Author Topic: New low-cost ($170) 100MHz Differential scope probe from Micsig  (Read 40269 times)

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Online Hydron

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Re: New low-cost ($170) 100MHz Differential scope probe from Micsig
« Reply #75 on: July 19, 2017, 08:04:53 am »
The input resistors shouldn't be a problem, both the voltage rating and power have to be considered. There are hv smd resistors, easy to buy 3kv ones at 2512 size, 1kV @1210 . At 2500V input however 5 1Meg resistors in series will dissipate 250mW each and those in the picture don't seem big enough. The input compensating ceramic caps would need to be upped in voltage rating too
I was deliberately being a bit pessimistic about the voltage ratings - was assuming that the working (not overload) voltage was used, and that you'd want a reasonable safety margin on top. At 1206 (my guess from the pictures) you can get 400V/1kV working/overload voltage at a reasonable cost, higher if you pay more, but I'd start to worry about creepage/clearance distance too much above this, and also power dissipation as you mentioned. If I were to push towards 2.5kV I'd count on changing a few things, though at the price it's OK to take a risk modding one!.

As for the gain question, I think the mystery will have to wait for better PCB pictures or for the courier to show up with mine (hopefully thursday).
All I can see is what looks to be 2 single ended input amplifiers (in the ADA4817), followed by traces going via the relay (appears to be 2 per side) to a differential-to-single ended amplifier in the SOT23-5 package driving the output. Unless I'm interpreting the signal flow incorrectly and it's actually connected like an instrumentation amplifier (with gain adjusted via a single resistor) then I don't understand how the gain could be switched by the relay.

Other than the questions of how high the input voltage can be pushed and how gain switching is done, I'm please to see that it looks like a promising design with some good parts, rather than just another dodgy clone made by Pintech etc. Hopefully this impression is proved right!
 

Offline MasterTech

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Re: New low-cost ($170) 100MHz Differential scope probe from Micsig
« Reply #76 on: July 19, 2017, 03:35:52 pm »
Other than the questions of how high the input voltage can be pushed and how gain switching is done, I'm please to see that it looks like a promising design with some good parts, rather than just another dodgy clone made by Pintech etc. Hopefully this impression is proved right!

Totally, it was time someone made this kind of probe at a reasonable price, given the electronics that these probes pack I never understood the high prices.

Now Micsig please, make a current probe similar to the Tek A6302  ;D
 

Offline alm

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Re: New low-cost ($170) 100MHz Differential scope probe from Micsig
« Reply #77 on: July 19, 2017, 04:03:56 pm »
Read some posts on the TekScopes list about the complexities of building a low noise DC-MHz current probe before expecting similar performance at an affordable price. As far as I know Tek is still selling pretty much the same probe with different (in some ways worse) electronics and connectors as the old P6042. And they have not been surpassed by competitors in size or noise level.
 

Offline MasterTech

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Re: New low-cost ($170) 100MHz Differential scope probe from Micsig
« Reply #78 on: July 19, 2017, 04:23:44 pm »
Read some posts on the TekScopes list about the complexities of building a low noise DC-MHz current probe before expecting similar performance at an affordable price.

Did you notice the laughing emoji next to my comment? I guess not..

Besides that, what is wrong with "expecting" a new design that lowers the cost dramatically compared to old products while maintaining fairly decent specs?
SDS1202X-E, SSA3021, TO1104.... ring any bells?
« Last Edit: July 19, 2017, 04:37:27 pm by MasterTech »
 

Offline alm

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Re: New low-cost ($170) 100MHz Differential scope probe from Micsig
« Reply #79 on: July 19, 2017, 04:46:05 pm »
Has there been a similar advance in ferrite technology and Hall sensors as in ADCs and memory? What would the modern way be of embedding a Hall sensor in a ferrite core with minimal gaps but maximum flux through the sensor?
 

Offline MasterTech

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Re: New low-cost ($170) 100MHz Differential scope probe from Micsig
« Reply #80 on: July 19, 2017, 07:43:40 pm »
I don't know, not my field of expertise. But that does not preclude a drop in price, all manufacturing areas have seen cost/quality/technological improvements. Take an X-ray tube, same vacuum and materials technology now and 40 years ago, but price has dropped a lot.
 
 

Offline supercilious

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Re: New low-cost ($170) 100MHz Differential scope probe... actually $160
« Reply #81 on: July 20, 2017, 01:22:23 am »
...or just over $100 each (plus 22% tax, ouch) if you order 2 for 185 EUR like I just did

Where did you get them at this price? Please PM me if you are not confortable posting it in the thread.
 

Online Hydron

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Re: New low-cost ($170) 100MHz Differential scope probe from Micsig
« Reply #82 on: July 20, 2017, 02:13:27 am »
In case of interest, we have the combo kit (2x) at Euro 185 + VAT
Link to this deal: http://www.batterfly.com/shop/micsig-combo-dp10013

As I haven't received my order yet (courier says tomorrow) I can't report on anything except Simone's communication and how long it took to ship, but those were both top notch (friendly and helpful via PM, shipped same day as I paid).
 

