ADS8900B: ADS8900B has DC bias

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Replies: 3

Views: 32

Part Number: ADS8900B

Hello,

Our customer used ADS8900B and found that it  has a little DC  bias,  the bias voltage is about 4mV. In addition, when there is no input signal, the bottom noise waveform is as following, the same as the sine waveform input signal. Could you please help to analysis? 

3 Replies

  • Hello Kailyn,

    The offset voltage of the ADS8900B is typically 3LSB.  The offset that the customer sees is likely due to the external amplifier.

    However, you do need to set the OFST_CAL register to the proper value depending on your voltage reference.  Power-on reset, it defaults for a 5V reference.  If using a 3.3V reference, then set this register to 0x03h.  (See Table 18 in the datasheet for other reference voltages.)

    Also, if I am reading the above waveform, it looks like there is about 1100LSB pk-pk of noise.  This is quite high compared to the evaluation board, which shows about 22LSB pk-pk of noise.  This could be due to several reasons:

    1.  Poor power supply filtering (the supply capacitors are either not present or too far from the ADC to be effective)

    2.  The input driver is not properly settling, which shows up as excessive noise.

    3.  External noise from some other part of the circuit.

    If possible, see if you can get a partial schematic that shows the ADS8900B, input amplifier, reference, and bypass capacitors.  A screenshot of the board layout would also be helpful as well.

    Regards,
    Keith Nicholas
    Precision ADC Applications

  • In reply to Keith Nicholas:

    thank for your advise! the board layout as follows:

  • In reply to user1827009:

    Hello Kailyn,

    From what I can see, the layout looks O.K.  The bypass caps are close to the ADC, and the input amplifier is reasonably close to the inputs.

    If possible, provide a partial schematic for the reference and the input amplifier.  The noise could be due to poor settling.

    Regards,
    Keith