Actually, what you want is for the total series impedance formed by R1 plus the natural series impedance RS of the driver, to equal half of R2. If the line is longer, then considering the power issue paramount, and assuming the signal quality were acceptable, I would prefer a simple, series-terminated driver. Depending on the application space, airborne, etc. At the source end of your connection, install a series resistor of approximately 22 ohms Figure 1. Our policy at Northrop is that all circuits must be designed to tolerate any power sequence. I find that when we make assumptions about the power sequence, or the initial logic state, someone inevitable violates those assumptions either during test or during design verification. 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!
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If I make the line sufficiently short, it might not need termination. Let’s begin by calculating the current required to make the original solution Figure 1 operate. Power sequencing is becoming more and more of an issue as we proliferate so many different power supply levels.
(PDF) 74S Datasheet PDF Download – Dual 4-Input NSND Line Driver
We just sliced the drive current requirement in half to only 33 mA. Figure 2 places a resistor R1 in series 74s1440 the driver, with the value of R1 selected such that the effective series impedance RS of the driver, plus R1, together equals about 50 ohms. What kind of termination can I use which does this attenuation? Depending on the application space, airborne, etc.
To meet our attenuation goal the resistor ratio R1: Your points are well taken, especially about the power sequencing mess. Message 2 of I have just got of the phone and NI have confirmed that it is definitely 75 ohm.
74S Datasheet(PDF) – Fairchild Semiconductor
I would think 75 ohms is correct, since it is a high speed card. This places a split end termination at the endpoint ohms to 3. What happens if we tweak the resistor values? Message 8 of The 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. I am trying to connect a ljne at 5V to a load at 3. You could use a TTL 7s140 50 ohm line driver ie 74Sit will invert your signal though.
At the destination end, install a shunt termination resistance of 50 ohms. Message 6 of In reaction to that response, my good friend James Buchanan, author of “Signal and Power Integrity in Digital Systems”, McGraw-Hill,and a highly experienced engineer at Northrop Grumman, writes to suggest that the solution I proposed to Mehran in “Law of Product Development” could be impossible!
Our policy at Northrop is that all circuits must be designed to tolerate any power sequence. Are you plannin on using 50 ohm coax cable to connect from the outputs to inputs of your equipment? The 74FD, which can sink 64 mA to 0. Adjust the ohm resistor to account for the series resistance of your driver, in order to perfect the received signal size. If that is the case, the problem with your suggested simple 22 and 50 ohm divider is that there are few logic devices I know of only one with enough drive strength to drive that low of an impedance to the proper 3.
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.
That makes quite a difference! Message 3 of I find that when we make assumptions about the power sequence, or the initial logic state, someone inevitable violates those assumptions either during test or during design verification.
If the layout is not “already done” I would consider specifying a higher impedance transmission line and raising R2 to reduce the driver current. The second possible problem with such a circuit, again assuming we are talking about going from a 5V logic device to a 3.
Thanks, I was intending on terminating the detector output signal with a 50ohm resistor terminator. The third problem is that the current drawn from the 5V part under this particular power sequencing scenario may burn out the 5V driver. Message 1 of