3.3Volts to 5 Volts level translator problem

Discussion in 'General Electronics Chat' started by tigerk2001, Jan 7, 2010.

  1. tigerk2001

    Thread Starter New Member

    Jan 7, 2010
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    Hi,
    I'm using a MCU working 0-3.3VDC and have to communicate with my DAC"s and ADC's that works on 0-5VDC. So, for my SPI(CS,SDI,SDO,SCK) I used a ST2378E level translator to allow the conversion from 3.3 to 5 and vice versa.
    However, it seems time to time I have some communication error. I tried to probe the VL or VCC side of the level translator but when I put it at hgh it shows a good value of 3.3 on VL side and 5 on VCC side. However, when I put it down to zero, sometime it goes down to zero as it should be but sometime it stays at 1.6Volts on VL side and 4.7Volts on VCC side. Why is time to time, the low value doesn't reach to zero?
    Is there any impedance problem?
    Could I have to add coupling capacitor?
    Could I have to add capacitor on each channel?
    I need your support
    Many thanks
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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  3. tigerk2001

    Thread Starter New Member

    Jan 7, 2010
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    Yes I read it carefully but can't find the reason of it.
    Do you have any idea of what could be the problem?
    I want to use that IC to convert from 0-3vdc on mcu side to 0-5vdc on my DAC & ADC side.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    I don't see your schematic posted.
    I don't see your board layout posted.
    You haven't mentioned the speed at which you're trying these communications.
    Do you have bypass capacitors to ground on both the VL and Vcc inputs?
     
  5. tigerk2001

    Thread Starter New Member

    Jan 7, 2010
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    I'll post my schematic tomorrow to give you a better idea of the problem. I don't have it with me at the moment.
    To answer your question, I have no bypass capacitors on the inputs and outputs. The VL side is directly connector to the GPIO's of my MCU and the VCC side is directly connector to the DAC and ADC for the SPI. Do I have to add some and what would be the value to install? Could it be the reason of my problem?
    Thanks for your good support. It is appreciate
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    I meant bypassing only the VL and Vcc terminals to ground, using 0.1uF (100nF) caps - not the logic level I/O lines. Adding capacitance to the I/O lines would make the problem much worse.
     
  7. tigerk2001

    Thread Starter New Member

    Jan 7, 2010
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    I'll try to add the 0.1uF coupling capacitors on VL and VCC tomorrow. I let you know the results and post the schematic if the problem is still there.
    thanks
     
  8. tigerk2001

    Thread Starter New Member

    Jan 7, 2010
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    It seems the problem is still there. What is strange, It seems when I probe an inlet with my voltmeter, the signal doesn't come to zero on the input at every time. It goes from 3.3volts to 1.6 volts. When I don't probe, it goes from 3.3 to 0 and time to time it stays at 1.6 volts.

    See attached schematic for reference. The page 7 shows the connections on the ST2378E.

    Let me know if you can find something.

    thanks
     
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