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Question for design 4 ADS1222 on board

Elite Hsieh
Prodigy 20 points
Other Parts Discussed in Thread: ADS1222, MSP430F5436A, MSP430F5436, ADS1224, REF5025

Hi,

Currently I design a data logger board which need 4 channel analog input. I use 4 ADS1222 and one MSP430F5436A. Now I have some questions need clarify. Please help, thanks in advance.

1. To accomplish analog input range +-5V, the Vref is 2.5V and VDD is 5V. According to ADS1222 spec page 4, VOH will be 0.8VDD min, that is 4.0V min in my design. Can the MSP430F5436 accept 4.0V digital input? I see reference schematic in ADS1224 is direct connect to MSP430. If not I will add a buffer between ADC and MSP430. 

2. Since there are 4 ADS1222 on board, do I need use 4 voltage reference (ref5025) to 4 ADS1222 or one ref5025 to 4 ADS1222?

Elite Hsieh

  • 0
    TI__Guru** 112765 points

    Hi Elite,

    The MSP430 inputs are not 5V tolerant, but this is not the only problem.  If you use a 3.3V digital supply for the MSP430, it will not be able to deliver an adequate logic high.  So you will need to use some means of level shifting.

    One other thing I want to clarify regarding your first question.  You stated measuring +/-5V.  From a device persepective this refers to one input relative to the other.  In no way can you measure below ground.  In other words you cannot measure a negative voltage.

    You can use one REF5025 for all four ADS1222, but you need to be careful with routing and making sure there is enough charge available for reference sampling (this assumes reference buffer is off).  You should have a cap near every ADS1222 reference pin and probably should be around 1uF.  You should also have 1uF (or larger) at the output pin of the REF5025 for stability.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 20 points

    Hi, Bob,

    Thanks for quick feedback. The analog input connect to AINP and AINN which is differential signal so I think no problem.