an input impedance ~ 10M and output impedance ~ 1k @ 1V/1mA and 100mV /50uA ~2k so it is does not reach negative rail to 0V. But perhaps close enough if your load is < 50uA or >> 2k. That is due to saturation of BJT’s.
Consider matching input R to V+,V- to null input bias current induced Vio with ~1M on each input.
Consider output current with voltage divider R’s with <1V = Vout. Raise the output R’s to reduce Iout. The clamp current from MCU diodes will limit the output voltage without a need for a Zener to << 1mA.
This ought to reduce open cct Vio and Vout .
A better solution might be to use a RRIO CMOS OpAmp with 10M on each input.