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| Differential preamplifier for oscilloscope |
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| tszaboo:
--- Quote from: David Hess on December 08, 2023, 12:41:20 am --- --- Quote from: TimFox on December 07, 2023, 07:57:09 pm ---The simple four-resistor-plus-op-amp differential amplifier does have a CMRR limited by the resistor matching. --- End quote --- There is going to be confusion if confusing terms are used. We should at least agree on definitions. Differential amplifiers have differential inputs and outputs. Difference and instrumentation amplifiers have differential inputs and single ended outputs. Instrumentation amplifiers have high impedance inputs which minimize the effects of source impedance. Texas Instruments among others likes to confuse the issue, but resist it. --- End quote --- I don't find it confusing, though I usually use the term 'fully differential amplifier' for the amplifier that has differential input and output (even though usually those are high speed). So why is CMR an issue? Let's say you put together an amplifier out of 1% resistors, and it has a CMR of -40dB. You want to use this amplifier to amplify 1mV signal by 1000. If your input signals are 0V and 1mV, your output is 1V, everything nice. Let's say your input is 1V and 1.001V. Your expected output is 1V, but because of resistor mismatch, you get 11V (-40dB is 1/100). And this might happen at DC, or it picks up 50Hz noise, or your signal has other sources of common mode noise. There are ways to improve this, for example ferrite cores and placing your wires through it, but ultimately, it's the best to use very much matched resistors. And you can either select these by hand, get expensive 0.001% resistors, or get one of these nice ICs, where these are internally matched, often times laser or electronically trimmed. But ultimately it's characterized, which is important. You don't get surprised, when the output is 11V, because the IC has -100dB CMR. And you might be familiar with this, and maybe there is no new information for you in my post. --- Quote from: rfguy2020 on December 08, 2023, 11:17:39 am ---I have been considering to use AD8129 with their evaluation board: https://www.analog.com/media/en/technical-documentation/user-guides/UG-133.pdf AD8129 is not probably lowest noise, but it might do the job? --- End quote --- Looks like a very useful board for this purpose. |
| ch_scr:
--- Quote from: tszaboo on December 08, 2023, 01:41:49 pm ---(...) I don't find it confusing, though I usually use the term 'fully differential amplifier' for the amplifier that has differential input and output (even though usually those are high speed). (...) --- End quote --- An example for a low speed, high voltage, cheap-ish (and precision to boot!) fully differential amplifier is TI's THP210 --- Quote from: tszaboo on December 08, 2023, 01:41:49 pm ---(...) but ultimately, it's the best to use very much matched resistors. (...) --- End quote --- To automate such selection tasks in general, I've made up a python script.For an arbitrary example of "tightest 8 values from the list of measured examples", it looks like this: --- Code: ---grouping_optimizer.py 8 6.932 6.939 6.9155 6.8915 6.905 6.909 6.936 6.943 6.9355 6.9355 6.933 6.943 Average value 6.93712 Error sum 3928.14 Average error 491.02 ppm With these elements and corresponding errors: (6.932, 6.939, 6.936, 6.943, 6.9355, 6.9355, 6.933, 6.943) ['-738.78', '270.28', '-162.17', '846.89', '-234.25', '-234.25', '-594.63', '846.89'] --- End code --- It can also be asked for a specific target value as well: --- Code: ---grouping_optimizer.py 8 6.932 6.939 6.9155 6.8915 6.905 6.909 6.936 6.943 6.9355 6.9355 6.933 6.943 --target 6.918 Average value 6.92519 Error sum 15394.62 Average error 1924.33 ppm With these elements and corresponding errors: (6.932, 6.9155, 6.905, 6.909, 6.936, 6.9355, 6.9355, 6.933) ['2023.71', '-361.38', '-1879.16', '-1300.95', '2601.91', '2529.63', '2529.63', '2168.26'] --- End code --- It works happily with multiple elements of the same value, can also show the iterations and handle priority elements much like the resistor divider ratio optimizer. With this and an grid of measured examples on a piece of paper, quite a tight match is easily achived, for small batches. The "divider optimizer" can be used in the same vain to automatically get multiple "equal ratio dividers" from "excessive examples of random measured resistors". If they are measured at the same time with a warmed up instrument under stable conditions, the absoulte errors (like calibration) cancel out. As a bonus the "arbitrary ratio" of the divider comes free, within the limits of gaussian distribution* of the resistor examples ;) (*not guaranteed to be gaussian) |
| hpw:
--- Quote from: rfguy2020 on December 07, 2023, 10:20:44 am ---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. --- End quote --- I am looking for such differential gear as 0.01Hz to xx-Mhz is my target as to measure ADC/DAC power noise/figures as most ADC/DAC are internally cap. switcher so the today's LDO regulators may at higher freq. on the limits. Even shunt regulators have some limits. While simulation tell not the full trough. Or may the new 12 bit DSO's using averaging, may help in this requirement! But still a differential probe is a must have. Hp |
| leowood:
This is the T100 freq response looks like, from 1M to 10M (they rated the useful BW up to 10M when gain setting is X1,) So there will be a serious design flaw, the 5M gain will be 3x high than 1Mhz, terrible |
| hpw:
--- Quote from: rfguy2020 on December 08, 2023, 11:17:39 am ---I have been considering to use AD8129 with their evaluation board: https://www.analog.com/media/en/technical-documentation/user-guides/UG-133.pdf AD8129 is not probably lowest noise, but it might do the job? --- End quote --- Well, it looks for me, that we / I need a LNA one for 1/f noise and one for RF noise :phew: Currently on DIYAUDIO https://www.diyaudio.com/community/threads/ad8428-low-noise-preamplifier.407270/ as for the 1/f project with 2000 gain chip as AD8428. As for the RF LNA & AD8129 testing: BW RF range ends soon about 20..200Mhz and CMR gets down to 40..50dB. Some other project as seen on TI forum. Important will be to protect the opamp input as using ADC & Capacitors as connecting various power sources as capacitor to charge and discharge. In addition the connecting cable should not pickup any RF :palm: so shielding is must apply. Hp |
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