SP9841 datasheet(22/32 Pages) SIPEX

22 / 32 page

288

through 19 show that for lower output current values

less than 1 mA, the SP9841/9842 can be used effec-

tively even with V

DD in the range of 2.7 to 3.3V.

Application Circuits

Figure 5 shows an inexpensive single–quadrant

DC source for generating voltages from near

ground to near 2.44V. When using a two–termi-

nal reference, the pull–up resistor should be

chosen so that the minimum input resistance of

5kOhms at each V

IN(X) can be driven at the

lowest expected V

DD. At VDD = 4.75V, and VREFL

= 1.25V, each input to be driven needs 0.248mA,

and the regulator needs 0.1mA to stay well

regulated. Thus, to drive all eight inputs, R

PULLUP

should be chosen to supply at least 2mA. To

operate at 4.75V, 1.75kOhms is required; the

1.5kOhms shown will suffice even if its value is

5% high. To drive a single input, a 10kOhm

value could be used. In order to reduce reference

and supply generated noise, an optional capaci-

tor of 1 to 100

µF bypasses the reference.

Figure 6 shows a circuit which generates DC

voltages

roughly

symmetric with respect to

2.446V. Two bandgap references are stacked to

first drive V

REFL to 1.223V, and the input to 2.446V.

The pull–up resistor value should again be scaled

for worst–case loading — in order to drive all eight

inputs at 4.75V, a value of 620 Ohms is required.

At fullscale, the DAC output is near 3.65V. While

typical units will source 5mA at an output voltage

Figure 5. Inexpensive DC Source.

–

OUT

1/8 of SP9841

+5V

1.22V

OUT

= 20mV to +2.44V for DATA = 00H to FFH.

At PRESET, DC

OUT = +1.22V

RPULLUP

1.5kOhms

ICL8069

45µF*

(*Optional Noise Reduction)

Figure 6. Pseudo Bipolar Source Generates Voltages Above and Below 2.44V.

–

OUT

1/8 of SP9841

+5V

1.22V

OUT

= +1.22V to +3.66V for DATA = 00H to FFH.

At PRESET, DC

OUT = +2.44V

2kOhm

ICL8069

1.22V

ICL8069

+1.22V

+2.44V

45µF*

(*Optional Noise Reduction)

45µF*

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