SP9840 datasheet(6/10 Pages) SIPEX

6 / 10 page

262

LAST

LSB

DATA

ADDRESS

MSB

DAC Output Voltage

VOUT =

–2VREFL

LSB

MSB

DAC Updated

FIRST

No Operation

DACA

DACB

DACC

DACD

DACE

DACF

DACG

DACH

No Operation

No operation

REFL

REF

128

−







 ∗−

() +

Table 1. Serial Input Decoded Truth Table

In reality, it is possible to define a DC voltage as a

signal ground in a single supply system. If the DAC's

REFL pin is driven to the voltage chosen as pseudo–

ground, then each voltage output will exhibit 4–

quadrant behavior with respect to pseudoground. For

codes greater than 128, the output voltage will enter

the quadrant below the pseudoground voltage when

the input voltage goes below pseudoground. For

codes less than 128, the output voltage will be below

pseudogroundwhentheinputisabovepseudoground.

When V

REFL is driven to some positive voltage and

IN(X) is grounded, the device performs as if it

were a buffered trimpot tied between ground and

a voltage equal to two times V

REFL. This mode of

operation can be used as an "inverted single–

quadrant" source with an approximate range of 0 to

REFL*2)Volts. Because the output voltage will

decrease as the code is increased, this mode is

considered to be inverted with respect to normal

single–quadrant operation. Note that the mini-

mum output voltage will be 1LSB above V

IN(X).

Figure 2a and 2b show "inverted single–quadrant"

and 4–quadrant performance of the SP9840/9843.

very precise when describing a system which runs

fromasinglepositivesupply.Traditionally,thequad-

rants have been defined with respect to 0V. A two–

quadrant multiplying DAC could produce negative

output voltages only if a negative voltage reference

were applied. A four–quadrant device could also

produce a code–controlled negative output from a

positive reference, or a code–controlled positive out-

put from a negative reference. If ground is used to

delineate the quadrants, then the SP9840/SP9843

should be considered single–quadrant multiplying

devices, as their output op amps cannot produce

voltages below ground.

–

DAC

REFL

+5V

IN(X)

OUT

REFL

=−

−







()

128

Figure 1. DAC and Output Amplifier Circuit

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