Datasheet

AD7679
Rev. A | Page 16 of 28
Transfer Functions
Except in 18-bit interface mode, the AD7679 offers straight
binary and twos complement output coding when using OB/
2C
.
See and for the ideal transfer characteristic. Figure 24 Table 8
000...000
000...001
000...010
111...101
111...110
111...111
ANALOG INPUT
+FS – 1.5 LSB
+FS – 1 LSB
–FS + 1 LSB–FS
–FS + 0.5 LSB
ADC CODE (Straight Binary)
03085-0-026
Figure 24. ADC Ideal Transfer Function
Table 8. Output Codes and Ideal Input Voltages
Description
Analog Input
V
REF
= 4.096 V
Straight
Binary
(Hex)
Twos
Complement
(Hex)
FSR –1 LSB 4.095962 V 3FFFF
1
1FFFF
1
FSR – 2 LSB 4.095924 V 3FFFE 1FFFE
Midscale +
1 LSB
31.25 μV 20001
00001
Midscale 0 V 20000
00000
Midscale –
1 LSB
–31.25 μV 1FFFF
3FFFF
–FSR + 1 LSB -4.095962 V 00001
20001
–FSR -4.096 V 00000
2
20000
2
1
This is also the code for overrange analog input (V
IN+
– V
IN–
above V
REF
– V
REFGND
).
2
This is also the code for underrange analog input (V
IN+
– V
IN–
below –V
REF
+ V
REFGND
).
AVDD AGND DGND
DVDD
OVDD OGND
CNVST
BUSY
SDOUT
SCLK
RD
CS
RESET
PD
2.5V REF
NOTE 1
REFBUFIN
20Ω
D
CLOCK
AD7679
μC/μP/DSP
SERIAL PORT
DIGITAL SUPPLY
(3.3V OR 5V)
ANALOG
SUPPLY
(5V)
DVDD
OB/2C
NOTE 6
PDBUF
DVDD
50kΩ
100nF
1MΩ
IN+
ANALOG INPUT+
C
C
2.7nF
U1
NOTE 3
NOTE 4
AD8021
–
+
30Ω
NOTE 2
NOTE 5
ADR421
10μF
100nF
+
10μ F 100nF
+
100nF
+
10μ F
IN–
ANALOG INPUT–
C
C
2.7nF
U2
NOTE 3
NOTE 4
AD8021
–
+
30Ω
50Ω
100nF
NOTES
1. SEE VOLTAGE REFERENCE INPUT SECTION.
2. OPTIONAL CIRCUITRY FOR HARDWARE GAIN CALIBRATION.
3.THE AD8021 IS RECOMMENDED. SEE DRIVER AMPLIFIER CHOICE SECTION.
4. SEE ANALOG INPUTS SECTION.
5. OPTION, SEE POWER SUPPLY SECTION.
6. OPTIONAL LOW JITTER CNVST, SEE CONVERSION CONTROL SECTION.
47μF
MODE1
MODE0
NOTE 1
C
REF
REF
REFGND
03085-0-027
Figure 25. Typical Connection Diagram (Internal Reference Buffer, Serial Interface)