ADS8866: Sometimes no data on DOUT

Hello Carsten,

Can you send details on how your are switching off the 3V reference?  It seems like the device is getting into a latchup mode, which could  be caused when the reference is switched off and a voltage is present on the analog inputs.

I am going to close the other post since it is a duplicate of this one.

Regards,
Keith Nicholas

Precision ADC Applications

Hello Keith,

we switch 3V for reference and opamps driving the analogue Inputs of ADC together.

For testing I cut Pin1 from switched 3V and connect it to Pin2. But the error remains sporadically.

In our design we have connected Pin2 and Pin10 (AVDD, DVDD) together on 3V.

If the error occurs only a power on reset solves the Problem, if not we can communicate for hours (4 times a second 20 measurments).

So in my opinion something went wrong at power up. We had a similar Problem with digital pressure sensor LSP22 from ST. That chip has

no reliable Reset circuit inside, so sometimes no valid pressure/temperature values are read. Only a power on reset solves that behaviour.

Because of that we cycle the LPS22 from a µC port pin (LPS22 draws only a few µA).

Best regards

Carsten Glahe

Hello Keith,

I made some measurments on 3V supply voltage. If I cycle power supply so fast that 3V supply will not go below approx. 1,2V, then DOUT of ADC is 0V.

But there is no RESET pin on ADS8866.

Hello Carsten,

There is an internal reset.  Section 11 of the datasheet has the following requirements.

During normal operation, if the voltage on the AVDD supply drops below the AVDD minimum specification (2.7V), then TI recommends ramping the AVDD supply down to ≤ 0.7 V before power up. Also, during power-up, AVDD must monotonously rise to the desired operating voltage above the minimum AVDD specification.

If you are not meeting the above requirements during power-up or normal operation, then a full power-down would be required.

Regards,
Keith

Hello Keith,

thank you for your answer. We supply the ADS8866 from TPS70930 LDO. That device meets all our requirements:

- Input voltage min. 24V

- 3V fixed output

- low Iq

- over temperature protection

- low dropout

- overcurrent protection

But the Feature "reverse current protection" leads to ADC failure as discussed here. I looked for another LDO with "Output discharge"

function and I only found LT3061 from Linear.

Another idea is to use a load Switch, such as TPS22917. I would like to place it between LDO Output and the complete circuit and connect "ON" Signal to

LDO Output (or via a voltage divider to set trip Point to approx. 2,7V). I hope that the load Switch disable 3V and discharge it, if 3V LDO is smaller than 2,7V.

Is it possible to use the load Switch for that purpose?

Best regards

Carsten Glahe