24-bit ADC with min 2 channels, simultaneous sampling with START input

Hello,

Would you take a look to see if the ADS131A04 (4-channel) would work for your application?  There is also the ADS131A02 (2-channel).  The ADS131M04 (4-channel) and ADS131M02 (2-channel) feature a sync pin and could also work. 

Both of these devices meet the resolution, channel count, simultaneous sampling, SPI interface, delta-sigma, single-ended input support, dedicated DRDY.  I'm not sure if either directly support the conversion mode you're interested in, but let us know if you have questions on these two devices and we'll go from there. 

Hi Collin,

Thanks for prompt answer.

Yes, I already checked those chips. The problem is with the ADS131A02 does not have SYNC pin while the ADS131M02 has a combined RESET/SYNC pin. Both converter can run in continuous conversion mode only from about 10 fixed output data rate. I do not know how could you start a conversion from a timer, which generates variable sampling frequency. 

Louis

Hello,

Understood and thank you for helping to clarify your expectations.   The ADS127L11 is a wide-bandwidth delta-sigma that can be operated in a more single-shot type mode at the requested sampling rates while achieving a good balance of performance and power. It's only a 1-channel device, but multiple can be simultaneously sampled as shown in this app note:

https://www.ti.com/lit/an/sbaa520/sbaa520.pdf

Hi Collin,

Thanks, that is a very nice chip and it really can be started externally. Using two chips from a single Start pin really solves the problem but the price almost doubled, like the space needed on PCB and then number of the external parts.

My question is: If I pick a simultaneously converting ADC with two channels and SYNC input would the following code work?

1. Disable DRDY interrupt pin of the CPU.
2. Initialize the ADC in continuous conversion mode.
3. When the sampling timer times out (in CPU) then SYNC the ADC using the3 SYNC input. This should restart the continuous conversion.
4. Enable the DRDY interrupt.
5. After the first conversion result is read from the ADC disable the interrupt pin of the CPU again
6. Continual from step 3.

Thank you for your help!

Best regards,
Louis

Hello Lajos,

A continuous conversion delta-sigma ADC can be used in the manner which you are describing. However, each time conversions are restarted, the internal digital filter requires multiple data rate periods to fully settle to within the specified accuracy. For a sinc3 filter, for example, this would require ignoring the first 3 DRDYn falling edges and capturing data on the 4th. If you do not need to acquire samples at a continuous data rate, this can be supported with manual control of the SYNC or START pins. You could also capture all of the data and discard the first three samples. See Figure 51 of the ADS131A04 data sheet as an example:

In Synchronous Slave Mode, the ADS131A04 actually uses the DRDYn pin as a SYNC input (see data sheet page 51). The intention is to connect DRDYn to the host controller or to an external event. A falling edge on DRDYn is expected at the programmed data rate or multiple thereof, such that it remains synchronous with the external clock input. You can essentially control when the ADC provides a conversion this way, but I believe it still needs to be periodic.  Would this be an option for your application?

Regards,

Ryan

Hello Ryan,

Thanks for the answer. It's clear now.

The reason why we need an ADC with SYNC or START pin is we have to take constant number (128) of samples from one full period of sine wave signal, regardless of the frequency of the input sine is 60, 75, 99 or 135Hz. This will not work with a continuously converting ADC on a fixed ODR. Is there better solution than using SYNC/START input ADC?

Thanks,

Lajos

Hello Lajos,

Is there a particular reason you are looking for a delta-sigma ADC? What you are describing sounds like a better fit for a SAR ADC application. Is it driven by the 24-bit resolution and DRDYn requirements?

Let me take another look at products which either support single-shot mode and/or single-cycle settling (referring to the digital filter response time) and see if any of these also support the rest of your requirements.

Regards,

Ryan

Hi Ryan,

Sorry for the long silence I was out of office.

The STM MCU (STM32H723ZG) what we use has 3 simultaneous sampling 16-bit SAR ADCs. It works but the conversion result is terrible noisy and we really need 24-bit resolution. The SAR ADCs are very sensitive to the noise and they might compare wrong.

It looks like we will use 2x ADS127L11 chips started simultaneously by connecting the START inputs to output of the sampling frequency timer in Single Shot mode..

Our analog front end is 2xINA126 Instrumentation Amplifiers but it is single ended (We need large CMRR). Because we measure phase difference between two, same frequency input signal we can't use low pass filters suggested in the user's manual of the ADS127L11 ADC. The low pass filters will cause addition phase shift and temperature dependency of the measured phase. Which is the best way to connect a single ended IN126 to ADS127L11  AD converter? The user's Manual does not contain single ended example schematic.

Many thanks for your help.

Best regards,

Lajos