Datasheet

AD7688
Rev. A | Page 13 of 28
Transfer Functions
The ideal transfer characteristic for the AD7688 is shown in
Figure 25 and Table 8.
100...000
100...001
100...010
011...101
011...110
011...111
ADC CODE (TWOS COMPLEMENT)
ANALOG INPUT
+FSR – 1.5 LSB
+FSR – 1 LSB
–FSR + 1 LSB
–FSR
–FSR + 0.5 LSB
02973-024
Figure 25. ADC Ideal Transfer Function
Table 8. Output Codes and Ideal Input Voltages
Description
Analog Input
V
REF
= 5 V Digital Output Code Hexa
FSR – 1 LSB +4.999847 V 7FFF
1
Midscale + 1 LSB +152.6 μV 0001
Midscale 0 V 0000
Midscale – 1 LSB −152.6 μV FFFF
–FSR + 1 LSB −4.999847 V 8001
–FSR −5 V 8000
2
TYPICAL CONNECTION DIAGRAM
Figure 26 shows an example of the recommended connection
diagram for the AD7688 when multiple supplies are available.
1.
This is also the code for an overranged analog input (V
IN+
− V
IN−
above V
REF
−
V
GND
).
2.
This is also the code for an underranged analog input (V
IN+
− V
IN−
below −V
REF
+ V
GND
).
AD7688
REF
GND
VDD
IN–
IN+
VIO
SDI
SCK
SDO
CNV
3- OR 4-WIRE INTERFACE
5
100nF
100nF
5V
10μF
2
≥7V
≥7V
≤–2V
1.8V TO VDD
REF
1
0 TO VREF
33Ω
2.7nF
3
4
≥7V
≤–2V
VREF TO 0
33Ω
2.7nF
3
4
02973-025
1
SEE REFERENCE SECTION FOR REFERENCE SELECTION.
2
C
REF
IS USUALLY A 10μF CERAMIC CAPACITOR (X5R).
3
SEE DRIVER AMPLIFIER CHOICE SECTION.
4
OPTIONAL FILTER. SEE ANALOG INPUT SECTION.
5
SEE DIGITAL INTERFACE FOR MOST CONVENIENT INTERFACE MODE.
Figure 26. Typical Application Diagram with Multiple Supplies