Datasheet
AD7864
Rev. D | Page 14 of 28
Table 6. Ideal Input/Output Code Table for the AD7864-2
Analog Input
1
Digital Output Code Transition
+FSR − 3/2 LSB
2
111...110 to 111...111
+FSR − 5/2 LSB 111...101 to 111...110
+FSR − 7/2 LSB 111...100 to 111...101
AGND + 5/2 LSB 000...010 to 000...011
AGND + 3/2 LSB 000...001 to 000...010
AGND + 1/2 LSB 000...000 to 000...001
1
FSR is the full-scale range and is 0 V to 2.5 V and 0 V to 5 V for the AD7864-2
with V
REF
= 2.5 V.
2
1 LSB = FSR/4096 and is 0.61 mV (0 V to 2.5 V) and 1.22 mV (0 V to 5 V) for the
AD7864-2 with V
REF
= 2.5 V.
AD7864-3
Figure 6 shows the analog input section of the AD7864-3. The
analog input range is ±2.5 V on the V
IN1A
input. The V
IN1B
input
can be left unconnected, but if it is connected to a potential,
that potential must be AGND.
T/H
R2
AD7864-3
R1
2.5V
REFERENCE
TO ADC
REFERENCE
CIRCUITRY
TO INTERNAL
COMPARATOR
6kΩ
V
IN1B
V
IN1A
V
REF
01341-006
Figure 6. AD7864-3 Analog Input Structure
For the AD7864-3, R1 = 6 kΩ and R2 = 6 kΩ. As a result, drive
the V
IN1A
input from a low impedance source. The resistor input
stage is followed by the high input impedance stage of the track-
and-hold amplifier.
The designed code transitions take place midway between
successive integer least significant bit values (that is, 1/2 LSB,
3/2 LSB, 5/2 LSB, and so on). Least significant bit size is given by
the formula 1 LSB = FSR/4096. Output coding is twos comple-
ment binary with 1 LSB = FSR/4096 = 5 V/4096 = 1.22 mV. The
ideal input/ output transfer function for the AD7864-3 is shown
in Table 7.
Table 7. Ideal Input/Output Code Table for the AD7864-3
Analog Input
1
Digital Output Code Transition
+FSR/2 − 3/2 LSB
2
011...110 to 011...111
+FSR/2 − 5/2 LSB 011...101 to 011...110
+FSR/2 − 7/2 LSB 011...100 to 011...101
AGND + 3/2 LSB 000...001 to 000...010
AGND + 1/2 LSB 000...000 to 000...001
AGND − 1/2 LSB 111...111 to 000...000
AGND − 3/2 LSB 111...110 to 111...111
−FSR/2 + 5/2 LSB 100...010 to 100...011
−FSR/2 + 3/2 LSB 100...001 to 100...010
−FSR/2 + 1/2 LSB 100...000 to 100...001
1
FSR is the full-scale range and is 5 V, with V
REF
= 2.5 V.
2
1 LSB = FSR/4096 = 1.22 mV (±2.5 V − AD7864-3) with V
REF
= 2.5 V.