Vref on 3.3V PIC18LF
Hi all
I want to use a PIC18LF4620 at 3.3V, but one of my sensors output an analog signal 0V-5V. Can I use a +5V Vref for the ADC on the PIC when the PIC is running at 3.3V?
Thanks,
The maximum voltage on any pin is Vdd. Why can't you use a voltage divider?
Mike.
Or, why can't you use the PIC at 5 V?
bananasiong
Does that mean that I can't use 5V as a ref V? I was thinking of using a simple resistor V divider, will I loose any accuracy/linearity (according to the maths it should be linear)? I suppose I would have to use 1% or better resistors to limit errors if the temperature change.Originally Posted by Pommie
I need to read/write 4 other sensors as well, and some of them only work with 3.3V max. It's only the one sensor that can only work with 5V, and I don't have to wtrite to that sensor.
IE: If I run the PIC at 5V, but the GPS can only work to 3.3V I will have to drop the V on the serial output to the GPS.
No, you can't use a Vref higher than Vdd (or at least not more than 0.7V higher, the datasheet gives the exact details).
But why would you want to anyway?, either use the 3.3V (which I presume is accurate enough?), or a 2.5V precision reference IC. Assuming you want to read 0-5V, a simple resistive attenuator on the input is all you require - but bear in mind the minimum source impedance requirements.
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk
Thanks for the replies.
On the sensor datasheet it says: Output resistive load, minimum 200K, and max is infinity. Sensor output is between 0.5V and 4.5V.
So with a 5V input to a devider with a 1K and a 2K resistors the output will be 3.33V, but how do I work out the impedance, and the sensor output says 200K minimum?
Please see the attached screenshot of the datasheet.
Thanks,
You need a buffer opamp between them, check my tutorials for how I solved a similar problem.
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk
Hi Nigel, thanks for the help. I searched you site, but can't find reference to the buffer op-amp.
Just a non-inverting configuration with unity gain.
bananasiong
I think this must be the tutorial Nigel mean.
Nigel's PIC Tutorial Analogue Board
That would be the one!
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk
The sensor has a 2.5V Vref output that I can connect to the ADC Vref, but how would I use that? The sensor output is 0-5V, with 2.5V at zero state. Do you mean I should drop the output from the sensor to give 0-2.5V and the use the 2.5V Vref?
Instead of having an op-amp buffer, can I put a small cap just before the AN input pin? To keep weight down I would prefer 2x resistors + 1x small cap rather than an op-amp circuit).
Edit to add: Just using a 3.3V swing on a 5V referenced ADC is not goin to be accurate enough.
Thanks,
Last edited by Boomslang; 21st June 2008 at 07:39 AM.
You can simply use an attenuator to drop 0-5V to 0-2.5V, with no loss of accuracy - but it's all dependent on impedances - which is why my tutorial uses opamps as buffers.
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk
Thanks Nigel!
I attached the datasheet for the device I am going to use, do you think I will be able to get away without using the op-amps to buffer the analog signal? (the important electrical specs are on page 5). Another spec: the max capacitive load is 100pf... the PIC ADC loads a 25pf cap, so I should be in spec on that one.
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