SLAAEE6 October   2023 MSPM0L1306 , MSPM0L1306

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
  5. 2Hardware Introduction
  6. 3Software Introduction
  7. 4Gauge GUI Introduction
  8. 5Current Detection and Calibration Method
    1. 5.1 MSPM0 OPA Introduction
      1. 5.1.1 OPA input and output limitation
      2. 5.1.2 OPA Accuracy Influence
    2. 5.2 Current Detection Method
    3. 5.3 Current Calibration Method
      1. 5.3.1 (R1+R2)/R2 calibration
      2. 5.3.2 OPA1 Voffset calibration
      3. 5.3.3 R3/(R4+R3) calibration
      4. 5.3.4 Vref calibration
  9. 6Solution Evaluation Steps
    1. 6.1 Step1: Hardware Preparation
    2. 6.2 Step2: Evaluation
  10. 7MSPM0 Gauge Solution Test Results
    1. 7.1 Calibration Test Result
    2. 7.2 Current Detection Result
      1. 7.2.1 Test Under 25°C
      2. 7.2.2 Test Under 0°C
      3. 7.2.3 Test Under 50°C
      4. 7.2.4 Conclusion
    3. 7.3 Current Consumption Test
  11. 8Solution Summery and Improvement Direction
    1. 8.1 Shunter Resistor
    2. 8.2 ADC and its Reference
    3. 8.3 Runtime Calibration

5.1.2 OPA Accuracy Influence

The OPA gain accuracy is influenced by three parameters:

  • The first is the input bias current.
  • The second is the input DC voltage offset.
  • The third is the OPA noise.

As the MSPM0 OPA uses CMOS technology, the gate is physically isolated from the conduction channel to create an input that is truly high impedance. Its input bias current parameter is mostly combined of the leakage from ESD structures, protection diodes, and/or parasitic junctions. That means, input bias current can have positive or negative flow depending on the conditions. Unlike with bipolar amplifiers implementation, in this hardware design, that is why a matching resistor is not used at the OPA noninverting input. From calculation, the bias current influence to the OPA output is below than 1 mV, which also can be calibrated later.

For input voltage offset, chopper function is more helpful to reduce its influence. After enabling the chopper function, the max of input offset voltage goes from 3.5 mv to 0.5 mV, shown in the chapter 7.17 of MSPM0L1306 datasheet. The chopper function realizes this result by modulating the input signal and demodulating the output signal. For the input signal, it will keep the same after modulation and demodulation. For input voltage offset, it starts to take effect after signal modulation, and will do demodulation with signal together. As a result, chopper function changes the DC voltage input offset to an AC voltage output offset and do no effect to the input signal.

For the OPA on MSPM0, it has an ADC assisted chopping mode. After enabling ADC hardware oversampling, the chopper frequency is controlled by ADC sampling frequency (see Figure 5-3). As the hardware sampling times is an even number, the sampling times at the positive voltage output offset and negative voltage output offset will be the same. After the final averaging, the AC voltage offset can be removed through the digital filter, which means you don’t need a hardware filter anymore! Because chopper technology cannot truly remove the voltage offset, when you calculate the output voltage range, you need to take the voltage offset into consideration, besides of the voltage output swing range limitation. As the voltage offset calibration shown in the current detection demo, the voltage offset parameter will mostly affect the detection current range.

GUID-A759A25F-CCA0-4853-8A41-264827AE4A30-low.pngFigure 5-3 MSPM0 Chopper Function

The OPA noise is the key parameter that mostly affects the current detection performance. However, after enabling the chopper function, you can efficiently reduce part of the noise (1/f noise), because the chopping shifts the base-band signal to the chopping frequency beyond the input stage’s 1/f region. Paired with software averaging filter, you can control the total AC noise to an acceptable level.