Datasheet

REV. B
AD7865
–10–
CIRCUIT DESCRIPTION
Analog Input Section
The AD7865 is offered as three part types, the AD7865-1 where
each input can be configured for ± 10 V or a ±5 V input voltage
range, the AD7865-3 which handles input voltage range ± 2.5 V
and the AD7865-2 which has an input voltage range of 0 V to
2.5 V or 0 V to 5 V. The amount of current flowing into the
analog input will depend on the analog input range and the analog
input voltage. The maximum current flows when negative full
scale is applied.
AD7865-1
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.
AD7865-1
V
INxA
TRACK/
HOLD
TO ADC
REFERENCE
CIRCUITRY
TO INTERNAL
COMPARATOR
R4
R1
R2
6k
2.5V
REFERENCE
R3
GND
V
INxB
V
REF
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
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
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.
AD7865-2
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
values. 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 respectively.
Table II shows the ideal input and output transfer function
for the AD7865-2.
AD7865-2
V
INxA
TRACK/
HOLD
TO ADC
REFERENCE
CIRCUITRY
TO INTERNAL
COMPARATOR
R1
6k
2.5V
REFERENCE
R2
V
INxB
V
REF
Figure 3. AD7865-2 Analog Input Structure
Table II. Ideal Input/Output Code Table for the AD7865-2
Analog Input
1
Digital Output Code Transition
+FSR/2 – 3/2 LSB
2
111 . . . 110 to 111 . . . 111
+FSR/2 – 5/2 LSB 111 . . . 101 to 111 . . . 110
+FSR/2 – 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
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.