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| Differential preamplifier for oscilloscope |
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| rfguy2020:
Hello, I would like to measure weak differential signal with oscilloscope. I have Rigol DS1054 and I was able to get some signal out by subtracting two channels, but as my signal is in level of few mV, it is not very good solution. My signal is around 10kHz - 600kHz. I have fairly significant common mode noise around 4-10MHz, which I need to filter out. Amplitude on my signal is around 1mVpp. I have been looking for some low cost preamplifiers like Alphalab LNA10 and Chinese T100. Those are a bit limited and I have been thinking to make my own preamplifier as it looks fairly simple. Has anyone done this project before or is there some ready made schematics of such board? I would need at least differential input, adjustable gain and low pass filter. |
| ch_scr:
In theory, all you'll need is a differential amplifier. In practice, look at the CMRR of Opamps in the 4-10MHz range. That's before the additional perfomance hit from impedance/phase mismatch in the two input paths... So it's (quite) a project, not insurmounteable, but probably you'll learn something. Do you need any gain? How big is the voltage swing on those signals? Maybe putting a (faster) fully differential amplifier at the input and a (slower) single ended one behind it, that's able to drive 50R with the required amplitude, you might get some benefit in "stacking" the CMRR's like that? |
| rfguy2020:
--- Quote from: ch_scr on December 07, 2023, 12:23:32 pm ---In theory, all you'll need is a differential amplifier. In practice, look at the CMRR of Opamps in the 4-10MHz range. That's before the additional perfomance hit from impedance/phase mismatch in the two input paths... So it's (quite) a project, not insurmounteable, but probably you'll learn something. Do you need any gain? How big is the voltage swing on those signals? Maybe putting a (faster) fully differential amplifier at the input and a (slower) single ended one behind it, that's able to drive 50R with the required amplitude, you might get some benefit in "stacking" the CMRR's like that? --- End quote --- My differential signal is around 1mVpp so I need probably 1000x gain. On the other hand, my common mode noise is level of 10mVpp. |
| tszaboo:
It's not that difficult to build an amplifier like that. You start with an Instrumentation amplifier, like the classic INA128, select a gain for it that will amplify the signal enough, and give it a dual supply. The REF pin should be either ground, or driven by a low impedance source. The gain is selected by one resistor, you can place a dip switch to select the multiple ones, maybe a pot. The power supply can be two 9V batteries. Look at the gain/bandwidth graph, and make sure you don't use too much gain for your INA. |
| David Hess:
I could not find the Chinese T100, but what did you find limiting about the Alphalab LNA10? The old Tektronix AM502 will do what you want, and they usually work despite their age. --- Quote from: ch_scr on December 07, 2023, 12:23:32 pm ---In theory, all you'll need is a differential amplifier. --- End quote --- Oh, I like that idea, and it is simpler than a complete instrumentation or difference amplifier. A two operational amplifier differential amplifier has a common mode gain of 1, and adjustable differential gain, and only requires three resistors. Pick some operational amplifiers with reasonable common mode rejection at 10 MHz and you are all set. Let the oscilloscope do the subtraction. |
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