Datasheet

AD7678
Rev. A | Page 16 of 28
Transfer Functions
Except in 18-bit interface mode, the AD7678 offers straight
binary and twos complement output coding when using OB/
2C
.
See Figure 20 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)
03084-0-026
Figure 20. 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
20
CLOCK
AD7678
C/P/DSP
SERIAL PORT
DIGITAL SUPPLY
(3.3V OR 5V)
ANALOG
SUPPLY
(5V)
DVDD
OB/2C
PDBUF
DVDD
50k
100nF
1M
IN+
ANALOG INPUT+
C
C
U1
NOTE 4
50
AD8021
–
+
NOTE 3
NOTE 5
ADR421
10F
100nF
+
10F 100nF
+
100nF
+
10F
IN–
ANALOG INPUT–
C
C
U2
NOTE 4
50
AD8021
–
+
100nF
10F
MODE1
MODE0
NOTE 2
C
REF
REF
REFGND
03084-0-027
NOTES
1. SEE VOLTAGE REFERENCE SECTION.
2. C
REF
is 10F CERAMIC CAPACITOR OR LOW ESR TANTALUM. CERAMIC SIZE
1206 PANASONIC ECJ-3xB0J106 IS RECOMMENDED. SEE VOLTAGE REFERENCE SECTION.
3. OPTIONAL CIRCUITRY FOR HARDWARE GAIN CALIBRATION.
4.THE AD8021 IS RECOMMENDED. SEE DRIVER AMPLIFIER CHOICE SECTION.
5. OPTION, SEE POWER SUPPLY SECTION.
Figure 21. Typical Connection Diagram (Internal Reference Buffer, Serial Interface)