impedance configuration for ADG3304 Level translator

I'm using the ADG3304 to translate the level of four input signals from 5V to 3.3V, but I have some problems.

The configuration is as follows:

I interface the input signals with HCPL-063A optocouplers. The output signals from this optocouplers have 1K pull-up resistors to 5V.

Then the signals are connectet to the Y pins of ADG3304 (the Vccy and EN are connected to 5V, while the Vcca is at 3.3V; also I have 100nF capacitor between Vccy and ground). Then the outputs of ADG3304 are connected to GPIOs of a BF518 dsp. (see attached image)

Between the HCPL-063A and the ADG3304 I had 1K resistors, but the signals that arrived at the DSP was always neither high nor low (a broad signal about 1.5V).

Now I removed those resistors but the signals at the DSP are always like this.

I'm not skilled in electronics, I hope someone can help me to better understand how to correctly configure this connection.

Thank you

Marco

  • 0
    •  Analog Employees 
    on Jun 18, 2012 8:39 PM

    Hi marco55,

    Can you please send me an oscilloscope plot with the signals from the A and Y side I/O pins of the ADG3304?

    Regards,

    FEB

  • This is the signal at Y1 pin

    this is the signal at A1 pin:

    I expected a signal of 3.3V.

    As I said before I'm working without those 1K resistors.

    Finally this is the signal again at Y1 pin after I measured A1 pin.

    I suppose that the oscilloscope probe changed the impedance and the direction of ADG3304.

    Marco

  • I measured signals with another oscilloscpe.

    Moreover I connected switches to the inputs.

    the first plot shows the signals with the switch off (yellow is Y4 pin, blue is A4 pin): the 5V signal is correctly translated to 3.3V:

    the second image is the signal with switch on: the input signal goes to zero, but the output A4 pin is at 1V):

    Moreover sometimes the two signals are very noisy, without changing any condition, as shown in the last plot.

    In addition I noticed that the ADG3304 on my board is extremely hot (I cannot put my finger on it).

    I'm waiting any suggestion about properly configure ADG3304 impedance.

    Thank you

    Marco

  • 0
    •  Analog Employees 
    on Jun 21, 2012 7:38 PM

    Hi Marco,

    Assuming that the optocouplers you use have open drain outputs, you need the pull-up resistors, but you don't need the series resistors (R111 to R114) and the capacitors between the Y I/o pins of the ADG3304 and ground. Also, you are missing a decoupling capacitor on the VCCA supply pin of the level translator.

    The output impedance of the I/O pins is about 6k, so the load current should be low enough to ensure the correct output voltage levels.

    A similar rule should be used for the Y side of the device, where the pull-up resistors and the optocoupler pull-down resistance  need to ensure the correct VIL/VIH when the signal lines are loaded with 6k resistors to GND for a high logic level and VCCY for a low logic level.

    Regards,

    FEB

  • Hello FEB,

    I'm using ADG3304 too, and I have a similar issue.

    The A-side of the ADG3304 is directly connected to a BF506F output pulled up with 10kOhm, while Y-side is connected to a logic port (74HC...).

    The BF506F drives the ADG3304 with a square wave at 60kHz with 50% duty cycle but on the Y-side translator this wave is "corrupted" because there is an unexpected commutation within each level (low and high).

    The ADG3304 datasheet  specifies that input driver must have an output impedance lower than 150 Ohm and a minimum peak current of 36mA: Blackfin meets this requirement?

    Thanks in advance

    Regards

    Maurizio