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I have two digital signals, I perform a AND operation on these two signal with an AND gate (TI SN74LS08N), and want to use the output signal to trigger a function generator which will generate another signal.

So I did get the AND signal from the gate, and when I connect the output signal from AND gate to the oscilloscope, the signal looks good and have 0 and 5V for low and high. But when I connect this signal to the function generator, and put the generator to external trigger mode, the function generator is not triggered. I used the oscilloscope to check this signal, and found when the function generator is connected, the output signal from the AND gate dropped to 0 and 500mV for low and high, which is absolutely not enough to trigger the function generator.

So my question is how to amplify this output signal from AND gate so that it is enough to trigger the function generator? Can I use an op-amp to amplify the signal?

If so, how can I do it? One thing I am thinking is if I use the op-amp, how should I choose the resistor ratio to get the proper gain? I mean, before I connect the output from AND gate to the function generator, the signal is 5V for high, only after I connect it to the function generator, it drops, so should I use the dropped signal, for example, 500mV to calculate the gain, which will be 10?

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Update:

The function generator is a DEI PDG-2515, the data sheet can be found here.

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    \$\begingroup\$ Why is the impedance on the function generator input so low? \$\endgroup\$
    – Ignacio Vazquez-Abrams
    Jul 18, 2013 at 1:56
  • \$\begingroup\$ @IgnacioVazquez-Abrams I don't know. I only have this function generator at hand... \$\endgroup\$
    – Jing
    Jul 18, 2013 at 1:59
  • \$\begingroup\$ Do you have a schematic or specifications of the function generator available? \$\endgroup\$
    – Ignacio Vazquez-Abrams
    Jul 18, 2013 at 2:00
  • \$\begingroup\$ @IgnacioVazquez-Abrams I don't have the function generator by my side now, I can look it up tomorrow, is there any way to change the impedance? \$\endgroup\$
    – Jing
    Jul 18, 2013 at 2:08

2 Answers 2

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Going purely by the (somewhat unlikely) results of connecting the trigger input, as reported in the question:

The problem is not a low output signal from the gate itself, but a lack of external pull-up resistor at the output. Note that the datasheet indicates an output current sink rating of 16 mA, but source rating of only -0.4 to -0.8 mA.

Add a pull-up resistor of around 1 kOhms from the output of the gate to Vcc, and the output should trigger the signal generator fine.

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I suspect that the signal generator's trigger input is terminated in 50 ohms - if so, there may be a switch to remove the termination, which should allow the TTL signal from the 74LS08 to work.

There is probably a trigger level control which adjusts the level at which the trigger signal is recognized. Adjusting this may allow it to work,even with the termination.

Could you give the make and model of the function generator, and, if possible, a link to a spec sheet or manual - this would allow us to make more informed suggestions to fix the problem.

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  • \$\begingroup\$ The function generator is DEI PDG-2515 \$\endgroup\$
    – Jing
    Jul 19, 2013 at 2:18
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    \$\begingroup\$ That spec sheet does indicate that the trigger input accepts TTL levels, but it also gives the input impedance as 1K. This is too low for the typical LS TTL pull-up capability, so I suspect it is intended to accept "plain" (non-LS) TTL signals, as from the original 7408, which has about 10 times more drive than LSTTL. If you can't get a 7408, adding a 1K pull-up as suggested by Anindo may work. \$\endgroup\$
    – Peter Bennett
    Jul 19, 2013 at 3:09

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