ADXL210: Oscillation

Q 

I am using your ADXL210E accelerometer together with the ADXRS300 Gyroscope in
a miniature 6-degrees-of-freedom inertial sensor unit.

There appears to be a problem with the signal of the accelerometers at the
Xfilt and Yfilt pins. I use 47nF for Cx and Cy and 100K Ohms for Rset.

The signals at Xfilt and Yfilt oscillate with a frequency of about 2 kHz and an
amplitude of about 0.2V about the centre 2.5V. The Vdd voltage at the input to
the accelerometer is +5V and rock steady.

This oscillation is transmitted to the PWM outputs resulting in unusable PWM
signals.

I need a bandwidth of about 100Hz , what can I do to eliminate this
oscillation.

 

A 

100kohm Rset and 47nF for Cx Cy is an acceptable combination and the ADXL210
outputs will not self oscillate for any value of capacitor above 1nF. However,
note that the analof output of the accelerometer is a high impedance note
(nominal 32kohms output impedance) and so the output should be buffered. 47nF
is probably in the region which will make a high impedance opamp oscillate when
placed on the input and configured for unity gain – unless you implement some
bandwidth compensation in your opamp.

Try operating the ADXL210 with only the Cx Cy caps connected,
Assuming you have an opamp buffering the ADXL, try operating the opamp with a
47nF cap on the input and review this circuit for stability.