Adjust analog input range

Forum 2005-2010 (read only) Hardware Interfacing
1

Dear Arduino User,

Is there a way I can adjust the Analog in range from 0-5 Volt to 0-2.5 Volt? I have a sensor which produces values between 0 and 2.5 and I need arduino to digitize the input with as much resolution as possible.

I understand that I can change the reference voltage, but how?

Thanks, Greg

2

One possibility is to use an opamp with a voltage gain of 2. You will need an opamp that can work from a single supply (+5v) and can go rail-to-rail. Something like a MAX473 or a TLV2372.

3

Hook the 2.5V up to AREF, and add this to your code:

#ifndef cbi
#define cbi(sfr, bit) (_SFR_BYTE(sfr) &= ~_BV(bit))
#endif
#ifndef sbi
#define sbi(sfr, bit) (_SFR_BYTE(sfr) |= _BV(bit))
#endif

and in setup...

cbi(ADMUX, REFS1); //clear REFS1 bit (0)
cbi(ADMUX, REFS0); //clear REFS0 bit (0)

Now, 2.5V will be your reference for ADC of 1023. It may be good to put a capacitor on the Aref pin to minimize noise if your voltage is noisy.

D.

4

Thanks to both or you for the excellent advice!
When I follow up about changing
the AREF, I find this:

Table 74. Voltage Reference Selections for ADC
REFS1 REFS0 Voltage Reference Selection
0 0 AREF, Internal Vref turned off
0 1 AVCC with external capacitor at AREF pin
1 0 Reserved
1 1 Internal 2.56V Voltage Reference with external capacitor at AREF pin

Does that mean if I change the bits to 11 I will get to use the INTERNAL 2.56 V (which would be close enough), and if so what would that code look like?

Thanks, Greg

5

Like so:

sbi(ADMUX, REFS1); //set REFS1 bit (1)
sbi(ADMUX, REFS0); //set REFS0 bit (1)

If I understand the datasheet correctly, in this case you wouldn't want anything connected to the AREF pin.

D.

6

Dear D.
Ok so that's how that code works.
It works great, no extra electronics or power supplies!

Thanks so much for you help.

Greg