Analog Devices AN-385 Application Notes

AN-385

a

ONE TECHNOLOGY WAY • P.O. BOX 9106

Make Wide Temperature Range, Ultralow Drift Accelerometers

Using Low Cost Crystal Ovens

Commercial crystal ovens have been used for many
years to stabilize radio frequency oscillators which need
to maintain a near constant frequency over wide tem­perature ranges. These same products can be used to
maintain an accelerometer, such as the ADXL50 and
ADXL05, at a constant operating temperature. This effec­tively provides a very low drift, wide temperature range
acceleration measurement circuit.

The ADXL50 and ADXL05 accelerometers provide a
linear voltage output that varies directly with applied ac­celeration. With the ADXL50, the nominal sensitivity is
19 mV per g centered around a +1.8 volt 0 g offset. The
change in output voltage over temperature (i.e., drift) of
this offset is very small compared with the amplitude of
high g level signals but becomes more significant when
the accelerometer is measuring low g levels. In cases
where a true dc (i.e., gravity measuring) response is
needed, such as when measuring tilt angle, the 0 g offset
drift needs to be kept very low.

The crystal oven specified here, an Isotemp #M050570 is
available for $15.00 (in 100s), is small—only 3/4" in diam­eter by 1/2" high, and operates from the same +5 V sup­ply voltage as the accelerometer. This particular model
maintains the accelerometer at +70 °C, which is just right
for our lowest cost “J” grade devices specified for op­eration over the 0 °C to +70°C commercial temperature
range. These ovens are available for operation at other
temperatures up to +95 °C and with different supply volt­ages. For an “A” –40 °C to +85°C industrial grade device,
an oven with an operating temperature of +85 °C should
be used; this will provide the greatest overall operating
temperature range. Note that, although the oven’s speci­fied tolerance is ±3° C, its typical performance is much
better as long as the accelerometer is carefully fitted to
the oven as described in this application note.

•

NORWOOD, MASSACHUSETTS 02062-9106

by Charles Kitchin

APPLICATION NOTE

617/329-4700

•

bias level is drifting at an average rate of 0.65 mV/ °C;
(note that the drift of this particular unit is a bit higher
than the 0.5 mV/ °C level of a typical ADXL50JH device).
With the accelerometer mounted inside the crystal oven,
the measured drift is reduced to less than 1 mV over
the entire –55 °C to +65°C range. Since the 0 g bias
drift of the ADXL05 is much lower than that of the
ADXL50, even better results can be obtained for low g
measurement.

+15
+10

+5

0

–5
–10
–15
–20
–25

MEASURED 0g UNIT – mV

–30
–35
–40
–45

Figure 1. ADXL50 0 g Drift With & Without Using the
Isotemp Model M050570 Crystal Oven

Because the oven can only apply power to
nal cavity (and cannot cool it down), its preset tempera­ture must be higher than the planned operating
temperature. When the outside ambient temperature
increases to greater than 10 °C below the oven’s preset
temperature—in this case 60 °C (70°C minus 10°)—the
accelerometer’s temperature now begins to vary and its
0 g offset starts to change.

XL50 AND OVEN

UNCOMPENSATED

"J" GRADE ADXL50

TEMPERATURE – °C

heat

+105–35–55 +85+65+45+25+5–15

its inter-

Figure 1 shows the measured 0 g drift of a "J" grade
ADXL50 accelerometer alone and when placed inside a
M050570 crystal oven. By itself, the accelerometer’s 0 g

The oven has a built-in proportional control that regu­lates its temperature: as the outside temperature drops,
more current is applied to its internal heater to maintain
a constant temperature inside the oven. This means
that the oven will consume more power at lower tem­peratures and less at higher approaching zero at its pre­set temperature. Figure 2 shows the measured current
consumption of a typical M050570 (+5 V, 70 °C) model
oven vs. temperature.

600

500

400

300

200

A typical low g measurement circuit using the acceler­ometer/oven combo is shown in Figure 3. In this circuit,
low-pass filtering provided by capacitor Cf, reduces the
measurement bandwidth to approximately 1 Hz, and
lowers the noise floor, improving resolution. The crys­tal oven and accelerometer are both powered by the
same +5 volt supply, although they should be con­nected to the supply using separate leads. This prevents
any transient signals from the current powering the
oven from interfering with the accelerometer's circuitry.

4

C2

0.022µF

C1

0.022µF

COM

0g OUTPUT – +2.5V
3dB Bw – 10Hz

ADXL50 OR ADXL05

2

3

5

6

+3.4V

REF

PRE-AMP

V

PR

8

V

PR

R1

100

TYPICAL CURRENT CONSUMPTION – Ma

0

TEMPERATURE – °C

+105–35–55 +85+65+45+25+5–15

Figure 2. Isotemp Model M050570 Crystal Oven
Typical Current Consumption vs. Temperature

1

1.8V

BUFFER

AMP

10

V

IN–

R2

R3

C

F

C3

0.1µF

9

V

OUT

+5V

V

OUT

ISOTEMP

M050570

+5V

1

3

+VDC

2NC

0VDC

DEVICE FS MEASUREMENT RANGE* OUTPUT SENSITIVITY BUFFER GAIN R1 R2 R3 CF
ADXL50 ±10g 100mV/g 5.26 26.1k 351k 137k 0.1 µF
ADXL05 ±2 g 500mV/g 2.50 40.2k 255k 100k 0.15µF

*FS RANGE NUMBERS ARE CONSERVATIVE TO ALLOW FOR V

0g TOLERANCE.

PR

Figure 3. Low g DC Coupled (Tilt) Circuit Using Crystal Oven Compensation

–2–