DAC1021 datasheet(4/14 Pages) NSC

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Typical Applications

The following applications are also valid for 12-bit systems

using the DAC1220 and 2 additional digital inputs

Operational Amplifier Bias Current

(

Figure 3 )

The op amp bias current Ib flows through the 15k internal

feedback resistor BI-FET op amps have low Ib and there-

fore the 15k c Ib error they introduce is negligible they are

strongly recommended for the DAC1020 applications

VOS Considerations

The output impedance ROUT of the DAC is modulated by

the digital input code which causes a modulation of the op-

erational amplifier output offset It is therefore recommend-

ed to adjust the op amp VOS ROUT is E15k if more than 4

digital inputs are high ROUT is E45k if a single digital input

is high and ROUT approaches infinity if all inputs are low

Operational Amplifier VOS Adjust (Figure 3 )

Connect all digital inputs A1 – A10 to ground and adjust the

potentiometer to bring the op amp VOUT pin to within g1

mV from ground potential If VREF is less than 10V a finer

VOS adjustment is required It is helpful to increase the reso-

lution of the VOS adjust procedure by connectinga1kX

resistor between the inverting input of the op amp to

ground After VOS has been adjusted remove the 1 kX

Full-Scale Adjust

(

Figure 4 )

Switch high all the digital inputs A1 – A10 and measure the

op amp output voltage Use a 500X potentiometer as

shown to bring

llVOUTll to a voltage equal to VREF c

10231024

SELECTING AND COMPENSATING THE OPERATIONAL AMPLIFIER

Op Amp Family

PVW

Circuit Settling

Circuit Small

Time ts

Signal BW

LF357

10 pF

24k

25k

15 ms1M

LF356

22 pF

25k

3 ms

05M

LF351

24 pF

10k

4 ms

05M

LM741

10k

40 ms

200 kHz

TLH5689 – 3

VOUT ebVREF

A12aA24aA38a A10

1024

10V s VREF s 10V

0 s VOUT s b

1023

1024

VREF

where AN e 1 if the AN digital input is high

AN e 0 if the AN digital input is low

FIGURE 3 Basic Connection Unipolar or 2-Quadrant Multiplying

Configuration (Digital Attenuator)

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