ADXL50

REV. B

–11–

As an example, if the desired span is

±2.0 V for a = ±50 g input,

then R3/R1 should be chosen such that

(1)

span is the buffer amplifier’s output, giving

R3 = 2.105

× R1

(2)

In noncritical applications, a resistor, R2, may simply be con-

0 g offset level (see Figure 19). In this simplified configuration

R2 is found using:

R2 = (1.8 V

× R3)/(V

When used with a trim potentiometer, as in Figure 20, resistor

R2 sets the 0 g offset range and also sets the resolution of the

offset trim. A value of 100 k

Ω is typical. Increasing R2 above

this value makes trimming the offset easier, but may not provide

To provide an output span of

±2.00 V, with a 0 g output of

+2.5 V, R1 could be set to the standard value of 49.9 k

Ω and

from Equation 2, R3 = 105 k

Ω.

For Figure 20, the circuit transfer function is:

V

R3

R1

(1.8V – V









+

R3

R2

(1.8V – V









+ 1.8V

The summing amplifier configuration allows noninteractive

1.8 V at 0 g, it will contribute to output offset. Therefore, span

must be trimmed first, followed by 0 g offset adjustment.

OUTPUTS

driving a load to voltage levels approaching that of the supply

rail. However, both outputs are limited in how much current

they can supply, affecting component selection.

µA but

only has a sinking capability of 30

µA which limits its ability to

drive loads. It is recommended that the buffer amplifier be used

±50 g applications, the resistor R1 from V

mended to have a value greater than 50 k

Ω to reduce loading

effects.

pin may be used to directly drive an A/D input or other source

as long as these sensitivities are taken into account. It is always

preferable to drive A/D converters or other sources using the

Buffer Amplifier Output

The buffer output can drive a load to within 0.25 V of either

power supply rail and is capable of driving 1000 pF capacitive

loads. Note that a capacitance connected across the buffer feed-

back resistor for low-pass filtering does not appear as a capaci-

tive load to the buffer. The buffer amplifier is limited to

sourcing or sinking a maximum of 100

µA. Component values

for the resistor network should be selected to ensure that the

buffer amplifier can drive the filter under worst case transient

conditions.

SELF-TEST FUNCTION

The digital self-test input is compatible with both CMOS and

TTL signals. A Logic “l” applied to the self-test (ST) input will

cause an electrostatic force to be applied to the sensor which

will cause it to deflect to the approximate negative full-scale out-

put of the device. Accordingly, a correctly functioning acceler-

ometer will respond by initiating an approximate –1 volt output

braic sum of the two inputs. The output will stay at the self-test

level as long as the ST input remains high and will return to the

0 g level when the ST voltage is removed.

A self-test output that varies more than

±10% from the nominal

–1.0 V change indicates a defective beam or a circuit problem

such as an open or shorted pin or component.

Operating the ADXL50’s buffer amplifier at Gains > 2, to pro-

vide full-scale outputs of less than

±50 g, may cause the self-test

output to overdrive the buffer into saturation. The self-test may

still be used in the case, but the change in the output must then

Note that the value of the self-test delta is not an exact indica-

tion of the sensitivity (mV/g) of the ADXL50 and, therefore,

may not be used to calibrate the device for sensitivity error.

In critical applications, it may be desirable to monitor shifts in

the zero-g bias voltage from its initial value. A shift in the 0 g

bias level may indicate that the 0 g level has shifted which may

warrant an alarm.

POWER SUPPLY DECOUPLING

The ADXL50 power supply should be decoupled with a 0.1

µF

ceramic capacitor from +5 V pin of the ADXL50 to common

using very short component leads. For other decoupling consid-

erations, see EMI/RFI section.

OSCILLATOR DECOUPLING CAPACITOR, C2

An oscillator decoupling capacitor, C2, is used to remove

1 MHz switching transients in the sensor excitation signal, and

is required for proper operation of the ADXL50. A ceramic ca-

pacitor with a minimum value of 0.022

µF is recommended

from the oscillator decoupling capacitor pin to common. Small

amounts of capacitor leakage due to a dc resistance greater than

l M

Ω will not affect operation (i.e., a high quality capacitor is

not needed here). As with the power supply bypass capacitor,

very short component leads are recommended. Although

0.022

µF is a good typical value, it may be increased for reasons

of convenience, but doing this will not improve the noise perfor-

mance of the ADXL50.