Datasheet

AD7661
Rev. 0 | Page 17 of 28
Transfer Functions
Table 7. Output Codes and Ideal Input Voltages
Digital Output Code (Hex)
Description
Analog
Input
Straight
Binary
Twos
Complement
FSR –1 LSB 2.499962 V FFFF
1
7FFF1
FSR – 2 LSB 2.499923 V FFFE 7FFE
Midscale + 1 LSB 1.250038 V 8001 0001
Midscale 1.25 V 8000 0000
Midscale – 1 LSB 1.249962 V 7FFF FFFF
–FSR + 1 LSB 38 µV 0001 8001
–FSR 0 V 0000
2
8000
2
Using the OB/
2C
digital input, the AD7661 offers two output
codings: straight binary and twos complement. The LSB size is
V
REF
/65536, which is about 38.15 µV. The AD7661’s ideal
transfer characteristic is shown in Figure 25 and Table 7.
000...000
000...001
000...010
111...101
111...110
111...111
ADC CODE (Straight Binary)
ANALOG INPUT
V
REF
– 1.5 LSB
V
REF
– 1 LSB
1LSB0V
0.5 LSB
1 LSB =V
REF
/65536
03033-0-021
1
This is also the code for overrange analog input (V
IN
– V
INGND
above
V
REF
– V
REFGND
).
2
This is also the code for underrange analog input (V
IN
below V
INGND
).
Figure 25. ADC Ideal Transfer Function
NOTES
1
THE CONFIGURATION SHOWN IS USING THE INTERNAL REFERENCE AND INTERNAL BUFFER.
2
THE AD8021 IS RECOMMENDED. SEE DRIVER AMPLIFIER CHOICE SECTION.
3
OPTIONAL LOW JITTER.
4
A 10 µ F CERAMIC CAPACITOR (X5R, 1206 SIZE) IS RECOMMENDED (e.g., PANASONIC ECJ3YB0J106M).
SEE VOLTAGE REFERENCE INPUT SECTION.
AD7661
D
3
CLOCK
µC/µP/DSP
SERIAL
PORT
DIGITAL SUPPLY
(3.3V OR 5V)
DVDD
100nF
+
10
µ F
100nF
+
10
µ F
20
Ω
100nF
+
10
µ F
ANALOG
SUPPLY
(5V)
C
C
A
NALOG INPU
T
(0VTO 2.5V)
PD RESET
SER/PAR
OB/2C
BUSY
SDOUT
SCLK
INGND
IN
REFGND
REF
AGND
AVDD DGND DVDD OVDD OGND
03033-0-022
U1
2
PDREF
PDBUF
RD
CS
CNVST
REFBUFIN
1
100nF
BYTESWAP
C
R
4
Figure 26. Typical Connection Diagram