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I am using an Arduino to capture an AC signal. However, the application is sensitive and needs a high resolution, I decided to use the internal 1.1 Volt as a voltage reference for the ADC.

In the case of using 5 volt as a voltage reference for the ADC :

The AC signal contains negative voltages which the Arduino can't capture ( the Arduino is capable of measuring 0-5 volts) so I used two resistors connected to the 5 volt from Arduino and then the ground of my signal is to be connected to the 2.5 volt.

(the upper left part of the picture).

And that worked fine

my question is, if I want to use the 1.1 Volt as a voltage source which is to be connected to the two resistors to provide .55V (1.1V /2) as a middle point for my AC instead of 2.5 volt, would connecting the VRef pin to the Resistors affect the ADC? will it make it unstable ?

enter image description here

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From the ATmega328P datasheet:

Note that VREF is a high impedance source, and only a capacitive load should be connected in a system.

You will need to use a voltage follower buffer in order to present a high impedance so that you disrupt the ADC as little as possible.

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  • \$\begingroup\$ in my actual circit I was using a voltage follower buffer which its input is connected to the output of the voltage divider(Because I needed a stable 2.5 volt) so that means now I need two voltage follower Buffer, one connected to Vref and another one connected to the output of the Voltage divider ? @Ignacio Vazquez-Abrams \$\endgroup\$ – Sabir Moglad Jul 22 '15 at 2:21
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    \$\begingroup\$ Yes, which means that either your BOM doesn't expand or you move up to a dual chip. \$\endgroup\$ – Ignacio Vazquez-Abrams Jul 22 '15 at 2:22
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The ATmega328 datasheet says (emphasis added):

24.5.2 ADC Voltage Reference The reference voltage for the ADC (VREF) indicates the conversion range for the ADC. Single ended channels that exceed VREF will result in codes close to 0x3FF. VREF can be selected as either AVCC, internal 1.1V reference, or external AREF pin. AVCC is connected to the ADC through a passive switch. The internal 1.1V reference is generated from the internal bandgap reference (VBG) through an internal amplifier. In either case, the external AREF pin is directly connected to the ADC, and the reference voltage can be made more immune to noise by connecting a capacitor between the AREF pin and ground. VREF can also be measured at the AREF pin with a high impedance voltmeter. Note that VREF is a high impedance source, and only a capacitive load should be connected in a system.

Whilst you need not take that completely literally, if you want to use that voltage you should add a buffer that has as low bias current as a capacitor would have leakage. A CMOS-input op-amp connected as a voltage follower would suffice. Make sure that 1.1V is well within the common mode input and outupt range of the op-amp (usually not an issue).

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  • \$\begingroup\$ is it necessary to be a "A CMOS-input op-amp" and Ua741 is not a CMOS? @Spehro Pefhany \$\endgroup\$ – Sabir Moglad Jul 22 '15 at 2:25
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    \$\begingroup\$ @SabirMoglad: LM741 is bipolar. A great many CMOS op amps start with "LMC". \$\endgroup\$ – Ignacio Vazquez-Abrams Jul 22 '15 at 2:28
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    \$\begingroup\$ A 741 is not CMOS. There is no information as to how much the error the bias current would cause. It would also need a negative supply, so not a good choice. \$\endgroup\$ – Spehro Pefhany Jul 22 '15 at 2:28

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