AD7891

–13–

REV. A

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

Input Voltage

Twos Complement

Straight Binary

(9.99268 V,

4.99634 V

or

011...110 to 011...111

111...110 to 111...111

+FSR/2 – 5/2 LSBs

(9.98779 V,

4.99390 V

or

2.49695 V)

011...101 to 011...110

111...101 to 111...110

+FSR/2 – 7/2 LSBs

(9.99145 V,

4.99146 V

or

2.49573 V)

011...100 to 011...101

111...100 to 111...101

AGND + 3/2 LSBs

(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 LSBs

(–7.3242 mV, –3.6621 mV or

–1.8310 mV)

111...110 to 111...111

011...110 to 011...111

–FSR/2 + 5/2 LSBs

(–9.98779 V, –4.99390 V or

–2.49695 V)

100...010 to 100...011

000...010 to 000...011

–FSR/2 + 3/2 LSBs

(–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

±10 V range, 10 V for the ±5 V range and 5 V for the ± 2.5 V range, with REFIN = +2.5 V .

±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

Input Voltage

Twos Complement

Straight Binary

+FSR

(4.99817 V

or 2.49908 V)

4

011...110 to 011...111

111...110 to 111...111

+FSR – 5/2 LSBs

(4.99695 V

or 2.49847 V)

011...101 to 011...110

111...101 to 111...110

+FSR – 7/2 LSBs

(4.99573 V

or 2.49786 V)

011...100 to 011...101

111...100 to 111...101

AGND + 5/2 LSBs

(3.0518 mV

or 1.52588 mV)

100...010 to 000...011

000...010 to 000...011

AGND + 3/2 LSBs

(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

µV (0 to 2.5 V range), with REF IN = 2.5 V.

Transfer Function of the AD7891-1 and AD7891-2

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.

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