### DN229 - Don't Be Fooled By Voltage Reference Long-Term Drift and Hysteresis

```Don’t Be Fooled by Voltage Reference Long-Term Drift
and Hysteresis – Design Note 229
John Wright
Some manufacturers are now touting phenomenal
long-term drift speciﬁcations, based on accelerated
high temperature testing. THIS IS A DELIBERATE
LIE! Long-term drift cannot be extrapolated from
accelerated high temperature testing. The only way
long-term drift can be determined is to measure it over
the time interval of interest. The erroneous technique
produces numbers that are wildly optimistic and uses
the Arrhenius Equation to derive an acceleration factor
from elevated temperature readings. The equation is:
AF = e
EA ⎛ 1 1 ⎞
⎜ – ⎟
K ⎝ T1 T2⎠
where: E A = Activation Energy (Assume 0.7)
K = Boltzmann’s Constant
T2 = Test Condition in °Kelvin
T1 = Use Condition Temperature in °Kelvin
Equation the acceleration factor is 767 and the projected
“bogus” long-term drift is 0.156ppm/1000hr at 30°C.
For a 2.5V reference, this corresponds to a 0.39μV shift
after 1000 hours. This is pretty hard to determine (read
impossible) if the peak-to-peak output noise is larger
than this number. As a practical matter, one of the best
laboratory references available has long-term drift of
1.5μV/mo. This performance is only available from the
best subsurface zener references such as the LTZ1000,
utilizing specialized heating techniques.
Competitive Reference Measures 500 Times
Worse Than Claimed
Long-term drift data was taken with parts that were
soldered onto PC boards similar to a “real world” application. These boards were not preconditioned. They
were placed into a constant temperature oven with
TA = 30°C and their outputs were scanned regularly
and measured with an 8.5 digit DVM. Figures 1 and 2
show typical long-term drift of the LT1461S8-2.5 and
the SOT-23 LT1790S6-2.5. Initially, data was taken every
hour where the largest changes occur, but after several
hundred hours the frequency was lowered to reduce the
large number of data points. Figure 3 shows long-term
200
150
DRIFT (ppm)
The new micropower LT®1461 and LT1790 low dropout
bandgap voltage references excel not only in temperature coefﬁcient and accuracy, but also in longterm drift and hysteresis (output voltage shift due to
temperature cycling). Long-term drift and hysteresis,
which are sometimes ignored or wrongly speciﬁed by
other manufacturers, can be the accuracy limitations
of systems. System calibrations can remove TC and
initial accuracy errors, but only frequent calibration can
remove the long-term drift and hysteresis. Subsurface
Zener references, like the LT1236, have the best longterm drift and hysteresis, but they do not offer low
output voltage options, low supply current and low
operating supplies like these new bandgap references.
100
50
0
–50
0
200 400 600 800 1000 1200 1400 1600
DN229 F01
HOURS
Figure 1. LT1461S8-2.5V Long-Term Drift
200
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of Linear Technology Corporation. All other trademarks are the property of their
respective owners.
04/00/229_conv
DRIFT (ppm)
150
To show how absurd this technique is, compare this
calculation to real LT1461 data. Typical 1000 hour
long-term drift at 30°C = 60ppm. The typical 1000 hour
long-term drift at 130°C is 120ppm. From the Arrhenius
100
50
0
–50
0
200 400 600 800 1000 1200 1400 1600
DN229 F02
HOURS
Figure 2. LT1790SOT23-2.5V Long-Term Drift
200
DRIFT (ppm)
150
100
50
0
–50
0
200 400 600 800 1000 1200 1400 1600
DN229 F03
HOURS
85°C, 25°C and –40°C and all 25°C output voltages were
recorded. The stabilization time at each temperature was
30 minutes. The worst-case output voltage changes at
25°C are shown in Figures 4 and 5 for the LT1461S8-2.5
and the SOT-23 LT1790S6-2.5. A competitive reference,
which makes no mention of hysteresis on its data sheet,
was also measured and is shown in Figure 6.
Figure 3. XXX291S8-2.5V Long-Term Drift
Hysteresis Limits Repeatability
When a reference is soldered onto a PC board, the
elevated temperature and subsequent cooling cause
stress that inﬂuences the output. If the voltage reference
is repeatedly temperature cycled, inelastic stress is applied to the chip and the output voltage does not return
to the 25°C initial value. The mechanical stress is due
to the difference in thermal coefﬁcients of expansion
between the silicon chip, plastic package and PC board.
This error, known as “thermally induced hysteresis,” is
expressed in ppm and cannot be trimmed out because
it is variable and has memory of previous temperature
excursions. Hysteresis is always worse with higher
temperature excursions, and differs with die attach
and package type.
Hysteresis—Often the “Missing” Spec
Most manufacturers ignore hysteresis specifications, but they can be critical in precision designs. To
graphically show hysteresis, many references were
IR reﬂow soldered onto PC boards and the boards
underwent a “heat soak” at 85°C (this ensures that
they all had the same initializing temperature). The
temperature was then cycled multiple times between
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PERCENTAGE OF UNITS
60
85°C TO 25°C
– 40°C TO 25°C
50
40
30
20
10
0
–200
–160
–120
–80
–40
0
40
DISTRIBUTION (ppm)
80
120
160
200
DN229 F04
Figure 4. LT1461S8-2.5 Industrial Hysteresis
50
PERCENTAGE OF UNITS
Long-term drift can be reduced by preconditioning the
PC board after the reference has been soldered onto
the board. Operating the PC board at 25°C or elevated
temperature stabilizes initial drifts. This “burn-in” of
the PC board eliminates the output shift that occurs in
the ﬁrst several hundred hours of operation. Further
changes in output voltage are typically logarithmic and
changes after 1000 hours tend to be smaller than before
that time. Because of this decreasing characteristic,
long-term drift is speciﬁed in ppm/√kHr.
70
40
30
85°C TO 25°C
– 40°C TO 25°C
20
10
0
–200
–160
–120
–80
–40
0
40
DISTRIBUTION (ppm)
80
120
160
200
DN229 F05
Figure 5. LT1790S6-2.5 Industrial Hysteresis
50
PERCENTAGE OF UNITS
drift of a competitive reference that speciﬁes long-term
drift of 0.2ppm/kHr in its data sheet. Measured data
shows this reference to have drift between 60ppm/
kHr and 150ppm/kHr or 300 to 750 times worse than
claimed.
80
40
30
– 40°C TO 25°C
85°C TO 25°C
20
10
0
–200
–160
–120
–80
–40
0
40
DISTRIBUTION (ppm)
80
120
160
200
DN229 F06
Figure 6. XX780S8-2.5 Industrial Hysteresis
Conclusion
Voltage references from Linear Technology are conservatively and accurately speciﬁed, unlike those from other
manufacturers that intentionally mislead or eliminate key
speciﬁcations to cover shortcomings—shortcomings
that may cause large errors.
The new LT1461 and LT1790 excel in all speciﬁcations
that set system precision. There is nothing left out and
there is nothing hidden.
For applications help,
call (408) 432-1900
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(408) 432-1900
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