AD7865
AD7865-1
AD7865-2
Figure 2 shows the analog input section of the AD7865-1. Each
input can be configured for
±
5 V or
±10
V operation on the
AD7865-1. For
±
5 V operation, the V
INxA
and V
INxB
inputs are
tied together and the input voltage is applied to both. For
±
10 V
operation, the V
INxB
input is tied to AGND and the input volt-
age is applied to the V
INxA
input. The V
INxA
and V
INxB
inputs are
symmetrical and fully interchangeable. Thus for ease of 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.
+2.5V
REFERENCE
6k
V
REF
R1
R2
V
INxA
R3
V
INxB
R4
GND
TRACK/
HOLD
TO INTERNAL
COMPARATOR
TO ADC
REFERENCE
CIRCUITRY
Figure 3 shows the analog input section of the AD7865-2. Each
input can be configured for 0 V to +5 V operation or 0 V to +2.5 V
operation. For the 0 V to +5 V operation, the V
INxB
input is tied
to AGND and the input voltage is applied to 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.
For the AD7865-2, R1 = 4 kΩ and R2 = 4 kΩ. Once again, the
designed code transitions occur on successive integer LSB val-
ues. Output coding is straight (natural) binary with 1 LSB =
FSR/16384 = +2.5 V/16384 = 0.153 mV, and +5 V/16384 =
0.305 mV, for 0 V to +2.5 V and 0 V to +5 V options respec-
tively. Table II shows the ideal input and output transfer
function for the AD7865-2.
+2.5V
REFERENCE
6k
V
REF
R1
V
INxA
R2
V
INxB
TRACK/
HOLD
TO ADC
REFERENCE
CIRCUITRY
TO INTERNAL
COMPARATOR
AD7865-1
AD7865-2
Figure 2. AD7865-1 Analog Input Structure
For the AD7865-1, R1 = 4 kΩ, R2 = 16 kΩ, R3 = 16 kΩ and
R4 = 8 kΩ. The resistor input stage is followed by the high
input impedance stage of the track/hold amplifier.
The designed code transitions take place midway between suc-
cessive integer LSB values (i.e., 1/2 LSB, 3/2 LSBs, 5/2 LSBs
etc.) LSB size is given by the formula, 1 LSB = FSR/16384. For
the
±
5 V range, 1 LSB = 10 V/16384 = 610.4
µV.
For the
±
10 V
range, 1 LSB = 20 V/16384 = 1.22 mV. Output coding is twos
complement binary with 1 LSB = FSR/16384. The ideal input/
output transfer function for the AD7865-1 is shown in Table I.
Table I. Ideal Input/Output Code Table for the AD7865-1
Figure 3. AD7865-2 Analog Input Structure
Table II. Ideal Input/Output Code Table for the AD7865-2
Analog Input
1
+FSR/2 – 3/2 LSB
2
+FSR/2 – 5/2 LSB
+FSR/2 – 7/2 LSB
AGND + 5/2 LSB
AGND + 3/2 LSB
AGND – 1/2 LSB
Digital Output Code Transition
111 . . . 110 to 111 . . . 111
111 . . . 101 to 111 . . . 110
111 . . . 100 to 111 . . . 101
000 . . . 010 to 000 . . . 011
000 . . . 001 to 000 . . . 010
000 . . . 000 to 000 . . . 001
Analog Input
1
+FSR/2 – 3/2 LSB
2
+FSR/2 – 5/2 LSB
+FSR/2 – 7/2 LSB
AGND + 3/2 LSB
AGND + 1/2 LSB
AGND – 1/2 LSB
AGND – 3/2 LSB
–FSR/2 + 5/2 LSB
–FSR/2 + 3/2 LSB
–FSR/2 + 1/2 LSB
Digital Output Code Transition
011 . . . 110 to 011 . . . 111
011 . . . 101 to 011 . . . 110
011 . . . 100 to 011 . . . 101
000 . . . 001 to 000 . . . 010
000 . . . 000 to 000 . . . 001
111 . . . 111 to 000 . . . 000
111 . . . 110 to 111 . . . 111
100 . . . 010 to 100 . . . 011
100 . . . 001 to 100 . . . 010
100 . . . 000 to 100 . . . 001
NOTES
1
FSR is full-scale range and is 0 V to +2.5 V and 0 V to +5 V for AD7865-2
with V
REF
= +2.5 V.
2
1 LSB = FSR/16384 and is 0.153 mV (0 V to +2.5 V) and 0.305 mV (0 V to +5 V)
for AD7865-2) with V
REF
= +2.5 V.
NOTES
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/16384 = 1.22 mV (±10 V—AD7865-1) and 610.4 mV (± 5 V—
AD7865-1) with V
REF
= +2.5 V.
–10–
REV. A
AD [ ANALOG DEVICES ]
