Is it possible to have 2 separate ADC inputs AND an external Vref?

Hi to all,

I am trying to figure out how to build a circuit using a PIC16F628A or possibly a 16F887A to read two separate voltages accurately and display them on LCD, for accuracy using an external precision Voltage Reference.

I understand the LCD part but I am puzzled about the use of external Vref and two input channels. I do understand how to divide the voltages to levels the PIC is happy with.

I have never used an external Vref before nor have I measured two voltages at the same time.

I have tried to understand the datasheet for the PICs but they seem to assume I know something already and its obvious to me that I don't.

Can anyone explain the way (if it is possible at all) to do this? Or point me to a link that explains it better?

Thanks, Al

I don't the PIC16F628A has a ADC. So how are you going to measure voltages?

The 16F887A has a ADC. You can use Vref- at ground and Vref+ at 2.5 volts from a good source.

You can not really measure two voltages at the same time. You will need to measure the voltages one at a time as close together as possible. (in time)

Pics like the 16f88x series permit an external precision reference Voltage for accurate ADC conversion.

However, there is only one ADC converter. U can switch the input to the converter by assigning different PORT pins to it during the program execution. When u do that u can effectively sample different voltage sources at each of those pins. There is a short settling time and a suggested max impedance of 10k per input. The settling time is on the order of a few uSeconds. Thus u can sample thousands of times per second. When u work at 10bit ADC accuracy for precision, it is useful to do sample averaging to eliminate 'noise' for a better result. If u have rapidly changing sampling voltages, this may not be possible.

Just wanted to mention that the enhanced mid-range devices, like the 18-pin PIC16F1827, have an internal voltage reference module which can be used as the +vref for the ADC module without using any I/O pins. Raj Bhatt did a nice example and write-up on his Embedded-Lab blog; Voltage monitor for car's battery and its charging system.

Cheerful regards, Mike

Nigel's tutorial 11 is all about exactly that, using either a 16f876 or a 16f877.

16F876/F877 are both obsolete. 16F886/F887 supersedes these chips.

Jon Wilder said:

16F876/F877 are both obsolete. 16F886/F887 supersedes these chips.

Click to expand...

The exact same thing applies though, there's little difference across 14 bit PIC's.

Nigel Goodwin said:

The exact same thing applies though, there's little difference across 14 bit PIC's.

Click to expand...

Even though 16F PIC's use a 14 bit long word instruction, they are still 8 bit PIC's as the data buss width is only 8 bit.

Jon Wilder said:

Even though 16F PIC's use a 14 bit long word instruction, they are still 8 bit PIC's as the data buss width is only 8 bit.

Click to expand...

I know, I never suggested otherwise.

The original post was asking a question. If Big Al has a 16f877 to hand, then Nigel's tutorial 11 is just what is required. I have made the 'analogue board' mentioned, programmed the chip and it all works very well.
It's hardly important if the chip is old, dead, non-existant, dresses as a woman. . . etc..