Electronic – Controlling Voltage or Current with a PIC

you want a voltage divider. if you care about power dissipation, there are a couple of things you can do:

• use a fairly high impedance voltage divider, with a capacitor on the output terminal, and buffer it with a low-power op-amp. Make sure you use an RC filter between op-amp and ADC, values of this RC filter are typically around 50-200 ohms, 1000pf. This serves to keep voltage stable when the ADC is switching between channels and charge is transferred between the ADC's internal capacitor and the outside pin. An op-amp alone cannot do this. If you don't buffer the high-impedance voltage divider, you will get errors due to ADC leakage current and charge transfer.

• switch the voltage divider, e.g. connect/disconnect it to the supply voltage, so you can do this only occasionally when you want. A PFET would work, just be careful with how you drive it.

There are many current-to-voltage converters out there. The simplest is a resistor. Make it small to reduce losses or big to make the voltage big enough to cover your entire ADC range. Or, make it small to reduce losses and amplify it with an op-amp circuit to cover your entire ADC range.

Another often overlooked current-to-voltage converter is a MOSFET that's on, which is pretty close to a resistor. Perhaps not as accurate, but many circuits that require current measurement already have a switching MOSFET in them, so this method is cheap and convenient. The datasheet will list the on resistance as \$R_{DS\_ON}\$.

For a step up in sophistication, look for hall effect sensors. Hall effect sensors measure magnetic fields; some of them are configured to measure the magnetic flux associated with a current passing through a wire. Here's an example product page to give you an idea. However, at \$200mA\$, I don't see much need. Typically this sort of product comes into play when the current is high enough that a resistor, even a small one, would represent a significant loss.