Datasheet

AD7864
Rev. D | Page 13 of 28
CIRCUIT DESCRIPTION
ANALOG INPUT
Table 5. Ideal Input/Output Code Table for the AD7864-1
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
The AD7864 is offered in three models: the AD7864-1, where
each input can be configured for ±10 V or a ±5 V input voltage
range; the AD7864-3, which handles the input voltage range of
±2.5 V; and the AD7864-2, where each input can be configured
to have a 0 V to +2.5 V or 0 V to +5 V input voltage range.
AD7864-1
Figure 4 shows the analog input section of the AD7864-1. Each
input can be configured for ±5 V or ±10 V operation on the
AD7864-1. For ±5 V (AD7864-1) operation, the V
INxA
and V
INxB
inputs are tied together and the input voltage is applied to both.
For ±10 V (AD7864-1) operation, the V
INxB
input is tied to AGND
and the input voltage is applied to the V
INxA
input. The V
INxA
and
V
INxB
inputs are symmetrical and fully interchangeable. Thus for
ease of printed circuit board (PCB) layout on the ±10 V range,
the input voltage may be applied to the V
INxB
input while the
V
INxA
input is tied to AGND.
1
FSR is full-scale range and is 20 V for the ±10 V range and +10 V for the ±5 V
range, with V
REF
= 2.5 V.
2
1 LSB = FSR/4096 = 4.883 mV (±10 V for the AD7864-1) and 2.441 mV (±5 V
for the AD7864-1) with V
REF
= 2.5 V.
AD7864-2
Figure 5 shows the analog input section of the AD7864-2. Each
input can be configured for 0 V to 5 V operation or 0 V to 2.5 V
operation. For 0 V to 5 V operation, the V
INxB
input is tied to
AGND and the input voltage is applied to the V
INxA
input. For
0 V to 2.5 V operation, the V
INxA
and V
INxB
inputs are tied together
and the input voltage is applied to both. The V
INxA
and V
INxB
inputs are symmetrical and fully interchangeable. Thus for ease
of PCB layout on the 0 V to 5 V range, the input voltage may be
applied to the V
INxB
input while the V
INxA
input is tied to AGND.
2.5V
REFERENCE
T/H
TO ADC
REFERENCE
CIRCUITRY
6kΩ
R2
R3
TO INTERNAL
COMPARATOR
AD7864-1
R1
R4
AGND
V
IN1B
V
IN1A
V
REF
01341-004
For the AD7864-2, R1 = 6 kΩ and R2 = 6 kΩ. The designed
code transitions occur on successive integer least significant bit
values. Output coding is straight (natural) binary with 1 LSB =
FSR/4096 = 2.5 V/4096 = 0.61 mV, and 5 V/4096 = 1.22 mV, for
the 0 V to 2.5 V and 0 V to 5 V options, respectively.
Figure 4. AD7864-1 Analog Input Structure
Table 6 shows the ideal input and output transfer function for
the AD7864-2.
For the AD7864-1, R1 = 6 kΩ, R2 = 24 kΩ, R3 = 24 kΩ, and
R4 = 12 kΩ. The resistor input stage is followed by the high
input impedance stage of the track-and-hold amplifier.
2.5V
REFERENCE
T/H
TO ADC
REFERENCE
CIRCUITRY
R2
TO INTERNAL
COMPARATOR
AD7864-2
R1
6kΩ
V
IN1B
V
IN1A
V
REF
01341-005
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 forth). Least significant bit size is given
by the formula 1 LSB = FSR/4096. For the ±5 V range, 1 LSB =
10 V/4096 = 2.44 mV. For the ±10 V range, 1 LSB = 20 V/4096 =
4.88 mV. Output coding is twos complement binary with 1 LSB =
FSR/4096. The ideal input/output transfer function for the
AD7864-1 is shown in Table 5.
Figure 5. AD7864-2 Analog Input Structure