Datasheet

AD7985
Rev. A | Page 14 of 28
Transfer Functions
The ideal transfer characteristic for the AD7985 is shown in
Figure 22 and Table 7.
000 ... 000
000 ... 001
000 ... 010
ADC CODE (STRAIGHT BINARY)
ANALOG INPUT
+FSR – 1.5 LSB
+FSR – 1 LSB
–FSR + 1 LSB
–FSR
–FSR + 0.5 LSB
07947-006
111 ... 101
111 ... 110
111 ... 111
Figure 22. ADC Ideal Transfer Function
Table 7. Output Codes and Ideal Input Voltages
Description
Analog Input,
V
REF
= 4.096 V
Digital Output
Code (Hex)
FSR – 1 LSB 4.095938 V 0xFFFF
1
Midscale + 1 LSB 2.048063 V 0x8001
Midscale 2.048 V 0x8000
Midscale – 1 LSB 2.047938 V 0x7FFF
–FSR + 1 LSB 62.5 µV 0x0001
–FSR 0 V 0x0000
2
1
This is also the code for an overranged analog input (V
IN+
− V
IN−
above V
REF
−
REFGND).
2
This is also the code for an underranged analog input (V
IN+
− V
IN−
below REFGND).
TYPICAL CONNECTION DIAGRAM
Figure 23 shows an example of the recommended connection
diagram for the AD7985 when multiple supplies are available.
07947-007
NOTES
1. GND REFERS TO REFGND, AGND, AND DGND.
AD7985
GNDREF
AVDD,
DVDD
VIO
BVDD
5V 2.5V
1.8V
TO
2.7V
VIO
SDI
SCK
SDO
CNV
3- OR 4-WIRE
INTERFACE:
SPI, CS,
DAISY CHAIN
(TURBO = LOW)
TURBO
10µF
IN+
IN–
2.7nF
15Ω
V–
0V TO V
REF
V+
Figure 23. Typical Application Diagram with Multiple Supplies