AD7891
Table I. Ideal Code Transition Table for the AD7891-1,
10 V and
5 V Ranges and the AD7891-2,
2.5 V Range
Analog Input
+FSR
2
/2 – 3/2 LSBs
3
+FSR/2 – 5/2 LSBs
+FSR/2 – 7/2 LSBs
AGND + 3/2 LSBs
AGND + 1/2 LSB
AGND – 1/2 LSB
AGND – 3/2 LSBs
–FSR/2 + 5/2 LSBs
–FSR/2 + 3/2 LSBs
–FSR/2 + 1/2 LSB
Input Voltage
(9.99268 V,
(9.98779 V,
(9.99145 V,
(7.3242 mV,
(2.4414 mV,
(–2.4414 mV,
(–7.3242 mV,
4.99634 V
4.99390 V
4.99146 V
3.6621 mV
1.2207 mV
–1.2207 mV
–3.6621 mV
or
or
or
or
or
or
or
2.49817 V)
4
2.49695 V)
2.49573 V)
1.8310 mV)
0.6103 mV)
–0.6103 mV)
–1.8310 mV)
–2.49695 V)
–2.49817 V)
–2.49939 V)
Digital Output Code Transition
1
Twos Complement
Straight Binary
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
111...110 to 111...111
111...101 to 111...110
111...100 to 111...101
100...001 to 100...010
100...000 to 100...001
011...111 to 100...000
011...110 to 011...111
000...010 to 000...011
000...001 to 000...010
000...000 to 000...001
(–9.98779 V, –4.99390 V or
(–9.99268 V, –4.99634 V or
(–9.99756 V, –4.99878 V or
NOTES
1
Output Code format is determined by the FORMAT bit in the control register
2
FSR 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 REFIN = +2.5 V .
3
1 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
Analog Input
+FSR
2
– 3/2 LSBs
3
+FSR – 5/2 LSBs
+FSR – 7/2 LSBs
AGND + 5/2 LSBs
AGND + 3/2 LSBs
AGND + 1/2 LSB
Input Voltage
(4.99817 V
(4.99695 V
(4.99573 V
or 2.49908 V)
4
or 2.49847 V)
or 2.49786 V)
Digital Output Code Transition
1
Twos Complement
Straight Binary
011...110 to 011...111
011...101 to 011...110
011...100 to 011...101
100...010 to 000...011
100...001 to 000...010
100...000 to 000...001
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
(3.0518 mV or 1.52588 mV)
(1.83105 mV or 0.9155 mV)
(0.6103 mV or 0.3052 mV)
NOTES
1
Output Code format is determined by the FORMAT bit in the control register
2
FSR is 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.
3
1 LSB = FS/4096 = 1.22 mV (0 to 5 V range) or 610
µV
(0 to 2.5 V range), with REF IN = 2.5 V.
4
0 V to +5 V range or 0 V to + 2.5 V range.
Transfer Function of the AD7891-1 and AD7891-2
Table III. Transfer Function M and N Values
The transfer function of the AD7891-1 and AD7891-2 can be
expressed as follows:
Input Voltage = (M
×
REFIN
×
D/4096) + (N
×
REFIN)
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 volt-
age range and the output data format. These values are given in
Table III. REFIN is the reference voltage applied to the AD7891.
Range
AD7891-1
±
10 V
±
10 V
±
5 V
±
5 V
AD7891-2
0 V to +5 V
0 V to +5 V
0 V to +2.5 V
0 V to +2.5 V
±
2.5 V
±
2.5 V
Output Data Format
Straight Binary
Twos Complement
Straight Binary
Twos Complement
Straight Binary
Twos Complement
Straight Binary
Twos Complement
Straight Binary
Twos Complement
M
8
8
4
4
2
2
1
1
2
2
N
–4
0
–2
0
0
1
0
0.5
–1
0
REV. A
–13–
AD [ ANALOG DEVICES ]
