a ONE TECHNOLOGY WAY AN-380 APPLICATION NOTE * P.O. BOX 9106 * NORWOOD, MASSACHUSETTS 02062-9106 * 617/329-4700 Compensating for the 0 g Offset Drift of the ADXL50 Accelerometer by Charles Kitchin and Paul Brokaw A Software Approach Using a P Interface For those applications where the ADXL50 output drives a P, it can be used to subtract out the 0 g drift over temperature. This can be indirectly approximated by using the formula: INTRODUCTION The ADXL50 accelerometer has a nominal sensitivity of 19 mV per g of applied acceleration. This is centered around a +1.8 volt offset. The offset will typically drift 35 mV over a 0 to +70 C temperature range. This drift is very small compared with the amplitude of high g level signals but becomes more significant as the measured acceleration level decreases. For applications not needing a dc (i.e., gravity sensing) response, ac coupling between the preamplifier and the on-chip buffer amplifier will eliminate almost all of the 0 g drift. But, in cases where a dc response is needed, an external temperature compensation circuit will greatly improve the low g performance of the accelerometer. VO g in mV = ((1.3 x 10 -5) T 3 ) + ((2.3 x 10 -3) T 2) - (0.08 T ) - 0.29 where T is the temperature in degrees centigrade, or by directly digitizing the output of a temperature sensor, using an ADC. In the circuit of Figure 1, an AD590 temperature sensor and a 1 k resistor are added to the board containing the accelerometer. The AD590 provides a 1 A/K current output which, together with the 1 k resistor, provides a 1 mV/K output to the P. For best temperature tracking, the AD590 should be attached to the case of the ADXL50. The outputs of the ADXL50 and the AD590 both run to the P. The circuit is then placed in an oven and operated over temperature; the P then stores the drift curve in its memory and subtracts it out for all succeeding measurements. This application note shows how to compensate for the linear component of the 0 g drift, using either a software or a hardware approach. C2 4 C1 +5V 1 ADXL50 0.022F C3 0.1F +5V ACCELERATION OUTPUT TO P 1.8V 2 0.022F BUFFER AMP PRE-AMP AD7890 3 9 C1 12-BIT MULTIPLE INPUT ADC VOUT 5 +5V COM 6 +3.4V REF VPR 10 VIN- 8 49.9k 0g LEVEL 50k ADJUST AD590 1A/K 499k 100k 1k 1mV/K TEMPERATURE OUTPUT Figure 1. Acceleration & Temperature Outputs to P for Software Correction of 0 g Drift TO P C2 2 0.022F 3 9 TEMPERATURE COMPENSATED ACCELERATION OUTPUT 5 8 6 +3.4V REF VPR R5 310k R7 310k 10 R1 49.9k VIN- R3 499k VOUT +5V TEMP AD590 REFERENCE R4 500 1A/K +5V 3 RB 10k 2 7 6 AD820 1k TEST POINT "A" R2 49.9k TEMPCO AMPLIFIER BRIDGE BALANCE R6 30k AT TMIN OR LOWER TEMP CAL. POINT: 1. SET RB ALL THE WAY TO ONE SIDE. 2. ADJUST RA FOR +3.4V AT TEST POINT "A." 3. SET RC FOR +2.5V VOUT (AT PIN 9 OF ADXL50). TO TEST THE CIRCUIT: 4. TEMORARILY CONNECT A 1.5k RESISTOR BETWEEN THE WIPER OF RB AND COMMON. 5. ADJUST RB FOR +2.5V AT VOUT . 6. REMOVE THE 1.5k RESISTOR. VOUT SHOULD NOT CHANGE. 0.1F TC COMP SET 4 RA CALIBRATION PROCEDURE: BUFFER AMP PRE-AMP COM 0.1F C3 1.8V C1 C1 +5V 1 ADXL50 E1913-12-5/94 4 0.022F R8 30k AT TMAX OR UPPER TEMP CAL. POINT: 7. GO TO TMAX OR HIGH TEMP CAL. POINT. 8. READJUST RB FOR +2.5V @ VOUT. 9. CALIBRATION COMPLETE. 0g OUTPUT LEVEL R9 20k RC 20k R10 25k Figure 2. ADXL50 0 g Drift Compensation Circuit A Hardware Approach The circuit of Figure 2 provides a linear temperature compensation for the ADXL50. Figure 3 shows the 0 g drift over temperature for a typical ADXL50 with and without this circuit. As shown by Figure 3, the linear portion of the drift curve has been subtracted out. In effect, the curve has been rotated counter clockwise until it is horizontal, leaving just the bow of the curve: that portion which is not linear. C2 4 As shown in Figure 3, over a +25 C to +70 C range, a 10 x reduction in drift is achieved. The circuit of Figure 2 is essentially a temperature sensor coupled to a forcedbalance bridge. The AD590 provides a 1 A/K current output whose voltage scale factor is set by resistor RA. The bridge circuit subtracts out the nominal 298 mV output of the AD590 at +25 C and leaves only the change in temperature, which is what is needed. 0.022F ADXL50 The output from the AD590 connects to the wiper of trim potentiometer R B. Since R B is across the input terminals of the op amp, the circuit can provide a variable output with temperature in either the positive or negative direction. The op amp output is divided down by resistors R9 and R10 which limit the range of trim potentiometer RC and increase its resolution. Resistors R1 and R3 set the ADXL50 accelerometer's gain at ten (190 mV/g) which is appropriate at low g levels, while R2 and R3 set the gain of the compensation circuit. Resistors R5 and R6 form a resistor divider (one half of the bridge) which divides down the +3.4 V reference output of the ADXL50 to 0.3 V which appears at the noninverting input of the AD820 op amp. Resistors R7 and R8 form the other half of the bridge, and because they have the same ratio as R5 and R6 the op amp will have a +3.4 V output at room temperature. Therefore, the op amp is across the output of the bridge and any imbalance will cause its output to change enough to maintain the summing junction at 0.3 V, which keeps the bridge in balance. -2- PRINTED IN U.S.A. Figure 3. ADXL50 0 g Drift With & Without the Compensation Circuit of Figure 2