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MAX232: Output resistance do not match Load resistance

Eyal Tsin
Prodigy 10 points
Part Number: MAX232
Other Parts Discussed in Thread: SN74AUP1G07

Hi, 

I am trying to connect TTL output to a GPS generator device (by spirent). The generator needs trigger in with 50 OHm impedance. I connected the source (MAX232) and Load (GPS Trig in) with RG316 (50 Ohm) Coax cable of 6m length. The initial TTL signal i measured was 5V, 1PPS, after conneting the Load it was about 0.3V at it's highest. I was thinking to use an oped-drain buffer (SN74AUP1G07
) for a kind of impedance matching. Yet i'm really not sure it's the proper solution, can someone suggest a better solution?

  • 0
    TI__Guru 62920 points
    Hi Eyal,

    Typically TTL outputs are intended to drive high-impedance inputs and tend to be overloaded when connected to a 50-Ohm sink. A common technique for interfacing to 50-Ohm test equipment is to insert a 450-Ohm resistance in series with the TTL output. This typically limits the output current enough that the full voltage can be achieved across the 500-Ohm equivalent load (450 Ohm series resistance plus 50 Ohm input resistance), and 10% of the voltage shows up at the 50-Ohm input. This is OK for most trigger inputs, and if the waveforms are being observed on a scope it is often possible to introduce a 10x multiplier to compensate for the voltage division effect.

    If this voltage division method is not an option, then you are right that using a buffer device that is able to drive a 50-Ohm load is the simplest solution.

    Best regards,
    Max