Online Hydron

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Re: New low-cost ($170) 100MHz Differential scope probe from Micsig
« Reply #83 on: July 21, 2017, 07:15:05 am »
Just got mine today, have not had a chance to properly check them out but happy with what I have been able to quickly check. For now I've scanned the manual, has some extra info which could be of interest: http://imgur.com/a/Ctycc

Main new things I saw in the manual:
- Overload indication
- Offset self-cal procedure
- 1000V CAT II rating
 

Offline CustomEngineerer

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Re: New low-cost ($170) 100MHz Differential scope probe from Micsig
« Reply #84 on: July 21, 2017, 10:13:43 am »
Couldn't find the user manual on Micsig's site, but did manage to find it on Micsig's Amazon site in case anyone is interested and looking.
 
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Offline mikeselectricstuff

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Re: New low-cost ($170) 100MHz Differential scope probe from Micsig
« Reply #85 on: July 21, 2017, 06:52:44 pm »
Has there been a similar advance in ferrite technology and Hall sensors as in ADCs and memory? What would the modern way be of embedding a Hall sensor in a ferrite core with minimal gaps but maximum flux through the sensor?
https://www.aimtti.com/product-category/current-probes/aim-i-prober-520

This uses a fluxgate sensor.

I wonder if there would be scope to adapt a GMR hard-disk head.
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Offline alm

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Re: New low-cost ($170) 100MHz Differential scope probe from Micsig
« Reply #86 on: July 21, 2017, 08:25:56 pm »
https://www.aimtti.com/product-category/current-probes/aim-i-prober-520

This uses a fluxgate sensor.
The major advantage is that you can use it without enclosing the conductor. Price is not very low, bandwidth is too low for many applications (5 MHz vs 50 MHz for the Tek), and noise is much worse (8 mA from DC to 5 MHz vs approximately 0.15 mA RMS from DC to 100 MHz). The latter is a big deal in modern circuits that can fly to the moon on 1 mA ;).

I wonder if there would be scope to adapt a GMR hard-disk head.
This has been proposed before, but I am not aware of anyone actually pulling it off. I would be worried about linearity (both over the bandwidth and over the dynamic range) and dynamic range. A Hall sensor is not necessarily great for linearity and dynamic range either, but Tek got around that by feeding a DC current through the core and using the sensor only as a null detector. Trying to do the same with a disk head might turn out to be more difficult than a Hall sensor and would definitely require some custom magnetics.
 

Offline MasterTech

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Re: New low-cost ($170) 100MHz Differential scope probe from Micsig
« Reply #87 on: July 21, 2017, 08:41:48 pm »
Just received them. This may be the fastest take it apart that I've done on a new item :)



High voltage silicone cable




Notice the slot to provide isolation between inputs along input divider

 
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Offline MasterTech

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Re: New low-cost ($170) 100MHz Differential scope probe from Micsig
« Reply #88 on: July 21, 2017, 08:46:00 pm »
And the board, had to reduce the quality of the pic to meet the 1M limit

Looks like a good design, thanks micsig!

« Last Edit: July 21, 2017, 08:47:57 pm by MasterTech »
 
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Online EEVblog

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Re: New low-cost ($170) 100MHz Differential scope probe from Micsig
« Reply #89 on: July 21, 2017, 08:52:09 pm »


What's the J3 pin header on the bottom used for?
 
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Offline nctnico

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Re: New low-cost ($170) 100MHz Differential scope probe from Micsig
« Reply #90 on: July 21, 2017, 11:10:11 pm »
How about the other side of the PCB? I want to see the USB section!
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Online Hydron

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Re: New low-cost ($170) 100MHz Differential scope probe from Micsig
« Reply #91 on: July 22, 2017, 12:31:36 am »
I assume that J3 is for programming U9, which I think is a Silabs 8051 micro (does DC offset adjustment and controls the gain switching relay). The micro also does DC offset cal when you start it up while pressing both buttons (see manual).

The other side of the PCB is very boring - mostly ground plane, a bit of decoupling and a small circuit that doesn't seem to be part of the signal path. I've attached a (cellphone) pic.

Note that the power input seems to be isolated - there is no DC path from any pin of the USB input connector to the rest of the PCB. The output USB connector has one of it's data pins grounded, the other reads high-z (about 2 Meg) - no idea why they have done this!

IC part numbers I've found so far:
http://www.analog.com/media/en/technical-documentation/data-sheets/ADA4817-1_4817-2.pdf (main input buffer)
http://www.ti.com/lit/ds/symlink/opa2171.pdf (low freq op-amp, involved in offset circuit, marking OPMI)
http://www.analog.com/media/en/technical-documentation/data-sheets/AD8001.pdf (output diff-SE converter opamp)
DC-DC part number is AH4V55V5S-1W, made by "SUCCEED"

I haven't gotten any further working out how CMRR is maintained when switching gains - still looks to be done by switching the ouput resistors between the ADA4817 and the AD8001 from 100 to 1000 ohms. This continues to not make sense to me as i would have thought that CMRR matching would be a nightmare due to needing to match 2 resistors + relay contact resistance between 2 separate ranges. I'll look into it further to try and make some sense out of it.

