HF add-on amplifier to my oscilloscope + probes - diyAudio
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Old 2nd July 2013, 08:19 PM   #1
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Default HF add-on amplifier to my oscilloscope + probes

Hi Diy'ers,

I've been doing some measurements with my oscilloscope on the PSU rails going to a DAC I'm currently working on (DSD-1794 from TI).

And it's actually been quite a surprise to see that the digital noise on the supply rails is significant (many millivolts) - even if I seem to have been just reasonably fortunate with my decoupling capacitor arrangements. And also to learn that the "altogether" PSRR of the DSD1794 is relatively limited so that these noises show up at the outputs of the DSD1794.

I am intrigued to look further into this and would like to find - or build - a simple add-on amplifier to my oscilloscope that goes from e.g 20 kHz to around 300 MHz.

I'm guessing that a total amplification of ~ 200 will allow me to look sufficiently into the noise (the oscilloscope's 2 mV resolution limit/200 = 10 uV). This should be somewhat adjustable though so I've thought of e.g. two stages of x15 amplification or a suitable variable amplification solution.

My intention is to look mainly at waveforms and frequencies so I reckon that a design without very low distortion is acceptable.

I would appreciate if one of you have suggestions for an already made (and not too expensive) amplifier - or maybe a schematic/link to a schematic that is suitable in this context? If I am to build it myself I would prefer it to a simple & small design based on maybe one or two 9 volt batteries so that I don't have to have more AC wires running.

I have considered a stage built around TI's LMH6629:

http://www.ti.com/lit/ds/snosb18g/snosb18g.pdf

... but maybe there's a better solution?

Also, I have a couple of oscilloscope probes (40 MHz Cat II, & 60 MHz Cat I), yet measuring at these frequencies I may need something better/different or more widebanded ... are there any oscilloscope probes that are better suited to this purpose, maybe considered "good", not too expensive and maybe can be had second-hand?

Thanks for reading and possibly replying

Jesper
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Old 3rd July 2013, 12:33 AM   #2
DUG is offline DUG  Canada
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Keep the ground of the probe close to what you are probing.

I've seen noise "measured" because of the large area loop that the probe/ground path creates.

In some cases there was no significant noise at the measurement point...just loop pickup.

As a test, probe the same point the ground connection is on.

Have fun.

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Old 3rd July 2013, 06:40 AM   #3
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Hey DUG,

Thanks for answering and pointing out the loop that the ground probe path may cause. However, I already use the clip-on ground "spring" that comes with the probes I have so the effective length of the ground probe is about 9 mm. Also when I change the sampling frequency of the DAC signal the noise changes frequency and level so maybe there actually is digital noise here?

But I'd actually like to get around such issues in measuring digital noise so if you or someone else here knows how to do this with the amplifier circuit I'm looking for then that would be very interesting to me.

Greetings,

Jesper
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Old 3rd July 2013, 10:48 AM   #4
DUG is offline DUG  Canada
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Quote:
Originally Posted by gentlevoice View Post
Hey DUG,

... Also when I change the sampling frequency of the DAC signal the noise changes frequency and level so maybe there actually is digital noise here?

...

Greetings,

Jesper
If changing the sampling frequency changes the results then you need to go higher (faster) sweep times.

Look up aliasing.

Later
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Old 4th July 2013, 09:27 AM   #5
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Hi DUG,

Quote:
Look up aliasing.
The measurement frequencies & levels are changed on the supply rails to the DAC and to my knowledge this is plausible given that the frame sync, Master clock, data signal & bit clock changes frequencies. E.g. the bit clock goes from about 2.8 Mhz (44.1 kHz 32 bits) to 12.28 MHz (192 kHz 32 bits) when going from 44.1 kHz to 192 kHz sampling frequencies and I would expect this to change the noises present on the PSU rails (different timing of the current loads on the PSU rails).

But you are thinking something else?

Greetings,

Jesper
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Old 4th July 2013, 10:41 AM   #6
DUG is offline DUG  Canada
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I was thinking something else...If you sample a waveform too slow, you won't be getting all of it.

I forgot about the DAC operating changes.

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Old 4th July 2013, 10:46 AM   #7
Mooly is offline Mooly  United Kingdom
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Try this. If you connect the probe tip to the probe ground lead you should see nothing, just a perfect fine trace. Keeping those shorted, now touch the ground lead onto the ground point you have been using. Do you now see noise even though the tip is still connected to the ground lead ?
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Old 6th July 2013, 05:28 PM   #8
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Hi Mooly,

Didn't see your reply until now ... I have tried what you suggest and when connecting the ground probe to the ground "sense" pin there's no signal on the scope. The same happens when I place the ground pin and the "sense" pin on the ground I measured on previously.

On the other hand when I place the "sense" pin on the Vcc of the DAC and the ground pin on the ground the signal looks like the attached image. Which could be better but is also "ok" as I'm learning something from it ...

However, I'd like to do better in the future so (with some help from the LTspice forum) have experimented with the attached circuitry for the last couple of days. Its bandwidth goes from 10 kHz to appr. 160 MHz and according to the simulation there's no ringing on 10ns rise pulses (100 MHz). I intend to mount it so that the pins from the first LMH6629 to the measure circuit points are as short as possible (to reduce inductance). Amplification is 40 dBs which means that one div. resolution on my scope will correspond to ~ 20 uV.

I don't know if this is the best design possible for this purpose but it works and I reckon it can show even low noise levels on the DACs PSU rails. As a "need to know" but also out of interest

Greetings,

Jesper

P.S.: I haven't added a couple of reverse diodes to the input of the first LMH6629 yet (spike over-voltage protection). The 3 nH values in the circuitry are estimates for track inductances.
Attached Images
File Type: jpg ground+probe.jpg (533.6 KB, 223 views)
File Type: jpg meas_amp_ac_analysis.jpg (178.6 KB, 217 views)
File Type: jpg rail_noise.jpg (632.9 KB, 219 views)
File Type: jpg meas_amp_transient.jpg (182.6 KB, 209 views)
Attached Files
File Type: pdf LMH6629_meas_3.pdf (15.3 KB, 11 views)
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Last edited by gentlevoice; 6th July 2013 at 05:34 PM.
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Old 6th July 2013, 05:40 PM   #9
Mooly is offline Mooly  United Kingdom
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Layout and construction will be everything at those frequencies, and 40db gain too... its a tall order. Not to say its not do-able though
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Old 6th July 2013, 06:08 PM   #10
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why not get a good s/h tek analog scope for $100 ??

Digital sampling is the wrong tool for noise problems, unless you really know what you are doing.
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