Volume 44 – April 2010
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The 20-Bit DAC Is the Easiest Part of a 1-ppm-Accurate
Precision Voltage Source
use of high-resolution digital-to-analog converters (DACs) is providing
controllable precision voltages. Applications for DACs with resolution
up to 20 bits, precision up to 1 ppm, and reasonable speed include gradient
coil control in medical MRI systems; precision dc sources in test and
measurement; precision set-point and position control in mass spectrometry
and gas chromatography; and beamprobing in scientific applications.
Over time, the definition
of what constitutes a precision integrated-circuit DAC has changed rapidly
as semiconductor processing and on-chip calibration technologies have
advanced. Once, high-precision 12-bit DACs were considered to be hard
to achieve; in recent years, 16-bit accuracy has become widely available
for use in precision medical, instrumentation, and test and measurement
applications; and over the horizon, even greater resolutions and accuracies
are called for in control and instrumentation systems.
High precision applications
for integrated circuits now require 18-bit and 20-bit, 1-ppm-accurate
digital-to-analog converters, a performance level previously achieved
only with cumbersome, expensive, and slow Kelvin-Varley dividersthe
preserve of standards labs and hardly suitable for instrumentation systems
in the field. More convenient semiconductor-based, 1-ppm-accurate solutions
to such requirements using assemblies of IC DACs have been around for
some years; but these complex systems use many devices, require frequent
calibrations and great care to achieve accuracy, and are both bulky and
costly (see Appendix). The precision instrumentation
market has long needed a simpler, cost-effective DAC that doesn't require
calibration or constant monitoring, is easy to use, and offers guaranteed
specifications. A natural evolution from 16-bit and 18-bit monolithic
converterssuch a DAC is now a reality.
The AD5791 1-ppm
in semiconductor processing, DAC architecture design, and fast on-chip
calibration techniques make possible highly linear, stable, fast-settling
digital-to-analog converters that deliver better than 1-ppm relative accuracy,
0.05-ppm/°C temperature drift, 0.1-ppm p-p noise, better than 1-ppm
long-term stability, and 1-MHz throughput. These small, single chip devices
have guaranteed specifications, do not require calibration, and are easy
to use. A typical functional diagram for the AD5791 and its companion
reference- and output buffers is shown in Figure 1.
1. AD5791 typical operating block diagram.
single-chip, 20-bit, voltage-output digital-to-analog converter specifies
1-LSB (least-significant bit) integral nonlinearity (INL)
and differential nonlinearity (DNL), making it the world's first
monolithic 1-ppm-accurate digital-to-analog converter (1 LSB at 20 bits
is one part in 220 = one part in 1,048,576 = 1 ppm). Designed
for use in high-precision instrumentation and test and measurement systems,
it offers a significant leap in all-around performance compared to other
solutions, providing greater levels of accuracy and repeatability in less
space and at lower cost, permitting instrumentation applications that
previously would not have been economically feasible.
Its design, shown
in Figure 2, features precision voltage-mode R-2R architecture, exploits
state-of-the-art thin-film resistor-matching techniques, and employs on-chip
calibration routines to achieve 1-ppm accuracy levels. Because the device
is factory calibrated and, therefore, requires no run-time calibration
routines, its latency is no greater than 100 ns, so the AD5791 can be
used in waveform-generation applications and fast control loops.
2. DAC ladder structure.
Besides its impressive
linearity, the AD5791 combines 9 nV/√Hz noise density, 0.6-μV
peak-to-peak noise in the 0.1-Hz to 10-Hz frequency band, 0.05-ppm/°C
temperature drift, and better than 0.1-ppm long-term stability over 1000
A high-voltage device,
it operates from dual supplies of up to ±16.5
V. The output voltage span is set by the applied positive and negative
reference voltages, VREFP and VREFN,
offering a flexible choice of output range.
The precision architecture
of the AD5791 requires high-performance external amplifiers to buffer
the reference source from the 3.4 kΩ
DAC resistance and facilitate force-sensing at the reference input pins
to ensure the AD5791's 1-ppm linearity. An output buffer is required for
load driving to unburden the 3.4 kΩ
output impedance of the AD5791unless a very high-impedance, low-capacitance
load is being drivenor attenuation is tolerable and predictable.
