Datasheet

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APPLICATION INFORMATION
WAVEFORM GENERATION
GENERATING INDUSTRIAL VOLTAGE
GENERATING ± 5V, ± 10V, AND ± 12V
Loop Accuracy
DAC7551
V
REF
H
DAC7551
_
+
V
dac
R2
R1
REF3140
V
REF
V
tail
V
OUT
OPA4130
V
OUT
+ V
REF
ǒ
R2
R1
) 1
Ǔ
SDIN
4096
*V
tail
ǒ
R2
R1
Ǔ
(1)
Loop Speed
DAC7551
SLAS441E – MARCH 2005 – REVISED APRIL 2007
slow the loop down. With its 1MSPS (small-signal)
maximum data update rate, DAC7551 can support
As a result of the exceptional linearity and low glitch
high-speed control loops. Ultralow glitch energy of
of the DAC7551, the device is well-suited for
the DAC7551 significantly improves loop stability and
waveform generation (from DC to 10kHz). The
loop settling time.
DAC7551 large-signal settling time is 5 µ s, supporting
an update rate of 200kSPS. However, the update
rates can exceed 1MSPS if the waveform to be
RANGES
generated consists of small voltage steps between
consecutive DAC updates. To obtain a high dynamic
For control loop applications, DAC gain and offset
range, REF3140 (4.096V) or REF02 (5.0V) are
errors are not important parameters. This
recommended for reference voltage generation.
consideration could be exploited to lower trim and
calibration costs in a high-voltage control circuit
design. Using a quad operational amplifier
OUTPUTS FOR PRECISION INDUSTRIAL (OPA4130 ), and a voltage reference (REF3140 ), the
CONTROL DAC7551 can generate the wide voltage swings
required by the control loop.
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.
DAC offset, gain, and the integral linearity errors 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
Figure 28. Low-cost, Wide-swing Voltage
each DAC step determines the minimum incremental
Generator for Control Loop Applications
change the loop can generate. A DNL error less than
–1LSB (non-monotonicity) can create loop instability.
The output voltage of the configuration is given by:
A DNL error greater than +1LSB implies
unnecessarily large voltage steps and missed
voltage targets. With high DNL errors, the loop loses
its stability, resolution, and accuracy. Offering 12-bit
Fixed R1 and R2 resistors can be used to coarsely
ensured monotonicity and ± 0.08LSB typical DNL
set the gain required in the first term of the equation.
error, DAC755x devices are great choices for
Once R2 and R1 set the gain to include some
precision control loops.
minimal over-range, a single DAC7551 could be
used to set the required offset voltages. Residual
errors are not an issue for loop accuracy because
Many factors determine the control loop speed, such
offset and gain errors could be tolerated. One
as ADC conversion time, MCU speed, and DAC
DAC7551 can provide the V
tail
voltages, while four
settling time. Typically, the ADC conversion time,
additional DAC7551 devices can provide V
dac
and the MCU computation time are the two major
voltages to generate four high-voltage outputs. A
factors that dominate the time constant of the loop.
single SPI interface is sufficient to control all five
DAC settling time is rarely a dominant factor because
DAC7551 devices in a daisy-chain configuration.
ADC conversion times usually exceed DAC
For ± 5V operation:
conversion times. DAC offset, gain, and linearity
errors can slow the loop down only during the
R1 = 10k Ω, R2 = 15k Ω, V
tail
= 3.33V, V
REF
= 4.096V
start-up. Once the loop reaches its steady-state
For ± 10V operation:
operation, these errors do not affect loop speed any
further. Depending on the ringing characteristics of
R1 = 10k Ω, R2 = 39k Ω, V
tail
= 2.56V, V
REF
= 4.096V
the loop transfer function, DAC glitches can also
For ± 12V operation:
R1 = 10k Ω, R2 = 49k Ω, V
tail
= 2.45V, V
REF
= 4.096V
14
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