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| Oscilloscope 100x Pre-Amp for noise measurement? |
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| tggzzz:
There's a lot of useful information about this (and many other topics) in The Art Of Electronics - particularly chapters 4,5,8. You can see the table of contents in http://artofelectronics.net/wp-content/uploads/2016/02/AoE3_chapter9.pdf There's subject and parts indexes at https://artofelectronics.net/ Get that book and you'll save yourself a lot of time and effort, leaving you free to concentrate on your USP :) |
| RogerThat:
Aah, realised my formula misstake a few minutes after posting. Thanks Karel. "you have to be more clear on what performance you are for". Well, as good as possible. And what do I mean by that? In my current project I want to measure voltage noise/rippel down to 1uV. I'm starting to get a grip about what are the limiting factors regarding noise/BW. So what I understand now is that a resonable BW would be up to 10khz for 1uV. The input sampel cap and resistor on the ADC form a 32khz low pass filter so anything above say 40khz will make little impact on the measurement. So, let's limit to as good as possible with 40khz and components that can be had for thru hole mounting (for one-off things I really avoid SMD stuff). Ok, so the signal chain would be like this then (maybe just one amp if it can make 100 gain straight away): BNC->LPF (32khz)->AC couple 10x amp ->10x amp -> BNC Low resistor values and I guess film/tantal caps? tggzzz: great links, will read. |
| Marco:
It's hardly worth it to go two step for 100x40kHz, OPA847/LMH6624 combine low noise with way more GBW than you need. |
| RogerThat:
I hate when someone doesn’t follow up their thread, so… I built the thing :-/O. 100x Gain, 0,36hz-31.8kHz (-3db at 31.8khz that is). amp is good for 500khz @ 100x. BOM: LT1028 (DIP-8) C1: 2 x 22uF film cap (what I had in my box-of-many-components) R3: 10k R1: 10 R2: 1k C2 (parallel with R2): 5n/5000pF 2x 470nF decouple Film caps Power from two 9V batteries, +-9V. RG316 input cable….just put some solder on the measuring ends. Twinings tea metal box To-Do: will put BNC chassi contacts at the box input/output. Probe (@ 1x) measure directly on the protoboard. If I calculated correctly the theoretical noise would be less than 1uV. In reality my scope tells me around 20uVpp and 5uVrms when shorted with a 1k resistor. As you can see(picture two) the the FM Radio 100mhz is creating the noise floor. Picture one is the output from my voltage reference (the spike is caused by the ADC sending SPI). Picture two is the measuring ends shorted by a 1k resistor. Now off to part 2 of this, how do I reduce the noise from the voltage reference. I probably done something wrong, specs say 1uVpp. I'll create a new thread for that. Have a nice week-end everyone. |
| Kleinstein:
The LT1028 has quite some input current noise. With the 10 K at the input this gives quite some noise from the low frequency range, below the input coupling limit. Especially in the low frequency range the LT1028 is good only for low impedance sources (e.g. < 100 Ohms). So there is quite some low frequency noise - that is expected to come from the LT1028. So one could reduce this noise quite a bit if C1 is chosen even lager and the lower frequency limit set by another RC filter / AC coupling behind the amplifier. Another option would be to use an amplifier with less current noise, like AD8675, OPA827 or OPA140. |
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