Datasheet

AD7892
REV. C
–11–
The designed code transitions occur midway between successive
integer LSB values (i.e., 1/2 LSB, 3/2 LSBs, 5/2 LSBs). Output
coding is two’s complement binary with 1 LSB = FSR/4096 =
20 V/4096 = 4.88 mV for the ±10 V range and 1 LSB = FSR/
4096 = 10 V/4096 = 2.44 mV for the ± 5 V range. The ideal
input/output transfer function for the AD7892-1 is shown in
Table I.
AD7892-2
The analog input section for the AD7892-2 contains no biasing
resistors. The analog input looks directly into the track/hold
input stage. The analog input range on the V
IN1
input is 0 V to
+2.5 V. The V
IN2
input can be left unconnected but if it is
connected to a potential then that potential must be AGND.
The V
IN1
input connects directly to the input sampling capacitor
of the AD7892-2’s track/hold. The value of this input sampling
capacitor is nominally 10 pF.
Once again, the designed code transitions occur midway be-
tween successive integer LSB values (i.e., 1/2 LSB, 3/2 LSBs,
5/2 LSBs). Output coding is straight (natural) binary with
1 LSB = FSR/4096 = 2.5 V/4096 = 0.61 mV. The ideal input/
output transfer function for the AD7892-2 is shown in Table II.
Table I. Ideal Input/Output Code Table for the AD7892-1
Digital Output
Analog Input Code Transition
+FSR/2 – 3/2 LSB
1, 2
(9.99268 or 4.99634)
3
011 . . . 110 to 011 . . . 111
+FSR/2 – 5/2 LSBs (9.98779 or 4.99390) 011 . . . 101 to 011 . . . 110
+FSR/2 – 7/2 LSBs (9.98291 or 4.99146) 011 . . . 100 to 011 . . . 101
AGND + 3/2 LSB (0.00732 or 0.00366) 000 . . . 001 to 000 . . . 010
AGND + 1/2 LSB (0.00244 or 0.00122) 000 . . . 000 to 000 . . . 001
AGND – 1/2 LSB (–0.00244 or –0.00122) 111 . . . 111 to 000 . . . 000
AGND – 3/2 LSB (–0.00732 or –0.00366) 111 . . . 110 to 111 . . . 111
–FSR/2 + 5/2 LSB (–9.98779 or –4.99390) 100 . . . 010 to 100 . . . 011
–FSR/2 + 3/2 LSB (–9.99268 or –4.99634) 100 . . . 001 to 100 . . . 010
–FSR/2 + 1/2 LSB (–9.99756 or –4.99878) 100 . . . 000 to 100 . . . 001
NOTES
1
FSR is full-scale range and REF IN = +2.5 V, is 20 V for the ± 10 V range and 10 V
for the ± 5 V range.
2
1 LSB = FSR/4096 = 4.88 mV (± 10 V range) and 2.44 mV (± 5 V range) with REF
IN = +2.5 V.
3
± 10 V range or ± 5 V range.
Table II. Ideal Input/Output Code Table for the AD7892-2
Digital Output
Analog Input Code Transition
+FSR – 3/2 LSB
1, 2
(2.499084 V) 111 . . . 110 to 111 . . . 111
+FSR – 5/2 LSBs (2.498474 V) 111 . . . 110 to 111 . . . 110
+FSR – 7/2 LSBs (2.497864 V) 111 . . . 100 to 111 . . . 101
AGND + 5/2 LSB (0.001526 V) 000 . . . 010 to 010 . . . 011
AGND + 3/2 LSB (0.00916 V) 000 . . . 001 to 001 . . . 010
AGND + 1/2 LSB (0.000305 V) 000 . . . 000 to 000 . . . 001
NOTES
1
FSR is full-scale range and is 2.5 V with REF IN = +2.5 V.
2
1 LSB = FSR/4096 = 0.61 mV with REF IN = +2.5 V.
AD7892-3
Figure 5 shows the analog input section for the AD7892-3. The
analog input range is ±2.5 V on the V
IN1
input. The V
IN2
input
can be left unconnected but if it is connected to a potential then
that potential must be AGND. The input resistance on the V
IN1
is 1.8 kΩ nominal. As a result, the V
IN1
input should be driven
from a low impedance source. The resistor attenuator stage is
followed by the high input impedance stage of the track/hold
amplifier. This resistor attenuator stage allows the input voltage
to go to ± 7 V without damaging the AD7892-3.
The designed code transitions occur midway between succes-
sive integer LSB values (i.e., 1/2 LSB, 3/2 LSBs, 5/2 LSBs).
Output coding is two’s complement binary with 1 LSB = FSR/
4096 = 5 V/4096 = 1.22 mV with REF IN = +2.5 V. The ideal
input/output transfer function for the AD7892-3 is shown in
Table III.
+2.5
REFERENCE
REF OUT/
REF IN
V
IN1
V
IN2
*
AGND
TO HIGH
IMPEDANCE
SHA INPUT
TO ADC
REFERENCE
CIRCUITRY
2k⍀
3.25k⍀
3.25k⍀
* UNCONNECTED INTERNALLY ON THE AD7892-3
Figure 5. AD7892-3 Analog Input Structure
Table III. Ideal Input/Output Code Table for the AD7892-3
Digital Output
Analog Input Code Transition
+FSR/2 – 3/2 LSB
1, 2
(2.49817) 011 . . . 110 to 011 . . . 111
+FSR/2 – 5/2 LSBs (2.49695) 011 . . . 110 to 011 . . . 110
+FSR/2 – 7/2 LSBs (2.49573) 011 . . . 110 to 011 . . . 101
AGND + 3/2 LSB (0.00183) 000 . . . 001 to 000 . . . 010
AGND + 1/2 LSB (0.00061) 000 . . . 000 to 000 . . . 001
AGND – 1/2 LSB (–0.00061) 111 . . . 111 to 000 . . . 000
AGND – 3/2 LSB (–0.00183) 111 . . . 110 to 111 . . . 111
–FSR/2 + 5/2 LSB (–2.49695) 100 . . . 010 to 100 . . . 011
–FSR/2 + 3/2 LSB (–2.49817) 100 . . . 001 to 100 . . . 010
–FSR/2 + 1/2 LSB (–2.49939) 100 . . . 000 to 100 . . . 001
NOTES
1
FSR is full-scale range and is 5 V with REF IN = +2.5 V.
2
1 LSB = FSR/4096 = 1.22 mV with REF IN = +2.5 V.