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Compiler/ADS1232: Issue with ADS1232 with arduino

Part Number: ADS1232
Other Parts Discussed in Thread: LM317

Tool/software: TI C/C++ Compiler

Hello,

Currently, I am working on a project based on weight change measurements. I have tried some different ways to amplify the signal from 10kg load cell 1mv/V to be read using an Arduino UNO on my prototype. Finally, with ADS1232 (WCMCU-1232) I got the best measurements, but there was a little bit of noise. Sometimes with no reason weight measurements could gain or lose non-logical 20 or 30 grams. So, looking for the best step measurement I tried different electronical connections until I found the next schematic in a forum (https://e2e.ti.com/cfs-file/__key/communityserver-discussions-components-files/73/1817.SCH.jpgwith some changes that I applied like all supply connections are connected to 5v regulator (Lm7805), except DVDD (it is connected to 3.3V LM317 regulator). Finally, Arduino is connected to another 5v source (LM7805), and all ground connection between them.

This connection made that the measurements were a few better than before, now I have sometimes 0.3 or 0.5 gr in changes. But then, I found a new issue.

For some reason, where everything is connected to each supply source, the PDWN pin gives a 5v output that makes turning on the Arduino (when Arduino is off), what is worse, when the supply source of Arduino is turned on becomes the 3.3v input in a 4.2V input. This could be confusing, but it is happening, and as a result, if I am having e.g 5000 gr of weight, automatically gives 250000 gr or something crazy value like that.

I think of course could be a problem with the source supply, but I have verified all connections, and everything it is ok, and for that reason, I have used different supply sources.

So, my question is. Do you know how to solve this problem with the PDWN pin? maybe some different connection to try or what is the importance of this pin? may be connected with a diode or filter?

Thank you, so much for your time.

Duvan.

 

  • Hi Duvan,

    Welcome to the E2E forum!  The schematic that you referred to would not be appropriate for a load cell.  The schematic shows a gain of 1 and a single-ended measurement.  The load cell output will be differential and near mid-excitation of the load cell.  To get the best performance the gain pins should be set high, and the excitation, analog supply and ADC reference should all be 5V.  You should also have input filtering on the analog inputs.  Digital and analog ground should also be at the same potential (connected together).  Can you send me your precise schematic that you are using?

    The PDWN pin is a digital input pin and is referenced to the digital supply.  You may something incorrectly connected.  The PDWN pin is used to power down the device when set low.  Pulsing the pin will reset most of the device internal circuitry.  Also note that you are to make sure that the PDWN pin is held low for a minimum of 10us after both the analog and digital supplies have reached operating voltage.  This is explained in the datasheet on page 23 in the Power-up Sequence section.  You can do this with a pull-down resistor on the pin, and then the micro can set the pin high after it is operating.

    If you are seeing an output voltage on the PDWN pin this is concerning.  This is an input pin and should not output a voltage unless the part is incorrectly wired or the part is damaged.  Both are easy to do with a prototyping circuit.  Make sure that you are following the datasheet pin connections and have reviewed the Absolute Maximum Ratings table to determine if you have violated any operating conditions.

    Best regards,

    Bob B

  • Hi Bob,

    I really appreciate your help. I have forgotten to mention a very important part that I applied in the connections. The schematic that I shared is based on an amplifier used between the load cell and ads1232, but I am using it with the load cell directly using the first channel, with 128 GAIN selected from both gain pins in HIGH. It could not be appropriated to show the complete schematic here, but I think it is a good idea to show you. Do you have an email where I can send it to you?

    I have been checking the datasheet and also I found a simple RC circuit (page 23). Could I use a simple RC circuit to make sure this minimum of time to AVDD be powered up? and do not connect the pin directly to the micro. 

    Also, I made the connection within of limits of the Absolute Maximum Ratings table. At this point, I could think that something could be wrong with my microcontroller (internal working) or something is badly connected. I will check the circuit and will make sure the correct PDWN pin connection and I will tell you.

    Thanks, 

    Duvan.

  • Hi Duvan,

    Yes, you could use the RC circuit on page 23 for the PDWN pin.  Note however that both the AVDD and DVDD must be at nominal operating voltage before the PDWN pin goes high.  This is much easier to do if the supply to both AVDD and DVDD comes from the same voltage source.

    Maybe I misunderstood you original issue with PDWN.  As I said, PDWN is an input pin.  It should have no effect on the power supply of the Arduino.  If you want to send me your schematic, click on my name and start a private conversation.

    Best regards,

    Bob B

  • Hi Bob,

    I have been some tests with the system. Now I am testing with the circuit shown int page 23 of the datasheet. And of course, I have one source supply (LM7805) for the whole ADS1232 pins.

    You understood correctly, when I connect the ADS1232 to the supply, I measure the voltage between PDWN pin and GND and I have seen 5v AS A OUTPUT!!!. I will do some tests with another ADS1232 and prove if the chip is not working correctly.

    Finally, I am preparing the schematic to show you, because I have not found the exact ADS1232 library to use it.

    In the next days I will be sending you the schematic.

    Thanks,

    Duvan.