AN385 - Analog Devices - Datasheet.Live

Make Wide Temperature Range, Ultralow Drift Accelerometers

Using Low Cost Crystal Ovens

by Charles Kitchin

AN-385

APPLICATION NOTE

ONE TECHNOLOGY WAY

•

P.O. BOX 9106

•

NORWOOD, MASSACHUSETTS 02062-9106

•

617/329-4700

Commercial crystal ovens have been used for many

years to stabilize radio frequency oscillators which need

to maintain a near constant frequency over wide tem-

perature ranges. These same products can be used to

maintain an accelerometer, such as the ADXL50 and

ADXL05, at a constant operating temperature. This effec-

tively 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 ac-

celeration. 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 diam-

eter by 1/2" high, and operates from the same +5 V sup-

ply 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 op-

eration 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 volt-

ages. 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 speci-

fied 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

–45

–30

–40

–35

–15

–25

–20

–10

–5

+10

+105–35–55 +85+65+45+25+5–15

TEMPERATURE – °C

MEASURED 0g UNIT – mV

XL50 AND OVEN

UNCOMPENSATED

"J" GRADE ADXL50

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 inter-

nal cavity (and cannot cool it down), its preset tempera-

ture 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.

–2–

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

The oven has a built-in proportional control that regu-

lates 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 tem-

peratures and less at higher approaching zero at its pre-

set 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 acceler-

ometer/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 crys-

tal oven and accelerometer are both powered by the

same +5 volt supply, although they should be con-

nected to the supply using separate leads. This prevents

any transient signals from the current powering the

oven from interfering with the accelerometer's circuitry.

600

100

300

200

400

500

+105–35–55 +85+65+45+25+5–15

TEMPERATURE – °C

TYPICAL CURRENT CONSUMPTION – Ma

Figure 2. Isotemp Model M050570 Crystal Oven

Typical Current Consumption vs. Temperature

ISOTEMP

M050570

2NC

0VDC

+VDC

+5V

VIN–

VPR

VOUT

0.1µF

+5V

VOUT

BUFFER

AMP

1.8V

ADXL50 OR ADXL05

PRE-AMP

+3.4V

REF

COM

0.022µF

VPR

0g OUTPUT – +2.5V

3dB Bw – 10Hz

–3–

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 Fig-

ure 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.

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 com-

pound or a similar material.

The circuit’s scale factor may be user trimmed for

extremely high accuracy by substituting a trim potenti-

ometer 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.

ADXL50HX and ADXL05HX sample packs are available

from any Analog Devices sales office or distributor.

ADXL50 OR ADXL05

ISOTEMP

M050570

SEAL AIRSPACE BETWEEN

OUTSIDE OF OVEN AND

TOP OF PRINTED CIRCUIT

BOARD WITH RED RTV

COMPOUND

PRINTED CIRCUIT BOARD

USE THERMAL

HEAT SINK COMPOUND

ON TOP & SIDES OF

ACCELEROMETER'S CAN

–4–

PRINTED IN U.S.A. E1952–7.5–10/94