Datasheet

AD7643
Rev. 0 | Page 16 of 28
TRANSFER FUNCTIONS
Using the OB/
2C
digital input, except in 18-bit interface mode,
the AD7643 offers two output codings: straight binary and twos
complement. The LSB size with V
REF
= 2.048 V is 2 × V
REF
/
262,144, which is 15.623 μV. Refer to
Figure 22 and Table 8 for
the ideal transfer characteristic.
000...000
000...001
000...010
111. ..101
111...110
111. ..111
ADC CODE (Straight Binary)
ANALOG INPUT
+FSR – 1.5 LSB
+FSR – 1 LSB–FSR + 1 LSB
–FSR
–FSR + 0.5 LSB
06024-022
Figure 22. ADC Ideal Transfer Function
Table 8. Output Codes and Ideal Input Voltages
Digital Output Code (Hex)
Description
Analog Input
V
REF
= 2.048 V
Straight
Binary
Twos
Complement
FSR −1 LSB +2.0479844 V 0x3FFFF
1
0x1FFFF
1
FSR − 2 LSB +2.0479688 V 0x3FFFE 0x1FFFE
Midscale + 1 LSB +15.625 μV 0x20001 0x00001
Midscale 0 V 0x20000 0x00000
Midscale − 1 LSB −15.625 μV 0x1FFFF 0x3FFFF
−FSR + 1 LSB −2.0479844 V 0x00001 0x20001
−FSR −2.048 V 0x00000
2
0x20000
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
).
RD
CS
100nF 100nF
AVDD
10µF
100nF
AGND DGND DVDD OVDD OGND
CNVST
BUSY
SDOUT
SCLK
RESET
PD
REFBUFIN
10Ω
D
CLOCK
AD7643
MICROCONVERTER/
MICROPROCESSOR/
DSP
SERIAL
PORT
DIGITAL
INTERFACE
SUPPLY
(2.5V OR 3.3V)
ANALOG
SUPPLY (2.5V)
OVDD
DIGIT
A
L
SUPPLY (2.5V)
IN+
IN–
U2
15Ω
NOTE 5
50Ω
50pF
NOTE 1
ANALOG
INPUT +
C
C
C
C
2.7nF
2.7nF
U1
15Ω
NOTE 1
MODE0
MODE1
D0/OB/2C
REFGND
REF
PDBUF
PDREF
100nF
ANALOG
INPUT –
NOTE 2
NOTE 2
NOTE 3
NOTE 4
NOTE 3
NOTE 7
NOTE 6
10µF
10µF
C
REF
10µF
10kΩ
50pF
1. SEE ANALOG INPUTS SECTION.
2
. THE AD8021 IS RECOMMENDED. SEE DRIVER AMPLIFIER CHOICE SECTION.
3
. THE CONFIGURATION SHOWN IS USING THE INTERNAL REFERENCE. SEE VOLTAGE REFERENCE INPUT SECTION.
4
. A 10µF CERAMIC CAPACITOR (X5R, 1206 SIZE) IS RECOMMENDED (FOR EXAMPLE, PANASONIC ECJ3YB0J106M).
SEE VOLTAGE REFERENCE INPUT SECTION.
5. OPTION, SEE POWER SUPPLY SECTION.
6. OPTION, SEE POWER-UP SECTION.
7. OPTIONAL LOW JITTER CNVST, SEE CONVERSION CONTROL SECTION.
06024-023
Figure 23. Typical Connection Diagram