As mentioned in the initial post I will start to add some ideas/proposals for the next HW design.
All my comments are based on the dismantled DSO nano (3 modules : main board with ARM Cortex, plugin board with probe connector/card connector/USB connector/switches and display board).
As of my impression the DSO nano is based upon a multimedia player device in housing and plugin board. This was very good thought to keep cost low but also means some compromises
if one looks out of the perspective of a measurement instrument.
So here are my first proposals :
-
Design a new plugin board as a 4-layer-PCB to enable good signal shielding (at least one ground layer);
move the OpAmps, prescaler chip (yet : HC4051) and related R’s, C’s, L’s as close as possible towards the probe connector.
The stereo plug/jack connection can stay but might be prepared to be used as diffential probe connection (as the jack/plug provide 3 connections).
This would allow to make a move towards AC coupling easier. As long as we stay with the ARM internal ADC we can not exceed 500 kHz bandwidth anyhow.
For the PCB design provide full symmetric layout at least until the inputs of the Op Amps to lower noise impact. As the housing of the DSO nano is not really
an EMC shielding it might make sense to add a small metal shield over the signal input section - this would also help “against” impacts from the USB port and/or card slot.
Such shielding material is widely used in the mobile phone industry - so it should be availbale on the market.
btw : this does not automatically mean a redesign of the main board but would be a good idea instead of using “flying wires”. -
Use a better OpAmp; even if the TL082 is a really good and well known workhorse (and therefore also cheap) it is used at its low end of operating voltage in the present
design; it’s also more of a general purpose chip and not so much trimmed for measurement purposes. I have read of sync problems with the DSO nano - those can be based
on production lot variations of the chips which will show up very much if the device is driven close to its operating limits (here : close or maybe below the lower Vop limit of 6V acc. the datasheet I have from ST)
There is a large selection of high precision, high slew rate, low input current etc.pp. OpAmps out there.
So here is my quote : Texas Instruments OPA 2141 - for sure would need a new calculation of the input voltage divider chain - but I think it’s worth it in regards to measurement
accuracy and flexiblity. I assume that the other passive SMD components have been selected with a very low tolerance on values. -
In regards to add extensions to the DSO nano I recommend to bundle the yet unused I/O port connections of the ARM Cortex and add something like a “panelmate connector” (TM of Molex) for them
to the mainboard. This opens the opportunity [together with some SW enhancements!] to add other ADC boards, special probes, data interchange capability with other
DSO nanos, Logic analyzer functions…) -
Add full realtime USB interface capability to the DSO nano (is more of a SW issue I think) - this would allow to integrate the DSO nano into an automated measurement setup which might
be based on the National Instruments LabView suite(s).
In regards to the production of the DSO nano I recognized quite a bit of solder tin on the connectors of the ARM chip <=> points to hand soldering. This should be done via (hot air) reflow
to lower the risk of shortages. Even lead free tin can still “flow” some µm if the chip gets heated up and cause shortages or increased crosstalk.
Hope my inputs can help and are interesting to the (desireably) growing DSO nano fan community
All the best
Traugott