Because the amplifiers
are external, they can be selected to optimize for noise, temperature
drift, and speedand the scale factor can be adjusteddepending
on the needs of the application. For the reference buffers, the AD8676
dual amplifier is recommended, based on its low noise, low offset error,
low offset error drift, and low input bias currents. The input bias current
specification of the reference buffers is important, as excessive bias
currents will degrade the dc linearity. The degradation of integral nonlinearity,
in ppm, as a function of input bias current, is typically:
is in nA; VREFP and VREFN are in volts.
For example, with a ±10-V reference input span,
an input bias current of 100 nA will increase the INL by 0.05 ppm.
The key requirements
for an output buffer are similar to those for the reference buffersexcept
for bias current, which does not affect the AD5791's linearity. Offset
voltage and input bias current can affect output offset voltage, though.
To maintain dc precision, the AD8675
is recommended as an output buffer. High-throughput applications require
a fast output buffer amplifier with higher slew rate.
1 lists the key specifications of a few appropriate precision amplifiers.
1. Precision Amplifier Key Specifications
(μV p-p - 0.1 Hz to 10 Hz)
Voltage Error Drift
The AD5791 offers
reduced design time, reduced design risk, reduced cost, reduced board
size, increased reliability, and guaranteed specifications.
Figure 3 is a circuit
schematic implementing the AD5791 (U1) as a precision digitally controlled
1-ppm voltage source with a ±10 V range in
increments using the AD8676 (U2) as reference buffers and the AD8675 (U3)
as the output buffer. The absolute accuracy is determined by the choice
of the external 10 V references.
3. A 1-ppm accurate system using the AD5791 digital-to-analog converter.
measures of this circuit are integral nonlinearity, differential nonlinearity,
and 0.1-Hz to 10-Hz peak-to-peak noise. Figure 4 shows that typical INL
is within ±0.6 LSB.
4. Integral nonlinearity plot.
Figure 5 shows a
typical DNL of ±0.5 LSB; the output is guaranteed
monotonic over the entire range of bit transitions.
5. Differential nonlinearity plot.
in the 0.1-Hz to 10-Hz bandwidth is about 700 nV, as shown in Figure 6.
6. Low-frequency noise.
The AD5791 Is
Only the Beginning:
precision sub-1-ppm components such as the AD5791 are available on the
market, building a 1-ppm system is not a task that should be taken lightly
or rushed into. Error sources that show up at this level of precision
must be carefully considered. The major contributors to errors in 1-ppm-accurate
circuits are noise, temperature drift, thermoelectric voltages, and physical
stress. Precision circuit construction techniques should be followed to
minimize the coupling and propagation of these errors throughout the circuit
and the introduction of external interference. These considerations will
be summarized here briefly. Further information can be found in the References.
at 1-ppm resolutions and accuracies, it is of utmost importance to keep
noise levels to a minimum. The noise spectral density of the AD5791 is
9 nV/√Hz, mostly from the Johnson noise of the 3.4-kΩ
DAC resistance. All peripheral components should have smaller noise contributions
to minimize increases to the system noise level. Resistor values should
be less than the DAC resistance to ensure that their Johnson noise contribution
will not significantly add to the root-sum-square overall noise level.
The AD8676 reference buffers and the AD8675 output buffer have a specified
noise density of 2.8 nV/√Hz, well below the DAC's contribution.
can be eliminated relatively easily with simple R-C filters, but low-frequency
1/f noise in the 0.1-Hz to 10-Hz range cannot be easily filtered without
affecting dc accuracy. The most effective method of minimizing 1/f noise
is to ensure that it is never introduced into the circuit. The AD5791
generates about 0.6 μV
p-p of noise in the 0.1-Hz to 10-Hz bandwidth, well below the 1-LSB level
(1 LSB = 19 μV
for a ±10-V
output span). The target for maximum 1/f noise in the entire circuit should
be about 0.1 LSB, or 2 μV;
this can be ensured through proper component choice. The amplifiers in
the circuit generate 0.1-μV
p-p 1/f noise; the three amplifiers in the signal chain generate a total
of approximately 0.2-μV
p-p noise at the circuit output. Add this to the 0.6-μV p-p from the AD5791, and the
total expected 1/f noise is about 0.8 μV
p-p, a figure that closely correlates with the measurement displayed in
Figure 5. This offers adequate margin for other circuitry that may be
added, such as amplifiers, resistors, and a voltage reference.
