Voltage Divider Measurement Problem

If you measuring voltage with voltage divider circuit, you should use not only resistor. Capacitors are very important thing for voltage divider. Especially, if you are using high frequency signals.

When you made a simple two resistor voltage divider for measuring simple voltages such as battery voltage, most probably you will measure voltage value without any problem. But if you measure any voltage that have a frequency, you may get a wrong or unstable values. Not only this; If you measure a stabil voltage like a battery voltage but there are some high frequency signal nearby, you will get unstable or wrong result. Particularly; CPU signals, switching signals, oscillators etc.. these signals causing electromagnetic interference.

To avoid unstability, you should use simple filter with a small capacitor. You can calculate this capacity value with filter equations. Also there is an issue that you should be careful, if you select too large capacity value, it will cause a time delay and extra filtering.

For example; when your battery voltage gone lower, your proccessor will warn user to recharge the device, with too large capacity your proccessor can not detect the low voltage immediatly and user will not charge the device and device turns off without any warning. So, you need use capacitor charge time equations and filtering equation for select the right capacitor value. Generally 100nF is enough for many not-complex experimental circuit.

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