REV. 0

AD7866

–11–

from the part will be straight binary (for the next conversion). If this

pin is at a logic high when

CS goes low, then the analog input

range will be 2

× V

be two’s complement. However, if after the falling edge of

CS,

the logic level of the RANGE pin has changed upon the eighth

falling SCLK edge, point B, the output coding will change to the

other option without any change in the analog input range. So for

the next conversion, two’s complement output coding could be selected

is low upon the falling edge of

CS and high upon the eighth falling

SCLK edge, as shown in Figure 7. Figures 5 through 8 show

examples of timing diagrams when selecting a particular analog input

range with a particular output coding format. Table I also summarizes

the required logic level of the RANGE pin for each selection.

The Logic Input A0 is used to select the pair of channels to be

converted simultaneously. The Logic state of this pin is also

checked upon the falling edge of

CS and the multiplexers are set

up for the next conversion. If it is low, the following conversion

STRAIGHT BINARY

0V TO V

REF

INPUT RANGE

CS

SCLK

RANGE

1

8

16

16

1

AB

CS

SCLK

RANGE

1

8

16

16

1

AB

V

REF

V

REF

INPUT RANGE

TWO’S COMPLEMENT

will be performed on Channel 1 of each ADC; if it is high,

the following conversion will be performed on Channel 2 of

each ADC.

Handling Bipolar Input Signals

Figure 9 shows how useful the combination of the 2

× V

range and the two’s complement output coding scheme is for

handling bipolar input signals. If the bipolar input signal is biased

range of 2

× V

Transfer Functions

The designed code transitions occur at successive integer LSB

The ideal transfer characteristic for the AD7866 when straight

binary coding is selected is shown in Figure 10 and the ideal

transfer characteristic for the AD7866 when two’s complement

coding is selected is shown in Figure 11.

Table I. Analog Input and Output Coding Selection

Range Level

Range Level

@ Point A

1

@ Point B

2

Input Range

3

Output Coding

3

Low

Low

Straight Binary

High

High

Two’s Complement

Low

High

Two’s Complement

High

Low

0 V to 2

× V

REF

Straight Binary

NOTES

CS.