AD7124 Always reads zero. RDY/IDLE bit is always 1

Hi, 

Apologies. This is just a common steps I prefer to check if the ADC is converting and communicating properly with the MCU. Upon power up, can you pull /CS low and just monitor the DRDY pin if it is pulsing at correct ODR? 

Once you see it is pulsing, can you try to change the ODR settings and then see again if it is now pulsing at the new ODR? Then you can also check if you configure the ADC correctly by writing and reading to the same register. Once you confirm that the ADC is converting and that the SPI interface is good. Then we can go to the hardware debugging.

I'm sorry I'm kinda loaded now, but I will review this and come back to you if I find anything else. 

Thanks,

Jellenie

Hi Jellenie,

I have powered on the ADC, pulled /CS low, and DOUT/RDY remains high. 

I'm pretty sure something else is the issue. Maybe even the ADC is broken? I'd be surprised though.

My reasoning is that when I communicate with the device, it appears to be functioning correctly.

For example, the ID register reads back correctly. If I enable the MCLK_COUNT_EN bit in the ERROR_EN_REGISTER, the MCLK_COUNT register begins to update as expected. 

Hi 

If it remains high then it seems the ADC is not converting. Just to confirm the above suggestion is upon power up when ADC is at default config is that correct? 

Because I reviewed your register settings. It appears that your gain is set to 16. So if you have an input of 398mV then multiplying it with 16 will exceed and saturate the ADC. May I know why you used 1000uA? I am assuming you are using PT100 right? Have you seen this tool? This would help you check if the config is acceptable for ADC. Please have a look and let me know your feedback. 

https://www.analog.com/media/en/designtools/calculators/rtd_configurator_and_error_budget_calculator-rel1.0.0.exe 

Thanks,

Jellenie

Hi

>Just to confirm the above suggestion is upon power up when ADC is at default config is that correct? 

I've tried both in default config, and after configuration the current source. 

Right after boot, using the default values, and enabling all the errors, I see that the error register is 0xA880, showing that reference is not detected, and and both inputs are over voltage, and the analog LDO is not happy.

If I enable the current sources, all errors go away except the Analog LDO error (ALDO_PSM_ERR).

 I've looked at the tool. Thanks! I believe I should change my reference and headroom resistor 2K and 330 ohm respectively.

I also believe setting a gain of 8 and a current of 500uA is better.

Hi, 

Yes because I think the default config uses external reference and if your REFIN pins has no voltage because of the absence of IOUT then the ADC won’t convert which is the reason why DRDY remains high or low. 

We have here our recent released article regarding RTD application including the tool. Hope you find this helpful.

https://www.analog.com/media/en/analog-dialogue/volume-55/number-2/how-to-select-and-design-the-best-rtd-temperature-sensing-system.pdf 

Thanks,

Jellenie

Hi,

Thanks, that's a helpful guide. From reading it, my circuit is correct, with the exception that the reference resistor and headroom resistor need to be changed.

The error bit that is still set is ALDO_PSM_ERR. This won't go away no matter what I do. If I measure the analog LDO output, it's around 1.9V. Is this okay? This is what I'm the most stuck on, since even with correct reference voltages greater than 0.7V, or using an internal reference, this bit remains high. 

If I disable the monitoring, then the bit goes away, but I believe the error condition is still present, just hidden.

Hi, 

That is weird. Do you read the error registers consecutively and does the error flag remains as always or is it only appeared once? 

It is curious as you've mentioned that the LDO output is around 1.9V so that should be okay. I am wondering what might have causing this issue as the LDO output appears to be fine. Are you using Eval+ software or your own code? 

Thanks,

Jellenie

If I read it repeatedly, it remains. 

I am using my own software.

I've considered soldering the SPI pins from EVAL-SDP-CB1Z and using the Eval+ to talk to the device on my circuit board. 

Hi, 

I was thinking if this is just a bug somewhere. Are you having issues with your measurements? or everything appears to be normal? 

Let me see if I can double check this. I am currently wfh but when I get a chance this week I'll try if I can replicate the issue. 

Thanks,

Jellenie

Hi,

I'm wondering if the usage of the analog pins matters. In the app note CN0383, the "Multiple 3-wire RTD Configuration" shows the following:

My design on the other hand is using AIN0 and AIN3 for IOUT. Maybe this causes a problem. 

I'm going to be ordering a new board within a week or so, and I'll also pick a new Rref and RHeadroom.

Here's a screenshot of the Excel tool: