43,000 count voltmeter from 10 bit ADC - how? - Page 1
Forum section: electronics / beginners

author: michaeliv
author_rank: Frequent Contributor
timestamp: September 01, 2015, 03:14:47 pm
content: Hi,EDIT: There's actually an 18 bit ADC on the board.I got a 5 digit pannel voltmeter and noticed that it only has a STM8S003F3 microcontroller(10 bit ADC), a few passives + what looks like an op-amp(couldn't find what the CAH4 SOT23-6 part is).Question is: how do they achieve this ?The front side doesn't have any components except for the LED display.Specs are:- 0-4.3000v, 0.0001v (0.1mV) resolution- 4.3-33.000v, 0.001v (1mV) resolution- ~300ms Update RateItem link:http://www.aliexpress.com/item/Green-LED-display-Color-DC-0-4-3000-33-000V-0-36-Digital-Voltmeter-three-Wires/1320678280.htmlThanks!

author: bktemp
author_rank: Super Contributor
timestamp: September 01, 2015, 03:16:25 pm
content: U2 is a MCP3421 18bit ADC.

author: michaeliv
author_rank: Frequent Contributor
timestamp: September 01, 2015, 03:31:38 pm
content: Quote from: bktemp on September 01, 2015, 03:16:25 pmThanks, how did you manage to find the part from the code ?

author: bktemp
author_rank: Super Contributor
timestamp: September 01, 2015, 03:39:05 pm
content: I have used this great little chip before.After seeing these modules and the SOT23-6 device on its back, the MCP3421 was my first guess and I was right.

author: Brutte
author_rank: Frequent Contributor
timestamp: September 01, 2015, 04:27:48 pm
content: Besides, this ADC is much faster than 3sps so you could get arbitrary resolution result from 10-bit ADC by using dithering.

author: michaeliv
author_rank: Frequent Contributor
timestamp: September 01, 2015, 05:22:34 pm
content: Quote from: Brutte on September 01, 2015, 04:27:48 pmYup, that's what I thought was happening, they were doing funky stuff with the 10 bit ADC to get more resolution but less samples.Do you have any links that describe how this is achieved ?

author: retrolefty
author_rank: Super Contributor
timestamp: September 01, 2015, 05:34:23 pm
content: Quote from: michaeliv on September 01, 2015, 05:22:34 pmDid you not understand that the MCP3421 is performing the A/D conversion? The 10 bit A/D on the micro is probably not even being used.http://www.microchip.com/wwwproducts/Devices.aspx?dDocName=en520011

author: michaeliv
author_rank: Frequent Contributor
timestamp: September 01, 2015, 05:44:15 pm
content: Quote from: retrolefty on September 01, 2015, 05:34:23 pmYup I did understand that that is what is going on in this particular circuit - The MCP3421 has 3.75 SPS.I was responding to Brutte who (I think) was saying that the on-board ADC is faster than 3SPS and could theoretically be used to get higher than 10 bit resolution with decreased SPS. I was asking if this is possible even though this is not what is going on in the voltmeter above.

author: bktemp
author_rank: Super Contributor
timestamp: September 01, 2015, 05:49:48 pm
content: Atmel has some application notes on oversampling:http://www.atmel.com/images/doc8003.pdfhttp://www.atmel.com/Images/doc8498.pdfIt can be used to increase the resolution by 1 or 2 bits. More makes no sense in most cases, because of the nonlinearities in the ADC.

author: michaeliv
author_rank: Frequent Contributor
timestamp: September 01, 2015, 06:13:02 pm
content: Thanks a lot, very interesting.Another question: Seller says that in order to get the full 0.1uV resolution you need to use a separate, isolated power supply ( from the voltage source you are measuring ). Why would this make a difference ( assuming that both have equal noise ). I can't find any mention of this requirement in MCP3421's datasheet - though it does have differential input.

author: edavid
author_rank: Super Contributor
timestamp: September 01, 2015, 06:25:44 pm
content: Quote from: michaeliv on September 01, 2015, 06:13:02 pmYou don't need a separate power supply, just a separate power supply ground (Kelvin connection).You might want to read the previous thread:https://www.eevblog.com/forum/projects/$5-voltmeter-with-5-digit-%280-1mv%29-resolution/

author: Kleinstein
author_rank: Super Contributor
timestamp: September 02, 2015, 07:57:52 am
content: The MCP3421 not only has the 18 Bit ADC, but also a resonably accurate reference voltage (typical specs are 5ppm/K and +-0.05% absolute).Oversampling the µC internal ADC does not help that much on a DC reading. However it's quite powerfull if it comes to RMS readings.

