AD7891
–13–
REV. D
Table I. Ideal Code Transition Table for the AD7891-1,
10 V and
5 V Ranges and the AD7891-2,
2.5 V Range
Digital Output Code Transition
1
Analog Input
Input Voltage
Twos Complement
Straight Binary
+FSR
2/2 – 3/2 LSB3
(9.99268 V, 4.99634 V or 2.49817 V)
4
011...110 to 011...111
111...110 to 111...111
+FSR/2 – 5/2 LSB
(9.98779 V, 4.99390 V or 2.49695 V)
011...101 to 011...110
111...101 to 111...110
+FSR/2 – 7/2 LSB
(9.99145 V, 4.99146 V or 2.49573 V)
011...100 to 011...101
111...100 to 111...101
AGND + 3/2 LSB
(7.3242 mV, 3.6621 mV or 1.8310 mV)
000...001 to 000...010
100...001 to 100...010
AGND + 1/2 LSB
(2.4414 mV, 1.2207 mV or 0.6103 mV)
000...000 to 000...001
100...000 to 100...001
AGND – 1/2 LSB
(–2.4414 mV, –1.2207 mV or –0.6103 mV)
111...111 to 000...000
011...111 to 100...000
AGND – 3/2 LSB
(–7.3242 mV, –3.6621 mV or –1.8310 mV)
111...110 to 111...111
011...110 to 011...111
–FSR/2 + 5/2 LSB
(–9.98779 V, –4.99390 V or –2.49695 V)
100...010 to 100...011
000...010 to 000...011
–FSR/2 + 3/2 LSB
(–9.99268 V, –4.99634 V or –2.49817 V)
100...001 to 100...010
000...001 to 000...010
–FSR/2 + 1/2 LSB
(–9.99756 V, –4.99878 V or –2.49939 V)
100...000 to 100...001
000...000 to 000...001
NOTES
1Output code format is determined by the FORMAT bit in the control register.
2FSR is full-scale range and is +20 V for the
±10 V range, +10 V for the ±5 V range, and +5 V for the ±2.5 V range, with REF IN = +2.5 V.
31 LSB = FSR/4096 = +4.88 mV (
±10 V range), +2.44 mV (±5 V range), and +1.22 mV (±2.5 V range), with REF IN = +2.5 V.
4
±10 V range, ±5 V range, or ±2.5 V range.
Table II. Ideal Code Transition Table for the AD7891-2, 0 V to 5 V and 0 V to 2.5 V Ranges
Digital Output Code Transition
1
Analog Input
Input Voltage
Twos Complement
Straight Binary
+FSR
2 – 3/2 LSB3
(4.99817 V or 2.49908 V)
4
011...110 to 011...111
111...110 to 111...111
+FSR – 5/2 LSB
(4.99695 V or 2.49847 V)
011...101 to 011...110
111...101 to 111...110
+FSR – 7/2 LSB
(4.99573 V or 2.49786 V)
011...100 to 011...101
111...100 to 111...101
AGND + 5/2 LSB
(3.0518 mV or 1.52588 mV)
100...010 to 000...011
000...010 to 000...011
AGND + 3/2 LSB
(1.83105 mV or 0.9155 mV)
100...001 to 000...010
000...001 to 000...010
AGND + 1/2 LSB
(0.6103 mV or 0.3052 mV)
100...000 to 000...001
000...000 to 000...001
NOTES
1Output code format is determined by the FORMAT bit in the control register.
2FSR is the full-scale range and is 5 V for the 0 to 5 V range and 2.5 V for the 0 to 2.5 V range, with REF IN = 2.5 V.
31 LSB = F
S/4096 = 1.22 mV (0 to 5 V range) or 610
mV (0 to 2.5 V range), with REF IN = 2.5 V.
40 V to 5 V range or 0 V to 2.5 V range.
Table III. Transfer Function M and N Values
Range
Output Data Format
M
N
AD7891-1
±10 V
Straight Binary
8
–4
±10 V
Twos Complement
8
0
±5 V
Straight Binary
4
–2
±5 V
Twos Complement
4
0
AD7891-2
0 V to +5 V
Straight Binary
2
0
0 V to +5 V
Twos Complement
2
1
0 V to +2.5 V
Straight Binary
1
0
0 V to +2.5 V
Twos Complement
1
0.5
±2.5 V
Straight Binary
2
–1
±2.5 V
Twos Complement
2
0
Transfer Function of the AD7891-1 and AD7891-2
The transfer function of the AD7891-1 and AD7891-2 can be
expressed as
Input Voltage
M
REF IN
D
N
REF IN
=
() +
()
/4096
D is the output data from the AD7891 and is in the range 0 to
4095 for straight binary encoding and from –2048 to +2047 for
twos complement encoding. Values for M depend upon the
input voltage range. Values for N depend upon the input voltage
range and the output data format. These values are given in
Table III. REF IN is the reference voltage applied to the AD7891.