Besides random noise,
it is important to avoid errors caused by radiated, conducted, and induced
electrical interference. Such techniques as shielding, guarding, and scrupulous
attention to grounding and proper printed-circuit-board wiring techniques
all precision circuits, drift of all components with temperature is a
major source of error. The key to minimizing the drift as much as possible
is to choose critical components with sub-1-ppm temperature coefficients.
The AD5791 exhibits a very low 0.05-ppm/°C temperature coefficient.
The AD8676 reference buffers drift at 0.6 μV/°C, introducing an overall
0.03-ppm/°C gain drift into the circuit; the AD8675 output buffer
contributes a further 0.03-ppm/°C output drift; this all adds up to
a figure of 0.11 ppm/°C. Low drift, thermally matched resistor networks
should be used for scaling and gain circuits. Vishay bulk metal-foil voltage-divider
resistors, series 300144Z and 300145Z, with a temperature coefficient
of resistance tracking to 0.1 ppm/°C, are recommended.
voltages are the result of the Seebeck effect: temperature-dependent voltages
are generated at dissimilar metal junctions. Depending on the metallic
components of the junction, the generated voltage can be anywhere from
to 1 mV/°C. The best case, a copper-to-copper junction, will generate
less than 0.2 μV/°C of
thermoelectric EMF. In the worst case, copper-to-copper-oxide can generate
up to 1 mV/°C of thermoelectric voltage. This sensitivity to even
small temperature fluctuations means that nearby dissipative elements
or slow-moving air currents crossing over a printed circuit board (PCB)
can create varying temperature gradients, which in turn generate varying
thermoelectric voltages that are manifested as a low-frequency drift similar
to low-frequency 1/f noise. Thermoelectric voltages can be avoided
by ensuring that there are no dissimilar junctions in the system and/or
eliminating thermal gradients. While it is virtually impossible to eliminate
dissimilar metal junctionsmany different metals exist in IC packaging,
PCB circuits, wiring, and connectorskeeping all connections clean
and oxide-free will go a long way to keeping thermoelectric voltages low.
Enclosing the circuit to shield circuitry from air currents would be an
effective thermoelectric voltage stabilizing method, and it could have
the added value of providing electrical shielding. Figure 7 shows the
difference in voltage drifts between a circuit that is open to air currents
and one that is enclosed.
7. Voltage drift vs. time for open- and enclosed systems.
To cancel out the
thermoelectric voltages, compensating junctions could be introduced into
the circuit, a task that would involve considerable trial and error and
iterative testing to ensure the correct pairing and positions of the inserted
junctions. By far the most efficient method is to reduce the number
of junctions in the circuit by minimizing component count in the signal
path and stabilizing the local and ambient temperatures.
analog semiconductor devices are sensitive to stress on their package.
Stress relief compounds used within the packaging have a settling effect,
but they cannot compensate for significant stress due to pressure exerted
directly on the package by local sources, such as flexing of the PCB.
The larger the printed circuit board, the more stress that a package could
potentially suffer, so sensitive circuitry should be placed on as small
a board as possiblewith connection to the larger system through
flexible or nonrigid connectors. If a large board cannot be avoided, stress
relief cuts should be made around sensitive components, on two or (preferably)
three sides of the component, greatly reducing the stress on the component
due to board flexing.
noise and temperature drift, long-term stability deserves consideration.
