DN337 - Multiple Output Range 16-Bit DAC Design Made Simple

Multiple Output Range 16-Bit DAC Design Made Simple
Design Note 337
Derek Redmayne
of output ranges. However, with the new LTC ®1592
multiple output range DAC, all of this complexity is
unnecessary. Figure 2 shows the compact simplicity of
an implementation based on the new LTC1592. All the
standard industrial ranges (0V to 5V, 0V to 10V, ±5V,
±10V, ±2.5V and –2.5V to 7.5V) are provided, accurately
and under software control.
Introduction
Precision 16-bit analog outputs with software-configurable output ranges are often needed in industrial
process control equipment, analytical and scientific
instruments and automatic test equipment. In the past,
designing a universal output module was a daunting task
and the cost and PCB real estate associated with this
function were problematic, if not prohibitive. Figure 1
shows an example of the circuitry formerly required
to produce a programmable 16-bit DAC with a variety
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are the property of their respective owners.
U7
U6
5V
REF
+
+
U3
U10
10V
U7
–
–
U6
SHDN
50k
1%
U8
U9
+
10k
0.1%
10ppm/°C
TRACKING
U6
–
100pF
100k
U6
U7
+
U4
5V
–
10k
U7
10k
5V
10k
–
U1
16-BIT CMOS DAC
U2
1/2 LT1469
DN337 F01
+
U8
+
U5
–
2.5V
15V
U6
U7
U8
U8
SHDN
5k
U6, U7, U8: DG411
U9: 74HC1G4066
Figure 1. How NOT to Build a Universal 16-Bit Analog Output
05/04/337_conv
OUT
15pF
10ppm
TRACKING
SHDN
10k
U8
U2
1/2 LT®1469
–15V
The Old Way
Figure 1 shows a pre-LTC1592 implementation of a
multiple output range DAC. The circuit can be made
to work, but only with costly components and a lot of
PCB real estate. The range switching capability requires
the addition of analog switches and precision resistors
to the basic DAC. Some of these analog switches are
required to compensate for the resistance of switches
at other points in the circuit. The circuit as shown, even
with its considerable complexity, is a compromise as
some of these analog switches are not paired with
counterparts in the same package. The analog switches
are expensive. They also require PCB real estate, bypassing and decoupling to compensate for poor PSRR
to mitigate digital noise. In addition, since they are not
switching at virtual ground, the analog switches exhibit
on-resistance variation with voltage which will degrade
linearity. Leakage can be an issue at high temperature.
Precision matched resistor pairs are shown, as they are
available from a number of sources. But unless very
expensive devices are used, they will degrade accuracy.
The New, Easy Way
In contrast is Figure 2 where the LTC1592 contains all
of the circuitry required to perform these functions—all
under processor control. All the ranges are accurate
with low drift, fast settling and low glitch operation
right out of the box. The LTC1592 incorporates all the
switches and precision resistors. A full implementation takes less than 0.5in × 0.5in including the dual
operational amplifier, bypass and compensation. This
analog output subsystem can be reconfigured in real
time and the serial interface makes optoisolation easy.
Conclusion
Building a precision, multiple output range, softwareconfigurable 16-bit DAC is no longer a complicated,
expensive design effort. Now a clean, simple design
yields smaller size, lower cost and much better accuracy.
The LTC1592 can also be used for embedded or fixed
range applications, where its 4-quadrant operation
with serial interface make it compelling even if range
changing may not be required.
VREF
5V
5
+
7
1/2 LT1469
6
–
C2
150pF
5V
0.1μF
9
2
1
16 15
R1
R1
RCOM
R2 REF ROFS RFB
R2
3
4
C1
30pF
VCC
5
2
–
IOUT2 6
3
+
IOUT1
14
13
12
11
10
CLR
16-BIT DAC WITH
SPAN ADJUST
CS/LD
SCK
AGND
SDI
SDO
LTC1592
GND
15V
8
0.1μF
1/2 LT1469
7
4
–15V
8
1
VOUT
0.1μF
DN337 F01
Figure 2. Programmable Output Range 16-Bit SoftSpan™ DAC
Data Sheet Download
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Linear Technology Corporation
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call (408) 432-1900
dn337f_conv LT/TP 0504 305K • PRINTED IN THE USA
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© LINEAR TECHNOLOGY CORPORATION 2004
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