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AD7708/AD7718

Converting Single-Ended Inputs

The AD7708/AD7718 generally operates in buffered mode. This

places a restriction of AGND + 100 mV to AVDD – 100 mV on

the absolute and common-mode voltages that can be applied to

any input on the AD7708/AD7718.

Some applications may require the measurement of analog

inputs with respect to AGND. To enable the AD7708/AD7718

to be used in these single-ended applications, the buffer on the

AINCOM can be bypassed. The NEGBUF bit in the mode

input when a channel is configured for pseudo-differential mode of

operation. If cleared, the analog negative input (AINCOM) is

unbuffered, allowing it to be tied to AGND in single-ended

input configuration. If this bit is set, the analog negative input

(AINCOM) is buffered, placing a restriction on its common-

mode input range. When AINCOM is unbuffered, signals with a

common-mode range from AGND – 30 mV to AVDD + 30 mV

can be accommodated on this input allowing the end user to

connect the AINCOM input to AGND and perform single-

ended measurements with respect to this input. This unbuffered

input path on the AINCOM provides a dynamic load to the

driving source. Therefore, resistor/capacitor combinations on

this input pin can cause dc gain errors, depending on the output

impedance of the source that is driving the AINCOM input. All

analog inputs still operate in buffered mode and their com-

mon-mode and absolute input voltage is restricted to a range

between AGND + 100 mV and AVDD – 100 mV.

Combined Ratiometric and Absolute Value Measurement

System

The AD7708/AD7718 when operated with CHCON = 0 can be

configured for operation as four fully-differential analog inputs or

eight pseudo-differential analog inputs with two fully-differential

reference inputs. Having the ability to use either REFIN1 or

REFIN2 with any channel during the conversion process allows

the end user to make both absolute and ratiometric measure-

ments as shown in Figure 27. In this example a fully-differential

analog input (AIN1–AIN2) is being converted from a bridge trans-

ducer in a ratiometric manner using REFIN1 as the reference

input for this channel. AIN3 is configured as a pseudo-differential

input channel using REFIN2 to perform an absolute measurement

on the potentiometer. The REFSEL bit in the mode register is

used to select which reference is used with the active channel

during the conversion process. When the AD7708/AD7718 is

configured with CHCON = 1, only one reference (REFIN1) is

available. The contents of the CHCON bit override the REFSEL

bit. If the ADC is configured in five fully-differential or 10

pseudo-differential input channel mode, the REFSEL bit setting

is irrelevant as only REFIN1 is available.

REFIN1(–)

AD7718

REFIN2(+)

REFIN1(+)

AINCOM

AIN1

AIN3

AIN2

REFIN2(–)

GND

EXCITATION VOLTAGE = 4V

WIPER

Figure 27. Absolute and Ratiometric Measurement

System Using AD7718

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