REV. 0

ADXL190

–4–

APPLICATIONS

All the circuitry needed to drive the sensor and convert the

capacitance change to voltage is incorporated on-chip requiring

no external components except for standard power supply de-

coupling. Both sensitivity and the zero-g value are ratiometric to

the supply voltage, so that ratiometric devices following the

accelerometer (such as an ADC, etc.) will track the accelerom-

a function of both the acceleration input (a) and the power

Adjusting the 0 g Bias Level

In some cases the user may have an asymmetrical input or may

want to fine adjust the zero-g output level to obtain maximum

dynamic range. The zero-g level is adjusted by supplying a

voltage to the zero-g adjustment pin (see Figure 2).

2

25k

5k

ADXL190

ZERO

FILTER

GAIN = 3

ACCELERATION

SIGNAL

200k

C2

0.1 F

Figure 2. Optional Zero-g Adjust Circuit Detail

Any voltage difference between the zero-g adjustment pin and

This is then gained by the factor of 3 in the output stage for a

total gain of 0.5 for the zero-g adjustment. (Note: The ratio of

the resistors in the divider is consistent from part-to-part; how-

ever, the absolute values can have a

± 30% tolerance). The

zero-g adjustment voltage can be set up by a variety of methods

including a potentiometer (as shown in Figure 2), a PWM sig-

nal, or with a simple three-state output.

The simplest way is by adding a resistor between the ZERO g

Offset (V) = (7.5

× V

where R is in k

Ω and connected to V

Offset (V) = (–7.5

× V

where R is in k

Ω and connected to ground.

Resistors may also be connected to microcontroller I/O pins as

or three-state, there are seven possibilities as shown in Table I

Using such a system, any ADXL190 may be user trimmed to

output 2.5 V

± 35 mV at zero g.

= 5 V

Offset Voltage

P1

P0

Produced

Offset in g

Three-State

Three-State

0 mV

0

Three-State

0

–71 mV

–4

0

Three-State

–134 mV

–7.4

0

0

–191 mV

–10.6

Three-State

1

71 mV

4

1

Three-State

134 mV

7.4

1

1

191 mV

10.6

Another way to adjust the zero g offset is to supply a voltage to

the voltage at the ZERO g ADJUST pin is reduced by a factor

of 6 (as a result of the internal 5 k

Ω and 25 kΩ voltage divider)

and then multiplied by a factor of 3 in the output stage of the

ADXL190 resulting in a total gain of 0.5. Offset is thus de-

scribed by the following equation:

This voltage may be produced by a variety of methods includ-

ing a PWM signal from a microcontroller. Care must be taken

that the output impedance of this voltage source is less than

5 k

Ω and that there is very little ripple (noise). Any noise at the

ZERO g ADJUST pin will cause output errors.

If an asymmetric range of acceleration is required (e.g., +75 g

to –125 g) a resistor may be connected between the ZERO g

For a range of +75 g to –125 g the offset required is –25 g.

–25 g at 18 mV/g = 450 mV of offset is required.

Rearranging the offset equations above:

R = [(7.5

× V

Ω connected to ground.

For asymmetric operation the g range midpoint may be shifted

up to

±80 g typically.

500k

250k

ZERO

g

ADJUST

ADXL190

MICROCONTROLLER

P1

P0

Figure 3. An Offset Adjustment Scheme