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ADS1119: ADS1119进行AC的数模转换是否可行性？(Is it feasible to use the ADS1119 for AC analog-to-digital conversion?)

Brokenleaf Fox
Prodigy 10 points
Part Number: ADS1119

我使用ADS1119对3.6V的AC进行采样的时候，读出值是0X000000.

When I use the ADS1119 to sample a 3.6V AC signal, the readout value is 0X000000.

随后，我调整信号发生器的参数，此时电路的电压发生突变(示波器通道设置为直流（DC），从示波器中观察到的现象为峰值电压在7V左右)，ADS1119此时的读出值为0xFFFFFF，在电路稳定后（示波器通道设置为直流（DC），从示波器中观察到的现象为偏置电压约为1V，峰峰值为3.6V的50Hz的正弦波），ADS1119的读出值又变为0x000000。

Subsequently, I adjusted the parameters of the signal generator, at which point the voltage in the circuit 突变 (the oscilloscope channel was set to DC, and the phenomenon observed on the oscilloscope was that the peak voltage was around 7V). The readout value of the ADS1119 at this time was 0xFFFFFF. After the circuit stabilized (the oscilloscope channel was set to DC, and the phenomenon observed on the oscilloscope was a 50Hz sine wave with an offset voltage of approximately 1V and a peak-to-peak value of 3.6V), the readout value of the ADS1119 changed back to 0x000000.

下面是该ADC的外围电路原理图。

Here is the schematic diagram of the peripheral circuit for this ADC.

VCC=3.3V。

AOUT1与采样的AC信号相连。

AOUT1 is connected to the AC signal being sampled.

  • 0
    TI__Genius 14530 points

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  • 0 回复 Brokenleaf Fox
    TI__Mastermind 36630 points

    Thank you for sharing the schematic. 

    From my understanding of the circuit the initial issue it that the input signal is bipolar where the ADS1119 is not designed to measure that signal. This is due to the analog input going below AGND. Please see Section 9.1.6 of the datasheet. 

    With a 1V offset and 3.6V peak to peak AC signal. The signal will have a min: 1-1.8V = -0.8V and a max: 1+1.8V = 2.8V.

    The signals minimum falls below AGND. The analog input is violating the minimum voltage in the Absolute Maximum Table. VINmax = AVDD + 0.3V and VINmin = AGND - 0.3V.

    The recommended operating conditions of the device specify VINmax = AVDD +0.1V and VINmin = AGND - 0.1V. When the AVDD is 3.3V the recommended range of operation is -0.1V to 3.4V. This is violated by the minimum of -0.8V for the analog input in the system above.

    When the input voltage peaked at 7V, it significantly exceeded both the measurement range and the absolute maximum input voltage of the device. This severe overvoltage condition saturates the ADC at its positive full-scale, which can explain the 0xFFFFFF reading. 

    Even if the negative voltage issue were resolved, the signal's positive peak of +2.8 V exceeds the measurement range when using the internal reference. The ADS1119 can measure single-ended signals from AGND up to VREF/GAIN. With the internal VREF of 2.048 V and a gain of 1, the maximum measurable input voltage is 2.048V. To measure voltages greater than 2.048 V, an external reference must be used. For example, using AVDD, 3.3V, as the reference would allow measurement up to 3.3V.

  • 0 回复 Vivian Gao
    Prodigy 10 points

    Okay, I understand. I think I missed providing some information—this is the schematic diagram of the inverting amplifier.

    ...

    I tried connecting an electrolytic capacitor in series between the input of the ADC and the output of the amplifier circuit, but at this point, the readout value of the ADC is always 0x000000. Additionally, your response gave me an idea for a solution: converting the AC signal to a DC signal using a full-bridge rectifier circuit and a capacitor.

  • 0 回复 Brokenleaf Fox
    TI__Mastermind 36630 points

    The AC coupling capacitor in series between the output of the amplifier and the input of the ADC could be paired with a voltage divider after the capacitor to  VDD and GND to set the signal at mid-supply. However this would require you to increase VDD to 3.6V to stay within the recommended operating conditions suggested in the datasheet.

    If you are unable to change the supply from 3.3V an additional voltage divider could be used before the series capacitor to scale the 3.6V signal down to within the 3.3V supply.