Author Topic: Testing amplifier w/ spectrum analyzer  (Read 1452 times)

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Offline molotov

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Testing amplifier w/ spectrum analyzer
« on: June 20, 2016, 05:03:20 pm »
I am designing an amplifier for a piezo based vibration monitor.

I want to connect the output of the amplifier to the 50 ohm input of a spectrum analyzer using a standard 50 ohm BNC cable.

My circuit is below. Would I just need to put a 50 ohm resistor in series with the output of the instrumentation amplifier?



 

Online tggzzz

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Re: Testing amplifier w/ spectrum analyzer
« Reply #1 on: June 20, 2016, 05:45:14 pm »
Would I just need to put a 50 ohm resistor in series with the output of the instrumentation amplifier?

More or less, but you should take the output impedance of the INA into account, e.g. if the output impedance is 10ohms, then you would add a 40ohm series resistor. Whether or no that matters depends on whether the interconnection is electrically "short" or "long" compared to the signal's wavelength, and that is frequency dependent.

But be careful: some spectrum analysers cannot tolerate any DC component, and many are very sensitive to too much input power. Too much of either and the unprotected front-end-mixer is instantly destroyed. Check the spec of your SA.
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Offline molotov

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Re: Testing amplifier w/ spectrum analyzer
« Reply #2 on: June 20, 2016, 06:57:34 pm »
Would I just need to put a 50 ohm resistor in series with the output of the instrumentation amplifier?

More or less, but you should take the output impedance of the INA into account, e.g. if the output impedance is 10ohms, then you would add a 40ohm series resistor. Whether or no that matters depends on whether the interconnection is electrically "short" or "long" compared to the signal's wavelength, and that is frequency dependent.

But be careful: some spectrum analysers cannot tolerate any DC component, and many are very sensitive to too much input power. Too much of either and the unprotected front-end-mixer is instantly destroyed. Check the spec of your SA.

Thanks for the reply.

The spectrum analyzer is an HP 3588A and it can tolerate up to 4V DC. So as long as I keep the amplifier supply below +/- 4V I should be OK.

The maximum frequency I will be analyzing is 2000 Hz so just matching the impedance with a resistor should be sufficient, right?
 

Offline w2aew

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Re: Testing amplifier w/ spectrum analyzer
« Reply #3 on: June 21, 2016, 01:13:24 am »
Would I just need to put a 50 ohm resistor in series with the output of the instrumentation amplifier?

More or less, but you should take the output impedance of the INA into account, e.g. if the output impedance is 10ohms, then you would add a 40ohm series resistor. Whether or no that matters depends on whether the interconnection is electrically "short" or "long" compared to the signal's wavelength, and that is frequency dependent.

But be careful: some spectrum analysers cannot tolerate any DC component, and many are very sensitive to too much input power. Too much of either and the unprotected front-end-mixer is instantly destroyed. Check the spec of your SA.

Thanks for the reply.

The spectrum analyzer is an HP 3588A and it can tolerate up to 4V DC. So as long as I keep the amplifier supply below +/- 4V I should be OK.

The maximum frequency I will be analyzing is 2000 Hz so just matching the impedance with a resistor should be sufficient, right?

The Ad8220 won't do well driving a 100 ohm load with such a low power supply. However, the good news is that with a 2kHz max frequency, you don't have to worry about impedance matching. That spectrum analyzer has a 1Mohm input impedance setting. Connect it up and go....
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