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Using a voltage reference

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Hi Nigel,

Just a quickie - when I build this circuit - am I still limited to 10k Pots, or can i use larger value pots as the inputs will be buffered through this module?

Only reason I ask is that there are a few 100K pots in my box.

Thanks
 
The pots in my board are 2.2K, if you wanted to add pots at the input to the opamp, you would need to alter the input impedance, or replace the attenuator entirely by the pot.
 
Hi Nigel,

Just a quickie - when I build this circuit - am I still limited to 10k Pots, or can i use larger value pots as the inputs will be buffered through this module?

Only reason I ask is that there are a few 100K pots in my box.

Thanks

hi,
I dont want to cut across the info that Nigel's posting, but could you give details of the voltage range you expect from the source and what is the source of the voltage you want to convert.

There may be simpler ways to do it.:)
 
hi,
I dont want to cut across the info that Nigel's posting, but could you give details of the voltage range you expect from the source and what is the source of the voltage you want to convert.

There may be simpler ways to do it.:)

There may well be, as you say it depends exactly what it is.
 
Basically I just want to be able to read in the position of 13 pots, using the the 13ADC ports on my pic16f777.

I have a load of 100K pots in my box, but the pic likes 10K max (thats what it says earlier in the thread)

Accurancy is quite important for this, but so is easy maths.

Thanks
 
Basically I just want to be able to read in the position of 13 pots, using the the 13ADC ports on my pic16f777.

I have a load of 100K pots in my box, but the pic likes 10K max (thats what it says earlier in the thread)

Accurancy is quite important for this, but so is easy maths.

Thanks

Hi,
Got that, but whats the voltages on the pots, is it just +5V for training purposes.?
 
it can be any voltage, its just a potential divider using an external voltage reference to improve accuracy.

i only need to read them 5 times a second
 
it can be any voltage, its just a potential divider using an external voltage reference to improve accuracy.

i only need to read them 5 times a second

hi,
The first part of your reply dosn't make any sense.:)
 
sry im confused, i dont understand what your asking.

I dont mind what voltage i have to use, whatever is easiest.

I use the internal 5V reference at the moment, but i have been told that using an external reference would improve accuracy.

The way I have it working just now is that pin 'A' of all my pots are connected to +5v, pin 'B' of all the pots are connected to gnd, and each wiper is connected to a different ADC on the pic
 
sry im confused, i dont understand what your asking.

I dont mind what voltage i have to use, whatever is easiest.

I use the internal 5V reference at the moment, but i have been told that using an external reference would improve accuracy.

The way I have it working just now is that pin 'A' of all my pots are connected to +5v, pin 'B' of all the pots are connected to gnd, and each wiper is connected to a different ADC on the pic

hi,
As the 100K pots are all connected across +5V and 0V, with the pot wiper as output, all you need for each input is a non inverting buffer OPA, between the wiper and the PIC's adc input.
This will accept the high 100K impedance and transform it down close to zero.

For cost I would just buy some 10K pots, no buffers required.

What +Vref source are you planning to use.?
 
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sry im confused, i dont understand what your asking.

I dont mind what voltage i have to use, whatever is easiest.

I use the internal 5V reference at the moment, but i have been told that using an external reference would improve accuracy.

The way I have it working just now is that pin 'A' of all my pots are connected to +5v, pin 'B' of all the pots are connected to gnd, and each wiper is connected to a different ADC on the pic

Makes no difference for that use - you're comparing like with like, if the 5V changes slightly, the reference changes as well - so no point in an external reference.

But what's the point of just hanging 14 pots on the 5V line?.
 
Makes no difference for that use - you're comparing like with like, if the 5V changes slightly, the reference changes as well - so no point in an external reference.

But what's the point of just hanging 14 pots on the 5V line?.

ahh, i get you now - i feel so stupid now!!

the pots are connected to various levers, gears, plus few of them are linear pots measuring travel.

I need to log all the information, but from what you have stated above, what I have just now will work fine, I was told the external reference would be more stable, therefore the measurements would be more accurate, guess that i never explained my problem very well.

Thanks to everyone who has helped me, I have to admit that im very digital - i really struggle with analog stuff.

Eric, even if I do buy 10K pots, its still very handy knowing how to connect larger value pots to the pic.
 
ahh, i get you now - i feel so stupid now!!

the pots are connected to various levers, gears, plus few of them are linear pots measuring travel.

You could do this without needing any analogue inputs on the PIC at all, check my Joystick tutorial, which works in the same way as a PC Joystick port, using a digital input to measure the position of a pot.
 
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