# 5V voltage reference for the AD7718?

My application requires converting voltages from (almost) 0 to 5V using fully differential probing. From the application sheet it seems like I need to use a 2.5V reference in order to cover the entire input range of the ADC?

Somehow it seems that in order to run the setup I need want RN2 = RN1 = RN0 = 1 in ADCCON & Vref2 = 2.5V & Vredf1 = GND (0V). Is this correct?

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• Hi,

The nominal reference voltage for specified operation, VREF, (REFIN1(+)–REFIN1(–) or REFIN2(+)–REFIN2(–)), is 2.5 V, but the AD7708/AD7718 is functional with reference voltages from 1 V to AVDD. With Vref = 2.5V, the part allows to handle unipolar analog input ranges from 0 mV to +20 mV to 0 V to +2.5 V and bipolar inputs of +/-20 mV to +/-2.5 V. Because the part operates from a single supply these bipolar ranges are with respect to a biased-up differential input. Meaning, if you want to measure an input voltage range of 0 to 5V, you have to operate in pseudo-differential mode  referenced to AINCOM = 2.5V. If AINCOM is 2.5 V and the AD7718 is configured for an analog input range of +/-2.56 V, the analog input range on the AIN1 input can cover 0V to 5V (i.e., 2.5 V +/-2.56V). Bipolar or unipolar options are chosen by programming U/B bit in the ADCCON register.

Thanks,

Jellenie

• Hi,

The nominal reference voltage for specified operation, VREF, (REFIN1(+)–REFIN1(–) or REFIN2(+)–REFIN2(–)), is 2.5 V, but the AD7708/AD7718 is functional with reference voltages from 1 V to AVDD. With Vref = 2.5V, the part allows to handle unipolar analog input ranges from 0 mV to +20 mV to 0 V to +2.5 V and bipolar inputs of +/-20 mV to +/-2.5 V. Because the part operates from a single supply these bipolar ranges are with respect to a biased-up differential input. Meaning, if you want to measure an input voltage range of 0 to 5V, you have to operate in pseudo-differential mode  referenced to AINCOM = 2.5V. If AINCOM is 2.5 V and the AD7718 is configured for an analog input range of +/-2.56 V, the analog input range on the AIN1 input can cover 0V to 5V (i.e., 2.5 V +/-2.56V). Bipolar or unipolar options are chosen by programming U/B bit in the ADCCON register.

Thanks,

Jellenie

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