ADS1299: Connecting two Devices without synchronized Clock

KAMRAN RASOOL

Part Number: ADS1299

Hello Everyone

We have developed a board comprising of two ADS1299 and controller. As per my understanding of guidelines given by datasheet and TI forums, for multiple device

Clock should be synchronized across all ADS1299. One posible way is to let one ADS1299 runs on internal oscillator and export its internal clock to other ADS1299.
Use START signal to synchronize conversion for all devices.
Check DRDY signal of any device for data availibility.

Unfortunately we initially designed our board in a way that both ADS1299 run on their internal oscillator independently. Both devices DRDY signal are checked for their respective data availability. Block diagram of connection is follow.

Now when we run our both ADS1299 at 250Hz rate, their is periodic jitter (expected time is 4000 usec between data packets) as shown below. Clearly this phenomenon causes packets to miss.

Latency

So now my question is that is their any thing i can do without redesigning my PCB to solve this issue or i must use synchronized clock source (Resulting in PCB redesign).

Really appreciate any help in this regard.

Thanks

Kamran

over 3 years ago

0 TCT over 3 years ago

TI__Genius 11550 points

Hi Kamran,

Thanks for the E2E post.

How do you control the conversion for the two ADS1299? Are you using the START signal or by using the SPI command?

If you apply the same START signal to both of the ADS1299, the DRDY pulse should still be within the +/-CLK period even though both ADS1299 operates with the internal oscillator. You can set the CLK_EN bit in the CONFIG1 register to enable the oscillator clock to be output in the CLK pin. Using an oscilloscope, you can observe the relationship of the CLK, START, and DRDY signals for both of the ADS1299 devices for clarification.

Thanks.

-TC

0 KAMRAN RASOOL over 3 years ago in reply to TCT

Prodigy 55 points

Thanks TCT for timely reply.

We are using START signal to control the conversion simultaneously for both ADS1299.

Well i do have some doubt that when both ADS1299 operates with internal oscillator, DRDY pulse will be within the +/-CLK period. I checked DRDY signals on scope for both ADS1299 while running on their respective internal oscillators. DRDY signal of one ADS1299 was continuously shifting in time with respect to other, although both were given START signal simultaneously. This makes sense as internal oscillator of ADS1299 runs at 2.048 MHz. Its accuracy given by datasheet (page 10, section 7.5) is +/- 0.5%.

So one ADS1299 may be running slightly faster than other ADS1299. Their DRDY signals won't be synchronized.

Also in the original question post ADS1299: ads1299. Connection 3 ADS1299 by one SPI - Data converters forum - Data converters - TI E2E support forums, it was suggested that " Please ensure the CLK signals across the devices are also synchronized". So synchronization of clock across the board can be achieved either by external oscillator or exporting one ADS1299 clock to all other ADS1299 devices.

I may be missing some point. Any help will be highly appreciated.

Kamran

0 TCT over 3 years ago in reply to KAMRAN RASOOL

TI__Genius 11550 points

Hi Kamran,

Thanks for the information. We'll get back to you this week.

Thanks.

-TC

0 TCT over 3 years ago in reply to TCT

TI__Genius 11550 points

Hi Kamran,

Thanks for the patience and understanding.

I agree with you on your description of the DRDY signal. With the internal oscillator, the DRDY signal will continuously be shifting in time to the other if you are trying to do continuous data capture with a single START pulse to both devices. The phase and frequency difference between the two internal oscillators will cause the devices' DRDY pulse to become unsynchronized eventually. To synchronize multiple converters, you will need to provide a synchronous CLK and START pulse to both devices, as shown in Figure 69 of the datasheet.

The DRDY pulse will be within the +/-CLK period for the first DRDY if you use the same START signal for both devices regardless of their internal oscillator phase difference. It is still useful for synchronizing multiple converters in the Single-Shot conversion mode.

Thanks.

-TC

0 KAMRAN RASOOL over 3 years ago in reply to TCT

Prodigy 55 points

Thanks TCT for clearing the point.

For multiple ADS1299 devices, a synchronized source of CLK is required for continuous data acquisition mode.

This solves my query.

Regards

Kamran