• State TI Thinks Resolved
  • Locked Locked
  • Replies 1 reply
  • Answers 1 answer
  • Subscribers 46 subscribers
  • Views 1038 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

ADS1158: ADS1158IRTC

zvi Peshkess
Prodigy 40 points
Part Number: ADS1158

we are using the ADS1158 ADC together with MAX4564 Analog switch to switch 'AinCOM'

between external floating device GND and local system measurements GND. We have observed the following pulses on the AinCom line when connected to the 

MAX4564.

See images:

The only noteworthy activity on the  chip is..   The negative 2.5V goes slightly positive for about 60ms before dropping to -2.5V.​

 However, MeasGnd  that goes to the ADC,  has quite an unusual spike at power-on, 

 

as well as 2 strange dips after about 6 seconds ( very consistent )

 

here is zoomed up image of the last two neg pulses train ..

 

Here is a link to a video of the screen showing the "pulses " are actually periods of ADC measurements and the GAP between the measurements.

https://goo.gl/wUcoEg​

 

A zoom of the first dip is here..     I'm thinking of lifting pin MeasGnd on the chip to see if the source is from u58 or the adc..  

 https://goo.gl/wUcoEg​

 We  have lifted the MAX4654 and measure the AinCOM  and there were NO pulses ... ALso, There is NOTHING strange on the MAX4654 when it is disconnected from the ADS1158 !!

What could be the problem ?  

Note:  We connected Pull down resitor on the ADC AinCom to GND and the problem disappeared ... however it happens on different test enviorement.

this is the schematics of the ADC section.

  • 0
    TI__Mastermind 44620 points
    Hi zvi,

    Thanks for sharing the schematic!

    The first thing that stands out to me is that there are separate nets for DGND and AVSS ("GND" an "GNDA", respectively) and no indication in the image of how these nets connect. Do these nets connect anywhere in your schematic?

    What I think may be happening is that the "GND" and "GNDA" grounds are floating with respect to one another, except for perhaps a high-impedance discharge path that takes time to settle. This may be the reason why AINCOM appears to have these odd pulses. When measuring these pulses on the oscilloscope, which ground net where you using as the reference?

    Somewhere in the circuit, the "GND" and "GNDA" nets need to be shorted. Generally, it is best to use a single ground plane to ensure the lowest impedance connection between the analog and digital grounds. We typically try to discourage splitting the analog and digital ground planes; however, if you must split these grounds, then make sure to short them together right at the ADC. You might try shorting the DGND and AVSS pins of the ADS1158 and checking again to see if these pulse still occur when there is a low-impedance connection between these ground planes.

    Best regards,
    Chris