1 Block Diagram

Figure 1-1. Plant BODE Measurement Setup

Figure 1-1 illustrates the integration of various components for measuring the open-loop frequency response of a system using a dsPIC microcontroller. The operational amplifier circuitry located on the DP PIM, adds a 1.65V DC offset to the AC signal generated by the network analyzer (such as the Bode 100). This offset is necessary because the dsPIC’s Analog-to-Digital Converter (ADC) can process only positive voltages. The AC signal, now with the DC offset, is digitized by the ADC (in this example, accessible via pin AN18 of the dsPIC33CK DP PIM). The firmware then subtracts the DC offset, leaving a digitized AC signal that disturbs the control input, thereby modulating the PWM duty cycle or the switching frequency of the system.

For accurate plant frequency response measurement, the control input disturbance and the output voltage (VOUT) are analyzed. Channel 1 (CH1) of the network analyzer should be connected near the control input disturbance, and Channel 2 (CH2) should be connected to the converter’s output. Connecting CH1 to the node at which the disturbance is injected provides accurate gain measurements but may result in phase inaccuracies. To mitigate this, the digitized signal from the ADC is converted back to an analog signal using a DAC on the dsPIC. This approach ensures more precise phase measurements by accounting for the ADC sampling delay.

A breakdown of the setup is provided below:

  1. DP PIM Test Points (TP1, TP2, TP3):
    1. TP1 and TP2: These test points are designated for disturbance injection. The BODE 100 injects a disturbance into the power supply circuit through these points.
    2. TP3: This test point is connected to the dsPIC33’s Digital-to-Analog Converter (DAC) output. The injected disturbance is sampled by a 12-bit ADC in the dsPIC. The sampled signal is then fed into the DAC to produce a reconstructed version of the injected disturbance at the DAC output, which includes the ADC sampling delay. This signal is then fed into the BODE 100 as the Channel 1 (CH1) input.
  2. BODE 100 Connections:
    1. CH1 Input: Connected to TP3, this channel receives the reconstructed version of the sampled, injected disturbance from the DAC output.
    2. CH2 Output: This channel is connected to the output voltage of the power supply. It measures the output response of the power supply to the injected disturbance.