DAC1006 datasheet(11/22 Pages) NSC

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FIGURE 10 Providing a Bipolar Output Voltage with a Single Op Amp

TLH5688 – 13

FIGURE 11 Increasing the Output Voltage Swing

The output voltage swing can be expanded by adding 2

resistors to

Figure 10 as shown in Figure 11 These added

resistors are used to attenuate the aV voltage The overall

gain AV(b) from the aV terminal to the output of the op

amp determines the most negative output voltage b4(aV)

(when the VREF voltage at the a input of the op amp is

zero) with the component values shown The complete dy-

namic range of VOUT is provided by the gain from the (a)

input of the op amp As the voltage at the VREF pin ranges

from 0V to aV(10231024) the output of the op amp will

range from b10 VDC to a10V (10231024) when using a

V voltage of a2500 VDC The 25 VDC reference voltage

can be easily developed by using the LM336 zener which

can be biased through the RFB internal resistor connected

to VCC

53 Op Amp VOS Adjust (Zero Adjust) for Current

Switching Mode

Proper operation of the ladder requires that all of the 2R

legs always go to exactly 0 VDC (ground) Therefore offset

voltage VOS of the external op amp cannot be tolerated as

every millivolt of VOS will introduce 001% of added linearity

error At first this seems unusually sensitive until it becomes

clear the 1 mV is 001% of the 10V reference High resolu-

tion converters of high accuracy require attention to every

detail in an application to achieve the available performance

which is inherent in the part To prevent this source of error

the VOS of the op amp has to be initially zeroed This is the

‘‘zero adjust’’ of the DAC calibration sequence and should

be done first

If the VOS is to be adjusted there are a few points to consid-

er Note that no ‘‘dc balancing’’ resistance should be used

in the grounded positive input lead of the op amp This re-

sistance and the input current of the op amp can also create

errors The low input biasing current of the BI-FET op amps

makes them ideal for use in DAC current to voltage applica-

tions The VOS of the op amp should be adjusted with a

digital input of all zeros to force IOUTe0mA A1kX resistor

can be temporarily connected from the inverting input to

ground to provide a dc gain of approximately 15 to the VOS

of the op amp and make the zeroing easier to sense

54 Full-Scale Adjust

The full-scale adjust procedure depends on the application

circuit and whether the DAC is operated in the current

switching mode or in the voltage switching mode Tech-

niques are given below for all of the possible application

circuits

541 Current Switching with Unipolar Output Voltage

After doing a ‘‘zero adjust’’ set all of the digital input levels

HIGH and adjust the magnitude of VREF for

VOUTeb(ideal VREF)

1023

1024

This completes the DAC calibration

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