Datasheet

REV. A
AD7899
–9–
AD7899-1
V
INA
TRACK/HOLD
TO ADC
REFERENCE
CIRCUITRY
TO INTERNAL
COMPARATOR
R4
R1
R2
6k
2.5V
REFERENCE
R3
GND
V
INB
V
REF
Figure 2. AD7899-1 Analog Input Structure
For the AD7899-1, R1 = 4 k, R2 = 16 k, R3 = 16 k and
R4 = 8 k. 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 AD7899-1 is shown in
Table I.
Table I. Ideal Input/Output Code Table for the AD7899-1
Digital Output
Analog Input
1
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
GND + 3/2 LSB 000 . . . 001 to 000 . . . 010
GND + 1/2 LSB 000 . . . 000 to 000 . . . 001
GND 1/2 LSB 111 . . . 111 to 000 . . . 000
GND 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 AD7899-1) and 610.4 µV (± 5 V
AD7899-1) with V
REF
= 2.5 V.
AD7899-2
Figure 3 shows the analog input section of the AD7899-2. 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
INB
input is tied to
GND and the input voltage is applied to the V
INA
input. For
0 V to 2.5 V operation, the V
INA
and V
INB
inputs are tied together
and the input voltage is applied to both. The V
INA
and V
INB
inputs are symmetrical and fully interchangeable.
For the AD7899-2, R1 = 4 k and R2 = 4 k. 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 the 0 to 2.5 V and the 0 to 5 V options respectively. Table
II shows the ideal input and output transfer function for the
AD7899-2.
AD7899-2
V
INA
TRACK/HOLD
TO ADC
REFERENCE
CIRCUITRY
TO INTERNAL
COMPARATOR
R1
6k
2.5V
REFERENCE
R2
V
INB
V
REF
Figure 3. AD7899-2 Analog Input Structure
Table II. Ideal Input/Output Code Table for the AD7899-2
Digital Output
Analog Input
1
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
GND + 5/2 LSB 000 . . . 010 to 000 . . . 011
GND + 3/2 LSB 000 . . . 001 to 000 . . . 010
GND + 1/2 LSB 000 . . . 000 to 000 . . . 001
NOTES
1
FSR is Full-Scale Range and is 0 to 2.5 V and 0 to 5 V for AD7899-2 with V
REF
= 2.5 V.
2
1 LSB = FSR/16384 and is 0.153 mV (0 to 2.5 V) and 0.305 mV (0 to 5 V) for
AD7899-2 with V
REF
= 2.5 V.