Precision analog ICs are very stable devices but do undergo long-term
age-related changes. Long-term stability for the AD5791 is typically better
than 0.1 ppm/1000 hours at 125°C. The aging is not cumulative but
follows a square root rule (if a device ages at 1 ppm/1000 hours, it ages
at √2 ppm/2000 hours, √3 ppm/3000 hours, ...), and the time
is typically 10 times longer for each 25°C reduction in temperature;
so, at 85°C operation, one can expect aging of 0.1 ppm over a 10,000
hour period, approximately 60 weeks. If this is extrapolated, 0.32 ppm
aging can be expected over a 10-year period, so the data sheet dc specifications
can be expected to drift by 0.32 ppm over a 10-year period when operating
In a circuit
where such a high level of accuracy is important, careful consideration
of the power supply and ground return layout helps to ensure the rated
performance. Design the PCB such that the analog and digital sections
are separated and confined to separate areas of the board. If the DAC
is in a system where multiple devices require an analog-to-digital ground
connection, establish the connection at one point only. Establish the
star-point ground as close as possible to the device. There should be
ample power supply bypassing of 10 μF
in parallel with 0.1 μF
on each supply terminal, as close to the package as possible, ideally
right up against the device. The 10-μF
capacitors should be of the tantalum bead type. The 0.1-μF
capacitor should have low effective series resistance (ESR) and low effective
series inductance (ESL), such as the common multilayer ceramic typesto
provide a low-impedance path to ground at high frequencies to handle transient
currents due to internal logic switching. A series ferrite bead on each
power supply line will further help to block high-frequency noise from
getting through to the device.
The power supply
traces should be as large as possible to provide low-impedance paths and
reduce the effects of glitches on the power-supply line. Shield fast-switching
signals, such as clocks, with digital ground to avoid radiating noise
to other parts of the board. They should never be run near the reference
inputs or under the package. It is essential to minimize noise on the
reference inputs because it couples right through to the DAC output. Avoid
crossover of digital and analog signals, and run traces on opposite sides
of the board at right angles to each other to reduce the effects of feedthrough
on the board.
the performance of the entire circuit firmly within its grasp is the external
voltage reference; its noise and temperature coefficient directly impact
the system's absolute accuracy. To capitalize on the challenge posed by
the 1-ppm AD5791 digital-to-analog converter, the reference and associated
components should have temperature drift and noise specifications comparable
to those of the DAC. Although a reference with temperature drift of 0.05
ppm/°C is nothing short of fantasy, 1 ppm/°C and 2 ppm/°C
voltage references with 0.1-Hz to 10-Hz noise of less than 1 μV
p-p do exist.
accuracy requirements of precision instrumentationand test and measurement
applicationsincrease, more accurate components are being developed
to meet these needs. They have guaranteed precision specifications at
the 1-ppm level without further user calibration and are easy to use.
However, when designing circuitry for this level of precision, one must
bear in mind the many environmental and design-related challenges that
exist. Successful precision-circuit performance will come as a result
of considering and understanding these challenges and making correct component
on all ADI components can be found at www.analog.com.)
- "The Long
Term Stability of Precision Analog ICs, or How to Age Gracefully and
Avoid Sudden Death." Analog Devices. Rarely Asked Questions.
- Low Level
Measurements Handbook. 6th Edition. Keithley.
- MT-031, Grounding
Data Converters and Solving the Mystery of "AGND"
8 shows a block diagram of a typical contemporary 1 ppm DAC solution.
The core of the circuit consists of two 16-bit digital-to-analog convertersa
major DAC and a minor DACthe outputs of which are scaled and combined
to yield an increased resolution. The major DAC output is summed with
the attenuated minor DAC output so that the minor DAC fills the resolution
gaps between the major DAC's LSB steps.
8. Discrete 1-ppm DAC solution.
The combined DAC
outputs need to be monotonic, but not extremely linear, because high performance
is achieved with constant voltage feedback via a precision analog-to-digital
converter, which corrects for the inherent component errors; thus, the
circuit accuracy is limited by the ADC rather than the DACs. However,
because of the requirement for constant voltage feedback and the inevitable
loop delay, the solution is slow, potentially requiring seconds to settle.
Although this circuit
can, with significant endeavor, ultimately achieve 1 ppm accuracy, it
is complex to design, likely to require multiple design iterations, and
requires a software engine and precision ADC to achieve accuracy. To guarantee
1-ppm accuracy the ADC will also require correctionsince an ADC
with guaranteed 1-ppm linearity is not available. The simple block diagram
of Figure 8 illustrates the concept, but the actual circuit in reality
is far more complex, with multiple gain, attenuation, and summing stages,
involving many components. Also required is significant digital circuitry
to facilitate the interface between both DACs and the ADCnot to
mention the software required for error correction. (return to text)
is an applications engineer with the Precision Converters Product
Technology Group based in Limerick. Maurice joined Analog Devices
in 1998 and holds a BEng in electronic engineering from the University
of Limerick, Ireland.
(return to top)
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