AD7866 datasheet(13/25 Pages) AD

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REV. A

–12–

AD7866

Analog Input Ranges

The analog input range for the AD7866 can be selected to be 0 V

to VREF or 2

VREF with either straight binary or twos complement

output coding. The RANGE pin is used to select both the analog

input range and the output coding, as shown in Figures 5 to 8.

On the falling edge of

CS, point A, the logic level of the RANGE

pin is checked to determine the analog input range of the next

conversion. If this pin is tied to a logic low, the analog input

range will be 0 V to VREF and the output coding from the part will

be straight binary (for the next conversion). If this pin is at a logic

high when

CS goes low, the analog input range will be 2

VREF and

the output coding for the part will be twos complement. How-

ever, 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,

twos complement output coding could be selected with a 0 V to

VREF input range, for example, if the RANGE pin is low upon

the falling edge of

CS and high upon the eighth falling SCLK

edge, as shown in Figure 7. Figures 5 to 8 show examples of

timing diagrams for selections of different analog input ranges

with various output coding formats. Table I summarizes the

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

that the analog input range selected must not exceed VDD. 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 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

VREF

input range and the twos complement output coding scheme is

for handling bipolar input signals. If the bipolar input signal

is biased about VREF and twos complement output coding is

selected, then VREF becomes the zero code point, –VREF is

negative full-scale, and +VREF becomes positive full-scale with a

dynamic range of 2

VREF.

Transfer Functions

The designed code transitions occur at successive integer LSB

values (i.e., 1 LSB, 2 LSB, and so on). The LSB size is VREF/4096.

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 twos complement

coding is selected is shown in Figure 11.

Table I. Analog Input and Output Coding Selection

Range Level

@ Point A

@ Point B

Input Range

Output Coding

Low

0 V to VREF

Straight Binary

High

VREF

± V

REF

Twos Complement

Low

High

VREF /2

± VREF /2

Twos Complement

High

Low

0 V to 2

VREF

Straight Binary

NOTES

1Point A = Falling edge of

CS.

2Point B = Eighth falling edge of SCLK.

3Selected for next conversion.

STRAIGHT BINARY

0V TO V

REF

INPUT RANGE

SCLK

RANGE

DOUTA

DOUTB

Figure 5. Selecting 0 V to VREF Input Range with Straight Binary Output Coding

SCLK

RANGE

DOUTA

DOUTB

REF

INPUT RANGE

TWOS COMPLEMENT

Figure 6. Selecting VREF

± VREF Input Range with Twos Complement Output Coding

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