ADS7142: is it normal to get a short time pulse on Autonomous mode?

Hello,

Would you share what is driving the ADC input? Also, would you recreate this using a known DC value?

I would like to see if the driving source may not be able to drive the input accurately, especially when the input channels are switching.

Regards

Cynthia

Dear Cynthia,

Thanks for your reply, my customer tried 0.7V input to ADC, there was around 1.28V short-time pulse similiar as the picture above, then back to normal 0.7V

It is only shown after Autonomous modes –> start sequence. If they set to manual mode, then there wasn't this pulse, always 0.7V.

They want to what's the difference bwteen the Autonomous modes and manual mode, is there possible reason to cauce this? Thanks a lot

Dear Cynthia,

May I know your advices for this case, if you have demo code of ADS7142 DWC configration, could you kindly send me for doule check?Thanks a lot.

Unfortunately, there is no sample code for this device. 

The difference from autonomous mode and manual mode is what starts a conversion. In autonomous mode the device starts a conversion internally while in manual mode the host start a conversion. When in autonomous mode, it is recommended that the input is driven by a high impedance source, usually an op amp.  Would you share  what the driving input circuit is? What configuration is the device being used in? If it is two single ended channel configuration, do you see this on only one channel or both? 

Would you share how the device is being set up to start auto sequence in autonomous mode. This spike is only seen at the beginning when the device is set up correct? It is no longer seen as the device continuous to be in autonomous after? Ddisabling the high threshold can help to confirm this

Regards

Cynthia

Dear Cynthia,

Thanks for your support. Below is the circiut for the test. Yes, the spike is only shown on the set up stage after start auto sequence in autonomous mode.

I'll check the configration and try your mentioned several test way to verify the result, then reply you later.

Dear Cynthia,

I am the customer of Bruce Li. 

As you said,  it is recommended that the input is driven by a high impedance source, usually an op amp. Is it necessary? However, We just built a  temporary circuit without driving by an op amp.  Could you tell the reasons why the input need driving by a high impedance source ?     

Could you tell the way of disabling the high threshold. I could not see any  setting for this in the datasheet.

Regards

Joury Lai

Hello Bruce,

The input of a SAR ADC in a switching capacitor circuit, which means the input circuits needs to be able to hold the voltage and charge the sample and hold capacitor within the acquisition time.

Have you tested the op amp circuit to see if this spike is still seen?

To learn more about how to drive a SAD ADC, we have put together  SAR ADC Input Driver Design as part of our video series: TI Precision Labs.

This will provide deeper detailed and how to design the driving circuit.

As for the high threshold, this was a suggestion for debugging. I was suggesting to not enable the high threshold or set at max input, this way the autonomous mode would not be stopped by an alarm, and the input signal would still be monitored for future spikes.

Regards

Cynthia

Dear Cynthia,

Thanks for your answer.  We will try to test the op amp circuit . But, I still have a question about  why the op amp sircuit could prevent the spike. Several days ago, I made a experiment that no circuits connected to the AINP/AIN0, and measure the waveform of AINP/AIN0.(as illustrated in following figure ) 

I found it still have  a short time pulse on the input pin(AINP/AIN0)when it works on Autonomous mode, after set "Start sequence"  though the ads7142 have no any input singnal. I could not understand why and how the spike appear.  Is there only one way to prevent the spike by adding the  op amp circuit? 

I looking forward to your reply. Thank you .

Regards

Joury

Hello,

I am reaching out the design team to get more details about the device behavior when a sequence is started, to see if there is any thing that might cause these.

Please let me know how the op am circuit performs

Regards

Cynthia

Hi, Cynthia

Our electronics engineer said it is abnormal situation for ads7142 that it has a spike when working on Autonomous mode, after set "Start sequence"  though the ads7142 have no any input singnal. He thought the spike is generated by ads7142 and he couldn't understand why adding the op am circuit could help to prevent the spike. It was a pity that he  refused to help adding  the op am circuit for test.  We may think another solution. 

Regards

Joury

Hello Joury

Apologies for the late response, but our team has not found that there is reason the device would behave this way

Have you been able to move forward with this

Regards

Cynthia

Hi， Cynthia

Sorry for that, I could not move forward with this. Because this problem is almost no influence for the moment  in our actual items.  So, have you ever  verified this problem ?  Maybe it was caused by abnormal circuit design.