Question
In our project we intent to use the ADXL210JE chip to measure the acceleration
of the body, arms and legs of a human been during a football game. We use one
MPU (Atmega128) located in the same PBC with each accelerometer. The MCU has an
internal ADC with 10bit resolution. We actually need data of only 8bit
resolution with rate up to 100S/S. Power supply is 3Vdc. It is O.K. for us to
read the accelerometer data through the PWM path, but we prefer to use two
channels of the internal ADC for this task.
1. Is there any problem to drive pins 6,7 of the accelerometer chip directly to
the ADC?
2. Is there any internal voltage reference for the analog outputs of the chip
(pin 6,7) or they do follow VDD?
3. Is there any reduction in the accuracy of the measurements using the analog
method instead of the PWM method.
Answer
1. Is there any problem to drive pins 6,7 of the accelerometer chip directly to
the ADC?
As the analog outputs of the ADXL2210E are unbuffered, I recommend an opamp to
buffer the outputs prior to the ADC. Suitable opamp would be the OP284.
2. Is there any internal voltage reference for the analog outputs of the chip
(pin 6,7) or they do follow VDD?
There’s no internal reference, so the analog output is a direct function of the
power supply (0dB PSRR). But the part does not need a lot of power so you could
a reference such as the REF195 to provide a stable power supply.
3. Is there any reduction in the accuracy of the measurements using the analog
method instead of the PWM method.
There’s no advantage in accuracy when using the analog output vs PWM. We added
the PWM output to make it simple to interface to micros – al you need is a port
pin and counter per channel.