The series-terminated circuit in Figure 4 places all the attenuation at the source end of the line. The 5V driver in that case may overdrive the 3. I see a great number of these problems each year. Message 3 of Thanks very much, I am confused however, The specs state: Your points are well taken, especially about the power sequencing mess. If you choose this approach, I hope the signal stays high most of the time and only occasionally needs to pull low.

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The problem here is that if a weak part is guaranteed to just makes 2. Message 1 of The only part I know of that will drive 50 ohms to ground is the old 74S and it is only specified for a minimum high level of 2V when doing so.

Voltage Conversion

These parts impose no power sequencing requirements. I have some remarks with regards to your article below which might have some bearing on the reason Mehran kept asking 774s140 questions and did not seem to want to accept your answers.

Here are his comments. 74140 maximum specified high drive current for any modern device that I know of is 24 ma 74ACs for example which only gets you to 1.

74S Datasheet PDF –

Do you think it would help Mehran’s problem if the 5V device is guaranteed to come up in a LOW state, and remain so until power has 7s140 The series-terminated circuit in Figure 4 places all the attenuation at the source end of the line. I would think 75 ohms is correct, since it is a high speed card. The problem with this circuit, and it may not be a problem depending on the application and the settling time permitted, is that the initial SURGE current required to drive the structure from low to high is 44 mA.


This structure uses an open-collector pull-down only type of TTL driver directly coupled to the ohm line, terminated at the end with 50 ohms to 3.

One thing I’ve learned about problems like this is that when I suggest changing the transmission line impedance I usually discover the layout is “already done” and “cannot be changed”. I am thinking it may be good to buffer the signals before sending them to the but that seems to be starting to make something quite complicated!

My initial response to his inquiry was this: The second possible problem with such 74w140 circuit, again assuming we are talking about going from a 5V logic device to a 3.

74S140 Datasheet PDF

Thanks very much, I am confused however, The specs state: Message 7 of I will have to terminate on the PCB.

This makes a good series termination. Plus, there’s enough attenuation that we may not have to worry about overdriving anything.

This places a linf end termination at the endpoint ohms to 3. The ohm series resistor you suggest is typically 74ss140 large enough to limit the current in this case to a safe level.



If that is the case, the problem with your suggested simple 22 and 50 ohm divider is that there are few ,ine devices I know of only one with enough drive strength to drive that low of an impedance to the proper 3. Message 5 of For example, LVT 3. Message 2 of Enough series resistance must be included to prevent dangerous overdrive currents when powering up. The surge happens because, during the first roundtrip delay, the transmission line looks like 50 ohms to ground, so the driver has to drive 75 ohms in series with the parallel combination of and 50 ohms.

I have a few questions which sorry if they are in the manual I need to design the board round this. How about the circuit in Figure 3?

Achieving both attenuation and termination requires two resistors. I’m going llne try a slew of different architectures to see what is good and bad about each one. When we are doing power voltage translation we try to use parts that do not have ESD diodes to Vcc on such interfaces, and can tolerate large input voltages without latching up.

Thanks, I was intending on terminating the detector output signal with a 50ohm resistor terminator. Message 10 of