Datasheet

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APPLICATION INFORMATION
Waveform Generation
Generating ±5-V, ±10-V, and ± 12-V Outputs For
DAC7558
REFIN
DAC7558
_
+
V
dac
R2
R1
REF3140
V
ref
V
tail
V
OUT
OPA4130
V
OUT
 V
REF

R2
R1
 1

Din
4096
 V
tail
R2
R1
(1)
DAC7558
SLAS435A – MAY 2005 – REVISED DECEMBER 2005
ringing characteristics of the loop's transfer function,
DAC glitches can also slow the loop down. With its 1
MSPS (small-signal) maximum data update rate,
DAC7558 can support high-speed control loops.
Due to its exceptional linearity, low glitch, and low
Ultra-low glitch energy of the DAC7558 significantly
crosstalk, the DAC7558 is well suited for waveform
improves loop stability and loop settling time.
generation (from DC to 10 kHz). The DAC7558
large-signal settling time is 5 µs, supporting an Generating Industrial Voltage Ranges:
update rate of 200 KSPS. However, the update rates
For control loop applications, DAC gain and offset
can exceed 1 MSPS if the waveform to be generated
errors are not important parameters. This could be
consists of small voltage steps between consecutive
exploited to lower trim and calibration costs in a
DAC updates. To obtain a high dynamic range,
high-voltage control circuit design. Using a quad
REF3140 (4.096 V) or REF02 (5.0 V) are
operational amplifier (OPA4130), and a voltage
recommended for reference voltage generation.
reference (REF3140), the DAC7558 can generate the
wide voltage swings required by the control loop.
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.
Figure 45. Low-cost, Wide-swing Voltage
As long as a voltage exists in the transfer curve of a
Generator for Control Loop Applications
monotonic DAC, the loop can find it and settle to it.
On the other hand, DAC resolution and differential
The output voltage of the configuration is given by:
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.
Fixed R1 and R2 resistors can be used to coarsely
A DNL error greater than +1 LSB implies
set the gain required in the first term of the equation.
unnecessarily large voltage steps and missed voltage
Once R2 and R1 set the gain to include some
targets. With high DNL errors, the loop looses its
minimal over-range, four DAC7558 channels could be
stability, resolution, and accuracy. Offering 12-bit
used to precisely set the required offset voltages.
ensured monotonicity and ± 0.08 LSB typical DNL
Residual errors are not an issue for loop accuracy
error, 755X DACs are great choices for precision
because offset and gain errors could be tolerated.
control loops.
Four DAC7558 channels can provide the V
tai
l
Loop Speed:
voltages to minimize offset error, while the other four
DAC7558 channels provide Vdac voltages to
Many factors determine control loop speed. Typically,
generate four high-voltage outputs.
the ADC's conversion time, and the MCU's
computation time are the two major factors that
For ±5-V operation: R1=10 k Ω, R2 = 15 k Ω, V
tail
=
dominate the time constant of the loop. DAC settling
3.33 V, V
REF
= 4.096 V
time is rarely a dominant factor because ADC
For ±10-V operation: R1=10 k Ω, R2 = 39 k Ω, V
tail
=
conversion times usually exceed DAC conversion
2.56 V, V
REF
= 4.096 V
times. DAC offset, gain, and linearity errors can slow
the loop down only during the start-up. Once the loop
For ±12-V operation: R1=10 k Ω, R2 = 49 k Ω, V
tail
=
reaches its steady-state operation, these errors do
2.45 V, V
REF
= 4.096 V
not affect loop speed any further. Depending on the
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