Datasheet

AD7674
Rev. A | Page 17 of 28
Transfer Functions
Except in 18-bit interface mode, the AD7674 offers straight
binary and twos complement output coding when using OB/
2C
.
See Figure 26 and Table 8 for the ideal transfer characteristic.
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)
03083-0-026
Figure 26. 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
AD7674
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
50Ω
AD8021
–
+
15Ω
NOTE 2
NOTE 5
ADR421
10μF
100nF
+
10μ F 100nF
+
100nF
+
10μF
IN–
ANALOG INPUT–
C
C
2.7nF
U2
NOTE 3
NOTE 4
50Ω
AD8021
–
+
15Ω
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
03083-0-027
Figure 27. Typical Connection Diagram (Internal Reference Buffer, Serial Interface)