Vref on 3.3V PIC18LF

[Boomslang]

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,

 

[Pommie]

The maximum voltage on any pin is Vdd. Why can't you use a voltage divider?

Mike.

 

[bananasiong]

Or, why can't you use the PIC at 5 V?

 

[Boomslang]

The maximum voltage on any pin is Vdd. Why can't you use a voltage divider?

Mike.

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.

 

[Boomslang]

Or, why can't you use the PIC at 5 V?

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.

 

[Nigel Goodwin]

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.

 

[Boomslang]

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,

 

[Nigel Goodwin]

You need a buffer opamp between them, check my tutorials for how I solved a similar problem.

 

[Boomslang]

Hi Nigel, thanks for the help. I searched you site, but can't find reference to the buffer op-amp.

 

[bananasiong]

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.

 

[OY2L]

I think this must be the tutorial Nigel mean.

 

[Nigel Goodwin]

I think this must be the tutorial Nigel mean.

That would be the one!

 

[Boomslang]

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.

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,

 

[Nigel Goodwin]

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.

 

[Boomslang]

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.

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.

 

[Nigel Goodwin]

It says minimum load 200K, so you need a buffer. I notice it also has a 2.5V reference output, you could use that to feed the Vref for the PIC.

 

[Boomslang]

So by the looks of it I must do this:

1: Use an op-amp buffer circuit like yours in the tutorial
2: Drop the 5V to 3.3V and used VDD and VCC as Vref
3: or drop it to 2.5V with the op-amp and use the 2.5V Vref from the gyro.

Thanks again,

 

[Nigel Goodwin]

I would suggest using the 2.5V reference from the gyro, using Vdd is about as poor as it gets.

 

[Boomslang]

If I build your circuit as-is, what kind of scaling can I expect? IE: if I input a 3.3V signal, can I have a 1:1 (3.3V buffered) output. Sorry for the many questions, I don't know much about op-amps.

 

[Nigel Goodwin]

You just need to use an opamp as a buffer (no gain), use the 2.5V reference you already have in the chip, and two equal value resistors as an attenuator feeding the input of the opamp - I would suggest using a couple of 1% 220K resistors, so you're well above the chips load requirements.

So on the top part of my circuit make R1 and R5 220K, remove R3, and make P1 a wire link.