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THVD1400: THVD1400DR Off State Output Impedance

lu tan
Prodigy 160 points
Part Number: THVD1400

Tool/software:

When THVD1400DR is disabled or when no power is applied to it, what is the output impedance on its pin 7 and pin6 B and A pins? The datasheet says "high impedance". I am asking on this forum to see if there is a more specific resistance value.

  • 0
    TI__Guru 69010 points

    When the device is disabled, the input impedance from the receiver is still present so the leakage is spec'd in the datasheet as bus input current. So if we use this number we can calculate the impedance value. At 12V/100uA = 120k ohms. During normal operation though the leakage should be lower (since this uses 12V as the voltage on A/B pins). 

    We don't spec the impedance value when the device has no power so I don't have a number to specifically call out. But I suspect it's probably in the Megaohm range. 

    -Bobby

  • 0 in reply to BOBBY
    Prodigy 160 points

    Hi Bobby, thank you.

    What about the output impedance on the A and B differential pins during disable or power down states? Sorry if I misunderstood and the input impedance of 120k and output impedance are the same.

  • +1 in reply to lu tan
    TI__Guru 69010 points

    I assumed the output impedance was bypassed by the input impedance when you state it's disabled. When it's enabled, I believe the output impedance is around 20~25 ohms for each driver (we don't really spec this in the datasheet but that's roughly what you would expect for RS485 devices). When the DE is disabled (held low) the output impedance should be in the Megaohm range. The receiver circuit sits in parallel with this circuit which is why I stated earlier in this post that it bypasses the output impedance (120k ohms is much smaller than Megaohms so we neglect it). 

    When the device has no power, the output impedance should also be in the Megaohm range since it is turned off. We use diodes infront of the PFET and NFET to ensure it doesn't back bias and generate leakage current into the device. 

    -Bobby