DAC1006LCWM datasheet(10/22 Pages) NSC

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52 Analog Operation in the Voltage Switching Mode

Some useful application circuits result if the R-2R ladder is

operated in the voltage switching mode There are two very

important things to remember when using the DAC in the

voltage mode The reference voltage (aV) must always be

positive since there are parasitic diodes to ground on the

IOUT1 pin which would turn on if the reference voltage went

negative To maintain a degradation of linearity less than

0005% keep aV s 3VDC and VCC at least 10V more

positive than aV

Figures 6 and 7 show these errors for the

voltage switching mode This operation appears unusual

since a reference voltage (aV) is applied to the IOUT1 pin

and the voltage output is the VREF pin This basic idea is

shown in

Figure 8

This VOUT range can be scaled by use of a non-inverting

gain stage as shown in

Figure 9

Notice that this is unipolar operation since all voltages are

positive A bipolar output voltage can be obtained by using a

single op amp as shown in

Figure 10 For a digital input

code of all zeros the output voltage from the VREF pin is

zero volts The external op amp now has a single input of

V and is operating with a gain of b1 to this input The

output of the op amp therefore will be at bV for a digital

input of all zeros As the digital code increases the output

voltage at the VREF pin increases

Notice that the gain of the op amp to voltages which are

applied to the (a) input is a2 and the gain to voltages

which are applied to the input resistor R is b1 The output

voltage of the op amp depends on both of these inputs and

is given by

VOUTe(aV) (b1)aVREF(a2)

FIGURE 6

FIGURE 7

DIGITAL INPUT CODE

FIGURE 8 Voltage Mode Switching

TLH5688 – 12

FIGURE 9 Amplifying the Voltage Mode Output (Single Supply Operation)

DAC1006LCWM Datasheet(PDF) 10 Page - National Semiconductor (TI)