As for CMRR measurements, I measured one probe at 1MHz as ~54/52 dB (50/500x) and the other as ~47/50 dB (i.e. not quite meeting spec). At 10MHz I got ~35dB for both ranges on the better of the two probes, but I'm less certain of those numbers.

Edit: forgot to mention that the screws are under the label (ugh) but it thankfully came off without visible damage. There are 6 (!!) of them going into brass threaded inserts, and the case is nice and solid with a decent chance of passing a Dave-test. Plastic fingers from both case halves go into the slot between the HV inputs - I'd say that the resistor/capacitor network would arc over before anything else in this thing. Ouput bnc/cable is not a strong point of this thing (feels cheap, especially compared to the nice case) but i guess it does the job and is easily replaced (at which point the 75R output could be made into 50R).
« Last Edit: July 22, 2017, 02:23:52 am by Hydron »
 

Offline IAmBack

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Re: New low-cost ($170) 100MHz Differential scope probe from Micsig
« Reply #92 on: July 22, 2017, 04:34:51 am »
I got mine yesterday. Batterfly claimed that it has newer firmware, hence little delay in shipping. In manual there is "CATII" statement.
 

Offline MasterTech

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Re: New low-cost ($170) 100MHz Differential scope probe from Micsig
« Reply #93 on: July 22, 2017, 04:44:15 am »
These resistors control the gain, they are 100 and 1K resistor pairs switched by the relay and connected to the output of each op-amp.
Still haven't figured the whole feedback loop though, but it seems that the gain could be controlled at the input stage
 

Offline exe

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Re: New low-cost ($170) 100MHz Differential scope probe from Micsig
« Reply #94 on: July 22, 2017, 05:04:43 am »
I got mine yesterday. Batterfly claimed that it has newer firmware, hence little delay in shipping. In manual there is "CATII" statement.

Wait-wait-wait, firmware? In the probe?? Why?
 

Offline wraper

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Re: New low-cost ($170) 100MHz Differential scope probe from Micsig
« Reply #95 on: July 22, 2017, 05:33:12 am »
I got mine yesterday. Batterfly claimed that it has newer firmware, hence little delay in shipping. In manual there is "CATII" statement.

Wait-wait-wait, firmware? In the probe?? Why?
Why not? Some chips might be digitally calibrated. Why would they put a number of pots and trimmer capacitors instead? Also didn't you notice there is no hard switch for X50/X500 selection so likely there is MCU for that.
« Last Edit: July 22, 2017, 05:37:15 am by wraper »
 

Online Hydron

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Re: New low-cost ($170) 100MHz Differential scope probe from Micsig
« Reply #96 on: July 22, 2017, 05:49:03 am »
These resistors control the gain, they are 100 and 1K resistor pairs switched by the relay and connected to the output of each op-amp.
Still haven't figured the whole feedback loop though, but it seems that the gain could be controlled at the input stage
These resistors do look like they control the gain, and are what confuse me. If the ratio between the 100 and 1k resistors on one op-amp varies compared to the other, then you could adjust the input stage to get good CMRR for one range, but on the other it would be terrible. Even 0.1% gain difference limits CMRR to 60dB, and these probes should do better than that at low freq.

There is definitely a MCU involved - it switches ranges (remembering which between power cycles), corrects for DC offset, and is able to perform a self-cal if the offset needs fixing.
 

Offline Electro Detective

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Re: New low-cost ($170) 100MHz Differential scope probe from Micsig
« Reply #97 on: July 22, 2017, 09:40:41 am »
The coax shield copper looks green-ish, especially on the bottom solder pad

I've seen that before, usually not good as time passes   :--

 

Offline joeqsmith

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Re: New low-cost ($170) 100MHz Differential scope probe from Micsig
« Reply #98 on: July 22, 2017, 11:44:55 am »
I was asked about running one of these across the small transient generator that I use to benchmark multimeters.   With the probes input, I am sure we could find out at what level it breaks down.   If people wanted to see both l1-l2, l1-gnd and l2-gnd, I could just cycle through them while increasing the level until the probe is damaged.   Like the meters, I could run some sort of highish voltage AC/DC test to make sure it at least handles that.  I didn't see any derate curves for it. 

If any of you have an interest in seeing one tested to failure, or have some specific potentially destructive test you would like to see ran, feel free to post.   

Looking at the pictures, my concern would be at the faster edge rates putting a lot of stress on the two resistors in series with the cap string.   Without someone sketching out the front end, hard to say but seem like these two resistors, assuming they clamp on the back side of the caps, would go first to go.  No schematic or probe, so a total guess on my part.   No derate, seems I would be allowed to hammer it and it should survive. 
How electrically robust is your meter?? http://www.youtube.com/channel/UCsK99WXk9VhcghnAauTBsbg
 
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Offline tautech

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Re: New low-cost ($170) 100MHz Differential scope probe from Micsig
« Reply #99 on: July 22, 2017, 12:11:03 pm »
DC-DC part number is AH4V55V5S-1W, made by "SUCCEED"
Google can't find it.
Wonder how much isolation it provides ? Could it be the limiting component for this units voltage ratings ?  :-//
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