DAC8574IPWR datasheet(32/45 Pages) TI

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GENERATING

±5-V, ±10-V, and ± 12-V OUTPUTS FOR PRECISION INDUSTRIAL CONTROL

DAC8574

VREFH

DAC7574

Vdac

REF3040

Vref

Vtail

VOUT

OPA4130

Vout + Vref

R1 )

Din

65536

–V

tail

DAC8574

SLAS377B – JANUARY 2003 – REVISED DECEMBER 2004

Industrial control applications can require multiple feedback loops consisting of sensors, ADCs, MCUs, DACs,

and actuators. Loop accuracy and loop speed are the two important parameters of such control loops.

Loop Accuracy:

In a control loop, the ADC has to be accurate. Offset, gain, and the integral linearity errors of the DAC are not

factors in determining the accuracy of the loop. As long as a voltage exists in the transfer curve of a monotonic

DAC, the loop can find it and settle to it. On the other hand, DAC resolution and differential linearity do determine

the loop accuracy, because each DAC step determines the minimum incremental change the loop can generate.

A DNL error less than -1 LSB (non-monotonicity) can create loop instability. A DNL error greater than +1 LSB

implies unnecessarily large voltage steps, and missed voltage targets. With high DNL errors, the loop looses its

stability, resolution, and accuracy. Offering 16-bit assured monotonicity and

± 0.25 LSB typical DNL error, 85XX

DACs are great choices for precision control loops.

Loop Speed:

Many factors determine control loop speed. Typically, the ADC's conversion time, and the MCU's computation

time are the two major factors that dominate the time contstant of the loop. DAC settling time is rarely a dominant

factor because ADC conversion times usually exceed DAC conversion times. DAC offset, gain, and linearity

errors can slow the loop down only during the start-up. Once the loop reaches its steady-state operation, these

errors do not affect loop speed any further. Depending on the ringing characteristics of the loop's transfer

function, DAC glitches can also slow the loop down. With its 188 ksps maximum data update rate, DAC8574 can

support high-speed control loops.

Generating Industrial Voltage Ranges:

For control loop applications, DAC gain and offset errors are not important parameters. This could be exploited to

lower trim and calibration costs in a high-voltage control circuit design. Using a quad op amp (OPA4130), a

voltage reference (REF3040) and a quad 12-bit DAC (DAC7574), the DAC8574 can generate the wide voltage

swings required by the control loop.

Figure 59. Low-cost, Wide-swing Voltage Generator for Control Loop Applications

The output voltage of the configuration is given by:

Fixed R1 and R2 resistors can be used to coarsely set the gain required in the first term of the equation. Once

R2 an R1 set the gain properly, a DAC7574 could be used to set the required offset voltages. Residual errors are

not an issue for loop accuracy because offset and gain errors could be tolerated.

For

±5-V operation: R1=10 kΩ, R2 = 15 kΩ, Vtail = 3.33 V, Vref = 4.096 V

For

±10-V operation: R1=10 kΩ, R2 = 39 kΩ, Vtail = 2.56 V, Vref = 4.096 V

For

±12-V operation: R1=10 kΩ, R2 = 49 kΩ, Vtail = 2.45 V, Vref = 4.096 V

DAC8574IPWR Datasheet(PDF) 32 Page - Texas Instruments

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