author: bktemp
author_rank: Super Contributor
timestamp: September 02, 2015, 08:28:37 am
content: Quote from: Kleinstein on September 02, 2015, 07:57:52 amThat was the spec from the old datasheet. In the more recent version it is 15ppm/K and no maximum value is specified, so use at your own risk if you need a certain accuracy...They are great little chips, but without any specifications for the maximum values, they are useless for any decent design without measuring the drift of each unit.

author: coppice
author_rank: Super Contributor
timestamp: September 02, 2015, 08:32:58 am
content: Quote from: Kleinstein on September 02, 2015, 07:57:52 amThe gain stability is good, but not quite that good. Gain error with temperature is 15ppm/K typical, with no worst case specified. The absolute gain error is 0.05% typical, and 0.35% worst case. There is an additional error of 0.1% typical when the PGA is used. No worst case is specified for this. There is also a DC offset of +-40uV +-50nV/K to allow for at x1 gain. They don't seem to specify what happens to that at higher gains.

author: JoeN
author_rank: Frequent Contributor
timestamp: September 02, 2015, 11:24:57 am
content: Quote from: bktemp on September 02, 2015, 08:28:37 amThe old Microchip switcharoo...

author: dom0
author_rank: Super Contributor
timestamp: September 02, 2015, 12:23:31 pm
content: Quote from: JoeN on September 02, 2015, 11:24:57 amHere, we have thisgreat(datasheet rev 2)very good(ds rev 3) good part withabsolute(rev 2) specs.

author: Kalvin
author_rank: Super Contributor
timestamp: September 02, 2015, 02:22:36 pm
content: The tracking speed of the panel meter is quite poor because of the 3 Hz update rate. However, if one can get access to the source code of the panel meter, one could possibly create a small hardware and software hack so that the on-chip 10-bit A/D could be used too. When the signal amplitude changes "very much", the on-chip 10-bit A/D could be used as an approximate A/D for the measurement value. And when the amplitude is stable the 18-bit A/D will be used. In that way one can increase panel's responsiveness.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 02, 2015, 03:01:28 pm
content: If one could change the SW, it would be allready a speedup, if the 15 SPS mode(still 16 Bit) of the AD is used. If 3 of these readings are avaraged even a noticable 50 Hz suppression is possible. 15 SPS is way fast enough for manual readings. Usually close to 4 readings per seconds is not that bad - thats abut the speed of normal handhald DMMs.

author: Kalvin
author_rank: Super Contributor
timestamp: September 02, 2015, 03:04:56 pm
content: Quote from: Kleinstein on September 02, 2015, 03:01:28 pmYour suggestion even better as no hardware modification is needed.

author: michaeliv
author_rank: Frequent Contributor
timestamp: September 02, 2015, 04:26:09 pm
content: Looking at the sister ammeter ( 0.0000-3.0000A, 30mR ) : 'CA2P' SOT23-6, I'm assuming it's the MCP3421 again with a different code.I've looked around AliExpress and the MCP3421 being sold have several different codes for the same part number : CACP / CAVV/ CA20 for MCP3421A0T-E/CHWhy do manufacturers do this - give different codes to the same part number ?

author: bktemp
author_rank: Super Contributor
timestamp: September 02, 2015, 04:33:43 pm
content: Quote from: michaeliv on September 02, 2015, 04:26:09 pmSee datasheet page 35: The last two digits are "Alphanumeric traceability code", probably a datecode/production lot marking.

author: michaeliv
author_rank: Frequent Contributor
timestamp: September 02, 2015, 04:50:53 pm
content: Quote from: bktemp on September 02, 2015, 04:33:43 pmThanks. Also, slightly unrelated - does the ammeter shunt in the above ammeter have a special name ( The wire-type shunt I mean ) ? What is the material called ? If I wanted to buy wire-shunt what would I search for ?

author: Kleinstein
author_rank: Super Contributor
timestamp: September 02, 2015, 05:08:10 pm
content: There are also versions of the MCP3421 with different I2C adress.There is resistor wire available. This can be Konstantan or the better Manganin type. However there is one difficulty: these materials are not easy to solder. On may need extra flux.