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BQ27Z561-R2-DEVICE-FW: ADS 1285 EVM-PDK testing problem

Honghui Chen
Prodigy 10 points
Part Number: BQ27Z561-R2-DEVICE-FW
Other Parts Discussed in Thread: ADS1285EVM-PDK, ADS1285

I'm testing the ADS1285EVM-PDK 32bits ADC board, but I met some problems. When I only connect the UBS wire to PC and do not connect sonser, I measure the voltage of  "+" pin (positive pin) and "-" pin (negtive pin)  of IN 1 that are same (the value is +2.5 V) while the voltage of GND of IN1 is zero, which is strange. In my view, the volgage of "+" pin and "-" pin is inverse. 

Another question is when I connect a sensor (the sensor is JAE JA-40GA  Accelerometer who has a yellow signal wile labled "signal " and a white signa lwire  labled " signal return" acutally connected to ground )  to IN 1 as the bellow manner:  JAE " signal" wire to "+" pin of IN1, and JAE " signal return "  wire to "-" pin of IN1, the measured singal is fluctuaded from  zere to the acutall value( for example 1.05V). If I change the conncet manner (connect JAE " signal return" wire to "GND" pin of IN1) , the measured signal is 1.45 V (2.5 V-1.05 V =1.45 V )  whose rms noise is even worse than I measured using 24 bits NI 9239 borad. But when I have a self-noise test and mesure the self DAC singal of ADS 1285 board, the self-noise and test signal is ok.

Idon't know how to solve theose problems. Could you help me and give some advices.Thanks.

  • 0
    TI__Mastermind 18671 points

    Hello,

    I am assigning this to the group that covers the ADS1285EVM. 

    Regards, 

    Jonny. 

  • 0
    TI__Mastermind 40950 points

    Hello Honghui,

    Channel 1 has the inputs biased to mid-supply voltage, or 2.5V.  With nothing connected to these inputs, you should measure 2.5V on both the positive and negative inputs.

    The output of the JAE JA-40GA is a single-ended, bipolar voltage, with a range up to +/-10V at full scale.  The inputs of the ADS1285EVM only support unipolar voltages from 0V to 5V, and are differential inputs that need a common mode voltage of 2.5V.  Any voltage less than 0V or greater than 5V can damage the ADS1285.

    The EVM will not directly support interface to this sensor without additional signal conditioning.

    This application note shows how to convert from single-ended to differential, but would need modified for your sensor.

    https://www.ti.com/lit/an/sbaa246a/sbaa246a.pdf?ts=1714427694716

    Regards,
    Keith Nicholas
    Precision ADC Applications