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Op Amp to drive a coax cable

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Galroc:

Hi,

New to this forum, and new to electronics in general. I am a software programmer but I needed and wanted to lean electronics to solve a problem at work. The arduino has made it extremely easy to learn and introduce myself to electronics. I am about halfway through Dave's youtube videos...

One part of my project is taking an incoming ±10 Volt signal on a coax cable and adding/subtracting ±1.6 V from it, and putting it back out on another coax.  <10Khz.

It is the output that I am having issues with, especially with longer coax cables. I guess it is the capacitance and/or inductance of the coax.

Initially I just used an op amp that we had available to combine the voltages and send the result down the coax. This did not work well at all with long coax cables (~10ft). I then added a voltage follower (2nd op amp) with a 220ohm resistor in the feedback loop. This worked better and is usable but I am not totally happy.

I saw that Dave used a 100 ohm resistor on the output of an op amp in his uCurrent, but he didn't put it inside the feed back loop which means it changes the voltage output by a very small amount. I am driving 100K ohm instrument and he was driving a voltmeter which would have a much higher resistance. His error would be much lower than mine and acceptable. 100 0hm/100K ohm would give me a 0.1% error at max voltage.

I am now redesigning the circuit to use a proper instrumentation op amp (AD8221).  I am not certain if it needs another voltage follower after it.

How do people get signals to propagate reliably down a coax post op amp?

Cheers

Mechatrommer:
human dont usually send long distance info using coax, it will has large cumulative capacitive coupling along the distance and expensive. try using twisted pair. DC bias offset is not much of a problem over 10ft distance, only few tenth volt drop depending on cable quality/resistance, the problem is with digital (square signal), capacitive will make it flatter. though i'm maybe mistaken and not an expert in this, someone else may answer it more accurately. dont worry, you are not the only software guy coming here to play with an opamp and coax cable ;) welcome to the forum and i hope you stick around.

edit: and some hi speed opamp will oscillate if driving capacitive load such as coax cable, usually 50ohm impedance (if coax is 50ohm type) is placed on the opamp's output before the coax to avoid this. better if you can afford another 50ohm ground termination, but you'll get half the amplitude, but max power transfer. YMMV

edit2: and i dont think changing to intrument amp will make any difference. the output is all that matters (albeit low noise level), not its intrumentation inputs.

saturation:
With coax, its better to impedance match the op amp output to the receiver input, even at these low frequencies.



If you're using 50 ohm coax, then put a series resistor Zs and a terminating resistor Zl as 50 ohms as in the above photo.  Now adjust the output gain of Vs, which I presume is your sender op amp, to the voltage you require assuming a 50 ohm divider.

Is the receiver an inverting op amp too? If so, the gain resistors will be affected by the input Zl interacting with the gain resistors causing the output voltage to be not what you calculated.  It'll be easier to use the non-inverting mode or a general fix is put another voltage follower at the receiver input after Zl to buffer it before the 100k ohm instrument.


Lightages:
Most op amps have problems driving a 50ohm load. A high current output buffer is usually required.

jahonen:
I think that 50 ohm series resistor at the driving end is enough, no need to terminate the receiving end. If the resulting voltage divider causes too much error, perhaps you could add slight gain to the driving end to compensate for that?

Regards,
Janne

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