DAC8574 Datasheet, PDF (32/40 Pages) Texas Instruments – QUAD, 16-BIT, LOW-POWER, VOLTAGE OUTPUT, I2C INTERFACE DIGITAL-TO-ANALOG CONVERTER

DAC8574
SLAS377A – JANUARY 2003 – REVISED JUNE 2003
www.ti.com
APPLICATION INFORMATION (continued)
GENERATING ±5 V, ±10 V, and ± 12 V OUTPUTS FOR PRECISION INDUSTRIAL CONTROL
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.
DAC7574
V_tail
REF3040
V_ref
V_REFH
DAC8574
R1
R2
_
V_{dac +}
OPA4130
V_OUT
Figure 59. Low-cost, Wide-swing Voltage Generator for Control Loop Applications
The output voltage of the configuration is given by:
ǒ Ǔ _{Vout + Vref}
R2
R1
)
1
Din
65536
–V_tail
R2
R1
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
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