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ezdsp f28335 and ADC

Meral Özarslan
Prodigy 210 points
Other Parts Discussed in Thread: TL431

Hello

I have problem with ADC again unfortunately. I use ezdsp f28335 for controlling dc dc boost converter and three phase inverter. At first ,if anyone has used this kit before, what is the the input range of ADC? Is it 3 V like 2812 or  1,9 V?

I use MAtlab Simulink to program DSP. When I change ADC input to 1 V, ePWM duty cycle changes but after the input of ADC is bigger than 1 V it begins to draw 30, 40, 50 mA and more (I tried to 70 mA).Is it normal? I have no other dsp to try.

The other question how can I protect the ADC? I designed a voltage sensing circuit. There is a zener diode, a serial resistor to limit the current, there are protection diodes (not to pass 2.7 V and not to go negative) . I send the schematic circuit below? In spite of these protection can the ADC modules be damaged. AS I said it makes normal control to 1 V.

I am very confused again. Thanks from now... 

 

  • 0
    Genius 5885 points

    Hi Meral

     

    The ADC input range for 28335 is the same as for 2812, that if from 0V to 3.0V.

    Looking at your schematic I have a couple of questions/suggestions for you.

    1. you say when the ADC input voltage is above 1V it start to sink quit a lot of current. I this with your circuit connected or not? If yes, could it be possible that all (or at leas most) of the current flows through Zener diode?

    2. To which supply rail is D1 connected to. If you want to protect ADC it should be connected to 3.3V supply rail, and in that case you don't need D2 (Zener diode). From my experience Zener diodes knee is not as sharp as you'd like and are unsuitable as protection devices.

    3. Is the output of the circuit connected directly to ADC? If so, don't you think that the output impedance of the circuit is (too) high?

    Best regards, Mitja

  • 0 in reply to Mitja Nemec
    Prodigy 210 points

    Thank you Mitja

    Is it the same for ezdsp F28335? (0 - 3 V)

    My circuit isn't connected to ADC . I only change a DC power supply finely 0 - 1 V  and change the duty cycle of PWM. After 1 V it sinks much more current. What is the sink current of the F28335?

    I designed a circuit by using the same switch mode power supply( +15, -15 +5, -5V)  with TL431 and form 5 V I obtained 2.5 V, so all of the GND are connected together you see in the circuit? Do you mean 3.3 V from DSP ?

    If I don't know wrong, the ADC has't got internal buffer amplifier.  Isn't it correct? To prevent crosstalk I added a buffer.

    So my ADC is damaged or not?

    Best regards,

  • 0 in reply to Meral Özarslan
    Genius 5885 points

    THE ADC range is the same as in eZdsp f28335

    What DC power supply are you talking about? Power supply for complete F28335 (3.3 and 1.8V rail), power supply for analog stage? Or did you connect DC source to ADC input when DSP is powered? The sink currents for F28335 are listed in its PDF file under electrical specification.

    Why do you need 5V and 2.5V? The DSP needs 3.3V and 1.9V? Use ezDsp28335 schematic as a reference

    ADC does not have internal buffer(s). But it requires low input impedance. Adding resistor after buffer without any capacitor will increase buffer output/ADC input impedance.

    Regards, Mitja

  • 0
    Prodigy 200 points

    Hey did you get to solve your problem. if possible can you let me know.

    Thanks

    Bhanu