5v and 7v on same board - question

My scenario involves a pre-amped circuit (3rd party) connected to my custom PIC project board via a 13-pin cable/connector. A number of analog signals will be coming into my board, and power will be sent across to the pre-amp.

Here's the challenge...

My PIC circuit is obviously powered at 5v. The pre-amp circuit requires 7v power to be delivered. I'm trying to determine if I need to add a 7v voltage regulator on my circuit board in order to send the appropriate voltage, or if I can somehow use the 5v power that's already on the board (for the MCU). I guess I can always have 2 voltage regulators (5v and 7v) connected to the main power source, but would rather not.

Also, the analog signals coming back across the 13-pin cable (into my board) are 7v - so should I assume that I need to "step down" to 5v before connecting them to the analog inputs on my PIC?

Is there an easier way to do this? Can I just use a single 7v voltage regulator and run my 18F452 (or any other PIC) at 7v to make things easier? Bad idea, huh?

Or should I try sending 5v power to the pre-amp? No harm, sending less power than required, correct?

I'm pretty much a newbie, so appreciate any help. Thanks!!

- Mark O

Hi Mark,
You shouldn't run the PIC on 7V. The datasheet for the 18F452 says that the supply voltage should be between 4.2 and 5.5V - and that's probably the same for most other PICs too (not LF). So if the external circuit can't operate correctly on 5V (try it) you need to figure out a way to supply it with 7V. If you can fit another regulator that is probably the easiest way.

Now, for the analog inputs. Yes you need to bring the 0-7V signals down bit. If you are using Vdd as reference for the ADC you need to bring it down to 0-5V (if Vdd is 5V). A simple voltage divider will work fine. If you are using an external reference for the ADC the input obviously needs to span whatever the voltage the refrerense is set to.

Hope it helps

/Henrik Olsson.