Datasheet AD7708, AD7718 (Analog Devices) - 42

AD7708/AD7718 Converting Single-Ended Inputs Combined Ratiometric and Absolute Value Measurement
The AD7708/AD7718 generally operates in buffered mode. This
System
places a restriction of AGND + 100 mV to AVDD – 100 mV on The AD7708/AD7718 when operated with CHCON = 0 can be the absolute and common-mode voltages that can be applied to configured for operation as four fully-differential analog inputs or any input on the AD7708/AD7718. eight pseudo-differential analog inputs with two fully-differential Some applications may require the measurement of analog reference inputs. Having the ability to use either REFIN1 or inputs with respect to AGND. To enable the AD7708/AD7718 REFIN2 with any channel during the conversion process allows to be used in these single-ended applications, the buffer on the the end user to make both absolute and ratiometric measure- AINCOM can be bypassed. The NEGBUF bit in the mode ments as shown in Figure 27. In this example a fully-differential register controls the operation of the input buffer on the AINCOM analog input (AIN1–AIN2) is being converted from a bridge trans- input when a channel is configured for pseudo-differential mode of ducer in a ratiometric manner using REFIN1 as the reference operation. If cleared, the analog negative input (AINCOM) is input for this channel. AIN3 is configured as a pseudo-differential unbuffered, allowing it to be tied to AGND in single-ended input channel using REFIN2 to perform an absolute measurement input configuration. If this bit is set, the analog negative input on the potentiometer. The REFSEL bit in the mode register is (AINCOM) is buffered, placing a restriction on its common- used to select which reference is used with the active channel mode input range. When AINCOM is unbuffered, signals with a during the conversion process. When the AD7708/AD7718 is common-mode range from AGND – 30 mV to AVDD + 30 mV configured with CHCON = 1, only one reference (REFIN1) is can be accommodated on this input allowing the end user to available. The contents of the CHCON bit override the REFSEL connect the AINCOM input to AGND and perform single- bit. If the ADC is configured in five fully-differential or 10 ended measurements with respect to this input. This unbuffered pseudo-differential input channel mode, the REFSEL bit setting input path on the AINCOM provides a dynamic load to the is irrelevant as only REFIN1 is available. 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
REFIN1(+)
analog inputs still operate in buffered mode and their com- mon-mode and absolute input voltage is restricted to a range
AIN1
between AGND + 100 mV and AV
AIN2
DD – 100 mV.
REFIN1(–) AD7718 EXCITATION VOLTAGE = 4V REFIN2(+) WIPER AIN3 AINCOM REFIN2(–) GND
Figure 27. Absolute and Ratiometric Measurement System Using AD7718 –42– REV. 0