AN-385: Wide Temp. Range, Ultralow Drift Accelerometers Using Low Cost Crystal Ovens PDF

a
ONE TECHNOLOGY WAY
AN-385
APPLICATION NOTE
• P.O. BOX 9106 • NORWOOD, MASSACHUSETTS 02062-9106 • 617/329-4700
Make Wide Temperature Range, Ultralow Drift Accelerometers
Using Low Cost Crystal Ovens
by Charles Kitchin
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.
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
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
MEASURED 0g UNIT – mV
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.
0
–5
XL50 AND OVEN
–10
–15
–20
–25
UNCOMPENSATED
"J" GRADE ADXL50
–30
–35
–40
–45
–55
–35
–15
+5
+25
+45
+65
+85
+105
TEMPERATURE – °C
Figure 1. ADXL50 0 g Drift With & Without Using the
Isotemp Model M050570 Crystal Oven
Because the oven can only apply power to heat its internal 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.
600
TYPICAL CURRENT CONSUMPTION – Ma
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.
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.
500
400
300
200
100
0
–55
–35
–15
+45
+65
+85
+5V
1
ADXL50 OR ADXL05
+25
Figure 2. Isotemp Model M050570 Crystal Oven
Typical Current Consumption vs. Temperature
4
C2
0.022µF
+5
TEMPERATURE – °C
+5V
1.8V
C3
0.1µF
2
PRE-AMP
BUFFER
AMP
C1
0.022µF
9
3
VOUT
VOUT
1 +VDC
COM
5
6
+3.4V
REF
10
8
3 0VDC
VIN–
VPR
2 NC
ISOTEMP
M050570
VPR
R3
R1
R2
0g OUTPUT – +2.5V
3dB Bw – 10Hz
CF
DEVICE
FS MEASUREMENT RANGE*
OUTPUT SENSITIVITY
BUFFER GAIN
R1
R2
R3
CF
ADXL50
±10g
100mV/g
5.26
26.1k
351k
137k
0.1µF
ADXL05
±2g
500mV/g
2.50
40.2k
255k
100k
0.15µF
*FS RANGE NUMBERS ARE CONSERVATIVE TO ALLOW FOR VPR 0g TOLERANCE.
Figure 3. Low g DC Coupled (Tilt) Circuit Using Crystal Oven Compensation
–2–
+105
ISOTEMP
M050570
USE THERMAL
HEAT SINK COMPOUND
ON TOP & SIDES OF
ACCELEROMETER'S CAN
SEAL AIRSPACE BETWEEN
OUTSIDE OF OVEN AND
TOP OF PRINTED CIRCUIT
BOARD WITH RED RTV
COMPOUND
ADXL50 OR ADXL05
PRINTED CIRCUIT BOARD
Figure 4. Mechanically Mounting the Crystal Oven to the Accelerometer
The circuit can be built on a small card such as the 0.8 ″
square ADXL50 evaluation card. After the circuit has
been built up and is operating properly, the crystal oven
can be installed over the accelerometer as shown in Figure 4. After applying silicon heat sink compound to the
top and sides of the accelerometer’s can (being careful
not to use too much or get any compound on the leads),
slide the oven over the accelerometer until it is firmly
seated against the bottom edge of the can.
The circuit’s scale factor may be user trimmed for
extremely high accuracy by substituting a trim potentiometer and metal film resistor combination for R1.
This adjustment should be made only after the
accelerometer’s temperature has settled out at the
oven’s preset temperature.
The crystal ovens may be purchased ( and Free samples
are available) from Isotemp Research, Inc., P.O. Box
3389, Charlottesville, VA. 22903. Phone: 804-295-3101,
Fax 804-977-1849.
Then run the oven’s leads through the breadboard and
solder them to the bottom side of the board. The oven
has three leads: one is +5 V, the center lead is not used,
and the third is power supply common (ground). Be sure
that the +5 V supply lead used for the oven is a separate
wire from that powering the accelerometer. The last
step is to seal any air gap between the bottom of the
oven and the top of the PC board using red RTV compound or a similar material.
ADXL50HX and ADXL05HX sample packs are available
from any Analog Devices sales office or distributor.
–3–
–4–
PRINTED IN U.S.A.
E1952–7.5–10/94
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