New approach to a differential probe. - Page 1
Forum section: electronics / projects

author: JS
author_rank: Frequent Contributor
timestamp: June 10, 2018, 07:56:38 am
content: I'm in the need of a differential probe with different specs than standard. I do mostly audio, sensors, etc. so I need low bandwidth, low common mode voltage, low noise and some gain, to be used mainly with a Rigol DS1054Z. With BW limited it has 3mVpp noise in normal mode and 400µV in high res, I don't care about the ~100µV with averaging or math LPF.So, for the noise gets tricky I don't really need to get any lower than 4µV EIN, but will depend on the bandwidth and I plan to add an output filter. As I said 1MHz at 1X, I don't expect more than 10kHz at X0.01. Also, the input resistance for the probe usually limits that, 1MΩ is already adding 13µV so not possible to get the 4µV with that input resistance at 10kHz but I'd still be interested in lower bandwidths for some applications.The rough specs I'm aiming to are:30V common mode.1MHz BW @X120µV EIN @10kHzMore likely powered from dual 9V bateries.>1MΩ common mode input impedance.Ideally using jelly beam opamps, while I'm open to different approaches I don't have access to shmick opamps or a big variety differential amplifiers.Extra features:0dB, 20dB and 40dB selection.Full BW, 100kHz, 10kHz, 1kHz LPF selection.External trim for CMRR and offset.Well, the idea is to avoid input attenuation for the normal mode while maintaining the attenuation for the common mode, in this way I can avoid the extra noise and bandwidth penalties to having to use a gain of 10 to retrieve the original input level. This can be archived with bootstrapping the differential mode signal with two added bonus, the first would be to get a much higher normal mode input impedance (the measurements for high output impedance sources are more precise) and having a lower input resistance seen by the input opamps so input current noise is less of a problem.Note that there are two sources of noise in the input stage, as the bootstrapping amps are also inducing a source of noise, with a noise gain of 2. But not only there's no more attenuation on the input, but the instrumentation amplifier has a much lower input resistance, so a wider variety of opamps might be suitable for the task.I leave an schematic After the bootstrapping I could use an instrumentation amplifier or individual opamps. I choose this approach as I could change the gain of the stage using a single resistor, if that wasn't required U1A and U1B wouldn't be necesary and the noise could be further reduced. CMRR would be limited by the resistor selection but that could be trimmed, in this cases is more usual the input unbalances affect CMRR even when using perfectly matched resistors, hence the desire of having external CMRR trim. The resistors at the input should be selected accordingly to the schematic ratios ensure stability, CMRR and high NM input impedance (about 10MΩ). The circuit is lacking capacitors in the input stage, paralleled with some of the resistors to improve HF response, those should also be selected and/or trimmed.I hope you catch some traps I'd missed.Regards.JSPS, I forgot to mention, I'm not even thinking in using TL074, was just one random pick at the simulator which doesn't seem to care about noise at all. Without going crazy and using CA3140 for all the opamps the 20µV EIN is about there, as mentioned it could be better selecting different amplifiers for each stage.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: June 10, 2018, 02:12:48 pm
content: What you want is pretty much what Tektronix did with the 1A7A and 7A22 plug-ins for the old scopes. I'd download the manuals for both and look at how they did the attenuation. It might be tough to duplicate the switching unless you were very lucky with rotary switch wafers and clever wiring, but the concepts would be useful. Those plug-ins are my standard ones for most all audio work, to the point where I consider them "secret weapons". I would definitely include the same switchable low pass filters as Tek did. I don't use offsets much, as anything at the high gains will usually use AC coupling.

author: JS
author_rank: Frequent Contributor
timestamp: June 10, 2018, 03:43:54 pm
content: Thanks for the tip, I'll look into it. For what I saw in the manual so far they use a quite convoluted ppwer supply to archive CMRR, using a floating input amplifier, I'm trying to avoid that, as it requires the common mode to be within the rails, but granted it gives much higher CMRR while mantaining a low input resistance in series with the input which is very nice for noise.A good transformer might do the task for audio applications, I might have one or two around the lab that could come handy from time to time, but with this I'm more towards the side of DC and sensoring. For low level exclusive audio applications I do have a really nice soundcard, probably more expensive than the scope...Not to speak the traps while building discrete but I have my fair share of discrete audio opamps so not a biggy. Good thing about those, if I dont like specs I can tweak them.JSEnviado desde mi LG-M250 mediante Tapatalk

author: tggzzz
author_rank: Super Contributor
timestamp: June 10, 2018, 04:25:57 pm
content: You might like to consider using "instrumentation amplifiers", which contain the principal three opamps and matching resistors.It is left as an exercise for the student as to whether they can meet the common mode voltage, sensitivity, and noise requirements.

author: JS
author_rank: Frequent Contributor
timestamp: June 10, 2018, 05:00:59 pm
content: Well, that pict looks upside down, the inclinomenter on my phone might be acting up.Quote from: tggzzz on June 10, 2018, 04:25:57 pmYes, I can, and I did consider them. I do have an INA163 in the bin and a few AD620 on the way, but the only real advantage on them is the matching resistors, and as I wan't to trim the CMRR from the front panel it might be harder to do with the IC than discrete opamps, as I need the feedback resistor to be smaller than the rest of them by design, so they might be better saved for some other project.The INA163 and the AD620 are optimized for lower impedance sources than what's optimal for this design, about 1.2kΩ. I want at least 10kΩ input impedance, even after the bootstrapping. I could use OP27 opamps there, but the noise from the input resistors is dominant there so I don't need crazy low noise as those provide. As explained, CA3140 add less noise than the input resistors, so it's a nice fit for the task and I have them on the bin as well. I might want to add a few lower noise fet input opamps to the bin, but not for this task I guess.JS

author: David Hess
author_rank: Super Contributor
timestamp: June 10, 2018, 08:25:44 pm
content: Tektronix made a stand alone version of the 5A22/7A22 named theAM502which likely does exactly what you want. Unfortunately they are not as common as the 7A22 on the used market.