Close

Analog frontend

A project log for minimal DSO

I need your help to develop the smallest and cheapest (yet useful) breadboard sniffing tool!

yann-guidon-ygdesYann Guidon / YGDES 11/29/2015 at 22:215 Comments

@Klima has designed the supercute #Small voltmeter and I finally took some time to look at his design.

The LED bubble display is cute but quite expensive. With a small LCD as intended, this should be easy to solve.

Now the interesting part is the sigma-delta ADC : the MCP3421 is small and cheap enough, it works as a voltmeter but could also be reused for current and resistance measurements with a bit more electronics (maybe with a MCP3422 that has 2 inputs ?).

The display and CPU aspects should be easy to merge with the rest of the project. An I2C interface is required but if we have a programmable MCU, it's possible to bitbang it.

Discussions

Yann Guidon / YGDES wrote 11/30/2015 at 02:16 point

MCP3421 is slow, yes, but that would be for the volt/ampere/ohm measurements.
The DSO138 uses a funky design and yes, the bipolar system (with the PWM-driven DC/DC) could be much better... But I suppose the people in Shenzen drown under dirt cheap parts and they're looking for ways to sell them :-D

  Are you sure? yes | no

K.C. Lee wrote 11/30/2015 at 00:47 point

BTW The STM32F103 has two 1 Msps 12-bit ADC (not MUX) and 20kB of SRAM with DMA.  That's what some of the Chinese "scope" kit uses.  Breakout boards are around $4 from China.  Also it comes with high speed USB, so you can hook it up to a PC.

I made a quick breakout board for it.
https://hackaday.io/project/6607-stm32f103-canadian-maid-breakout-board

  Are you sure? yes | no

Yann Guidon / YGDES wrote 11/30/2015 at 01:22 point

Hi Ottawa ! :-)

I will soon play with a DSO 138 and it comes with ncie schematics.

OTOH the MCP3421 has interesting PGAs and a good voltage reference, it is also pretty precise so this is another application range (low frequency vs high frequency). It can measure voltages out of its power supply range, even though it's not a requirement, but "you never know".

Ideally I'd like to also have a crude logic analyser that can sample at some tens of MHz so people can tie it to the #Discrete YASEP  :-)

  Are you sure? yes | no

K.C. Lee wrote 11/30/2015 at 01:29 point

MCP3421: 3.75 SPS (18 bits), 240 SPS (12 bits)   That is nice for a multimeter, but is kind of slow for everything else.

  Are you sure? yes | no

K.C. Lee wrote 11/30/2015 at 01:53 point

I looked at the DSO 138 schematic.  Not the greatest design, but it is a good start.  
There are much better part for the opamp and power supply in the last 20 years.  Could have made life a bit easier without needing a bipolar supply.

  Are you sure? yes | no