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Hello TI engineers and all others,
I have some follow up questions to issue of incorrect operation of the ADS1225/ADS1226 when repeatedly powered-up as stated in this previous forum posts (link1, link2).
We encounter the issue, that the conversion times are higher than stated in the datasheet with max. 13.3ms (measured up to 14.6ms) with some fluctuations at the beginning when repeatedly powered-up. This leaves us with permanent incorrect measurements when this issue appeared after power-up (START pin is not permanent high, see InitFailure.PNG and InitSuccess.PNG). It is not repeatable on all boards; on other boards between 10 and 100 repeated powers-up are necessary until this issue appears.
As it is stated in these forum posts, there might be an unwanted charge left in the POR circuitry when repeatedly powered-up (and I guess this causes the internal oscillator to run incorrectly) resulting in this issue. We put a 1k bleeding resistor on DVDD, which resulted in reaching 0V after ca. 500ms (see 3V3VDD_Shutdown_1k_Pulldown.png). Unfortunately, this did not improve the appearance of this issue. Same is true for also putting a pull-down resistor on AVDD.
Now I have some follow-up questions:
Schematic: We use a 3.3V LDO on AVDD and a 3.3V switching regulator on DVDD, which powers up cleanly 60ms after AVDD. AVDD and the reference voltage powers-up simultaneously. Buffer is not enabled and high-speed mode is selected.
Thanks in advance,
Benjamin
InitSuccess.PNG
InitFailure.PNG
3V3VDD_Shutdown_1k_Pulldown.png (yellow line with 1k pull-down)
Hi Joseph,
thanks for you detailed and fast reply. The other trace on the oscilloscope image shows the reference trace without the 1k pull down resistor on DVDD for comparison.
Attached you'll find snippets of the relevant schematic and layout parts. The added oscolliscope shows the voltage power-up of 2.048 reference voltage (CH1), AVDD (CH2) and DVDD (CH3).
We plan endurance testing (no. failures / power-ups) to get some quantitive data the next days to compare this assembly values with these possible improvements: 1k pull down resistor on DVDD, AVDD and reduced decoupling capacitance 10µf/1µf). I'll get back to you end of week.
BR Benjamin
Hi Joseph,
I've got a quite interesting development.. We were preparing for the endurance test (controlled re-powering to quantify the frequency of occurrence of this issue) and used a different initialization sequence. During our investigation we tried different initialization sequences (calibration, 25th clock before beginning of operation, etc..), but found out that with an initial calibration the issue still appeared and left it out. We do not need the calibration of the ADC for accuracy.
Now with this new sequence the issue did not appear any more (4000 automated tries on one board). On board with the same setup, the issue appears every 20-60 power-ups with the old sequence, but not once with the new sequence.
How do you explain that with the POR circuitry? Is the capacitance somehow drained away at the beginning when permanent conversion is activated at the beginning?
We have a hard time to continue investigating this and we would appreciate you looking a bit more into that and maybe explaining this reproducible behavior. Is this new sequence permanently resolving this issue? Thanks!
Old beginning sequence direct start of conversion: Issue often reproducible on multiple boards
New sequence with calibration and short period of permanent conversion: Issue not any more reproducible
Benjamin,
In my experience with this device, it has been the sizing of the capacitance and how fast these decoupling capacitances are discharged that allow for device to start up correctly once power is restored. I'm still surprised that changing the capacitances doesn't have any affect at all. With your change in sequence, it's possible that there is more current drawn on with the START pin initially high.
I can help in this investigation, but I'm not sure how to verify this problem and duplicate it. I can get my own board and see if I can power up the device in a similar manner, but I'm not sure if I have a reliable control to power up the device in any particular sequence. Additionally, I'll need your exact sequence and timing. I'm still a bit unclear as to when the power is turned on and how long the START is held before you run the calibration, and then how long you continue the conversion before you begin pulsing the START pin to acquire real data.
I'm also a bit unsure how the good power up sequence differs from the first failed sequence you show in your post. For the good power up sequence, you start with a calibration, a set of conversions, and then begin pulsing the START pin after a short delay:
For the first failed sequence you've shown, you have the same calibration, a set of conversions and then begin pulsing the START pin:
There isn't a delay, but by this time, the device should have already come up to a stable state. However, there isn't a /DRDY indication going low, even though the conversion time shown in the previous set of conversions should have shown the conversion to have completed (based on the alignment of the START pulses and the following SCLK pulses.
The one other thing different about the first failed sequence is that there is a wide initial /DRDY pulse. This might indicate when the power supply first started up, but I found it unusual that the /DRDY powered up in the high state.
As I mentioned earlier, I may be able to duplicate the problem, but I'll need as much information about how this board is constructed. The problems that you see are not digital problems but rather involve some sort of analog settling within the digital section.
On another note, I would consider the ADS1220 as the next generation device to the ADS1226 and it does have better specifications. I also know that it has a more reliable POR setup than the ADS1226. The POR circuitry was designed for different power-sequencing conditions including brownouts as well as standard power-up. It was tested more completely based on what we had learned from the other devices in the product line. However, it still has power supply recommendations to prevent the digital from coming up in an unknown state. Regardless, for all those reasons, I would use the ADS1220 over the ADS1226.
Joseph Wu