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ADS131E08: ADS131E08 Current fluctuation

Gary Zeng
Prodigy 270 points
Part Number: ADS131E08
Other Parts Discussed in Thread: TIDA-010086, ADS131M08, REF5025

Hi DCC Experts

Problem: There is a current fluctuation when using TI ADS131E08 to measure 4ch voltage and current. You can see the phenomenon as below.

               This machine is a Battery Test system to charge and discharge the power battery. It need to precisely control the current and voltage. After this process, power batteries will be assemble to modules and pack and put in electric car.

The red line is the current signal. From the value of -3000(3A) to -500(500mA), You can see the current fluctuation.

Could you please check the schematic of the ADC part as below:

  • 0
    TI__Mastermind 35570 points

    Hi Gary,

    The customer's design is generating +2.5V and -2.5V DC voltage for VREFP and VREFN as a reference, and also all AVSS pins are shorted to -2.5V as a ground for the ADC. However, the DGND pins of ADC are shorted to GND and they do not have a same potential level as AVSS. I will check with design team to see any potential issues for this configuration.

    ADS131E08 can support fully-differential and pseudo-differential input, so normally you can shift the signal from -2.5V ~+2.5V to 0~+5V by using INA amplifier(the customer may be using INA but they do not provide a whole schematic), then use the following circuits to connect the signal to the ADS131E08 ADC for data conversion (R1 and R2 are not needed). A 5V external reference cab be used for VREFP, the VREFN can be shorted to AVSS which can be directly connected to GND.

    Best regards,

    Dale

  • 0 in reply to Dale Li
    TI__Prodigy 270 points

    Hi Dale,

    Thanks for replying,

    TIDA-010086 is designed for Battery test system as below.

    Customer needs to monitor the forward current when charging the battery and monitor the reverse current when discharging the battery. So they need ±2.5V reference voltage in my opinion.

    In addition, I see that datasheet asks user to connect CLK to GND if not use this pin and customer let it floating. Do you know the risk? 

  • 0 in reply to Gary Zeng
    TI__Mastermind 35570 points

    Hi Gary,

    The reference design (TIDA-010086) uses ADS131M08 which can support negative single-ended input down to -1.6V. ADS131E08 ADC your customer is using is different.

    I know the customer is trying to monitor forward and reverse current (-58.1A~+58.1A) which leads to a differential ±2.5V signal. However, this does not need a ±2.5V reference voltage for conversion. The customer is using +2.5V for VREFP and -2.5V for VREFN, the Vref = VREFP - VREFN = 5V, so the input range will be -Vref/Gain~+Vref/Gain which is -5V~+5V if the gain is 1. This actually exceeds the supply range. The customer is not using full scale range and they are losing a bit.

    ADS131E08 supports a wide power supply range between AVDD and DVDD (-2.1V to 3.6V) and also supports a wide range between AVSS and DGND (-3V~0.2V), so the customer circuit works. However, the biggest concern is, they are using REF5025 to generate +2.5V for AVDD&VREFP and -2.5V with inverting amplifier for AVSS&VREFN. Because these voltages are used not only for reference but also for AVDD and AVSS, they may draw more current during conversion, any change or fluctuation on both voltages will definitely affect VREF voltage and can result in code change. I would suggest them to use a stable clean power supply for AVSS/VREFN and another one for AVDD for test purpose. LDOs will be better to generate them.

    I think the customer is using an INA to measure the current across shunt resistor with a high command-mode voltage, please confirm. If so, the best solution will be, use the same INA amplifier to convert the true differential input to a single-ended output signal, and also level shift the output signal to 0~+5V range. A single 5V power supply can be used for AVDD of ADS131E08 ADC,  and also VREFN and AVSS pins can be shorted to GND ground.

    We can have a call if needed, please let me know, thanks.

    Best regards,

    Dale

  • 0 in reply to Dale Li
    TI__Prodigy 270 points

    Got it. Thx. I have recommended customer to add a LDO for power the AVDD pins.