ADS131M08EVM: Connecting an Accelerometer

Hi Larry,

Thank you for your post. 

The ADS131M08 accepts an absolute input voltage from -1.3 V to AVDD at Gain < = 4. For Gain > 4, the upper range reduces to AVDD - 1.8 V. This is the voltage that can be safely applied to a single input pin. Differentially, the voltage between AINxP and AINxN must be less than +/- Vref / Gain. This is the voltage which the ADC will convert to a binary value.

The ground of your accelerometer breakout board should always be reference to the ADC analog ground (AGND) since these are analog input signals. 

Are you able to scale down the output of the accelerometer? For example, if the output scaled down to 0 V to 2.4 V over the span of -3g to +3g, then you could apply 1.2 V to AINxM (assuming 1.2 V = 0g) and the corresponding differential voltage (AINxP - AINxN) would range from -1.2 V to +1.2 V. You would then only use Gain = 1 in the ADS131M08.

Regards,

Ryan

Hi Ryan,

 Thanks for the quick reply. If I understand this correctly, as long as I keep the Gain <= 4 there should be no risk of damage to the ADS131M08EVM due to the accelerometer voltages.

 I mentioned previously that when I connect my accelerometer breakout to the ADS131M08EVM that the voltage drops significantly. My connections/jumpers are as follows:

- 5V supply to the accelerometer breakout comes from the Arduino (I would like to get this from the ADS131M08EVM if possible)

- ground for the accelerometer breakout comes from the ADS131M08EVM AGND

- measure one accelerometer with axis oriented at 0 g shows approx. 1.647 V on the voltmeter (as expected, approx. midpoint of 3.3 V)

- connect the accelerometer signal to the ADS131M08EVM J1 AIN0P

- jumper placed on JP1 5-6

- voltage level on the voltmeter drops to 0.096 V

- voltage level measured on the ADS131M08EVM shows 0.0965 V (4096 samples, 4k sampling rate, Vref = 1.2 V, OSR = 1024, PGAGAIN0 = 1, 24 bits)

 Is the reason for this voltage drop when the ADS131M08EVM is connected due to the interaction between the accelerometer output impedance (32k) and the two 1k resistors in the ADS131M08EVM (connected to ground by the JP1 jumper)? This 2k and 32 k voltage divider should reduce the accelerometer voltage from 1.647 V to 0.097 V, just like I measured. To explore this further, I replaced the accelerometer with a 10k pot (still using the 5 V supply from the Arduino) and adjusted it to vary the voltage input to the ADS131M08EVM from 0 to slightly higher than 1.2 V. The ADS131M08EVM read the same voltages as my voltmeter but only up to 1.2 V. Note that these voltages agreed with the voltage divider calculation using the pot and the two 1 k resistors. Note that these voltages were also much higher when the ADS131M08EVM was not connected.

 So, it seems like I do not need to scale the accelerometer signals because they are already scaled by the two 1k resistors. In fact, they are scaled much more than is necessary. If an accelerometer voltage of 1.647 V converts to about 0.097 V then a fullscale level of 3.3 V (for +3 g) will be about 0.193 V, well below the limit of Vref (1.2 V).

Does all of this make sense?

 If so, is there a way to boost the accelerometer signals instead of scaling them down (e.g. different resistors in the ADS131M08EVM)? I did try changing the PGAGAIN0 to 2 and 4, and the signal changed accordingly on the ADS131M08EVM (but of course not on the voltmeter). I suppose I could select a higher gain, but I will have to recalculate the safe levels first.

What is the reason for limiting the reference voltage to 1.2 V and can it be increased?

Thanks,

Larry

Hi Ryan.

If I leave the jumpers uninstalled, the voltage on my voltmeter drops from 1.647 V to 1.571 V (not sure why) and pins 1 and 3 are the same level. This means the ADS131M08EVM measures 0 V.

If instead I connect the jumper to JP1[3-4] then I am using only one of the 1k resistors and the voltage drops to 0.0497 V (i.e., about half of the previous 0.097 V). This agrees with my MicroCap simulation of the circuit. It seems I still need a jumper and putting it on JP1[5-6] provides the highest signal.

Thanks,

Larry