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GeekGirl:
Hi everyone,

I have not forgotten this project, just with the first week of Uni and a few urgent repairs, I have been waylaid. On Friday I hope to be able to do up a block diagram, so that we can split up the project in to manageable chunks ;)

If anyone has any ideas, or has started anything on this. speak up :)
free_electron:
Take a look at a 7129 for example. 4 1/2 digit dual slope converter.

Other solution may be to brew your own. These things are very sensitive and difficult to maintain. one good one is an ADuC624. these have 24 bit a/d convertor and integrated cpu that is running in time step with the convertor
( cpu sleeps while conversions is done).

The advantage is that you can do autoscaling algorithm and relay ( range select) on the cpu. You spit out your measurement through the built in uart , go over 2 optocouplers into the backend processor ( ground referenced electronics)

The backend processor handles user interface ( display / keyboard / usb interface / ethernet interface / gpib / whatever.

the measurement board has its own transformer winding powering it. communication goes between the measurement block and the main electronics through optocouplers. That was you have galvanic isolation. Your measurement electronics must be floating !. Otherwise there is no point starting this development at all. you will have groundloops all over the place.

if you want to learn how to make good input circuitry ( scalers / protection / current sense / true rms whatever ) go to agilents website and download the servicemanual for the 34401. they have fll schematics published. You won't be able to copy since there are a couple of asic's in it, but you can learn a lot from it.

i see a lot of posts in this topic about cpu selection. that is all irrelevant. the hardest part is getting the analog done right. You need to come up with a list of requirements there

voltage range for DC and AC ( flashover protection )

current range for AC and DC ( overcurrent protection. current sense resistor and gainstage ...

resistance range .. ( 4 wire ? ) protection against users that want to measure the internal resistance of their wall socket while it is live ...

Diode check ( use current source with voltage scaling so you can test zener diodes )

continuity check

Frequency count ?

capacitor / coil test ? ESr metering ?


I myself don't care about being able to measure 1 kilovolt and 10 ampere. give me something that is accurate in the 0 to 5 volt range with 3 useful digits after the comma.
current wise i'd like to see microamps to 1 amp range..

and then there is another biiiiiiig question. How do you calibrate this thing ?

The measurement logic should NOT use floating point anywhere. everything should be scaled integer math, preferrably even bcd encoded.
you can solve some of the problems by selecting the correct reference for tha a/d . analog devices has the REF19x family. there are reference diodes that produce exactly 2.048 and 4.096 volt. ideal to hook up to a/d convertors since they prodice directly  binary weighted scales.

feed that with a stable suppy from a current source (lm319 for example. that thing has an oven on board ) and you are now very accurate. You can self calibrate the inl dnl of the on board a/d and compensate with some smart software.

for current sensig you will need a kelvin contact setup with a gainstage. susumu makes great kelvin resistor in the 20 milliohm to 50 milliohm range with 0.01% accuracy. for half a dollar you have one.

sense the drop over it using something like an AD8220 instrumentation amp and you can get somewhere..

These are the first things to consider. the instrument stands or falls around the aquisition section. get that one right and the rest is smooth sailing.

dds:
Hi GeekGirl, there is already a block diagram at the TI site:

http://focus.ti.com/graphics/blockdiagram/blockdiagram_images/6279.gif (for handheld DMM)

http://focus.ti.com/docs/solution/folders/print/591.html

http://focus.ti.com/docs/solution/folders/print/593.html (bench DMM)


I don't know if you have already seen those docs, if yes - sorry :)



_Sync_:
Now that my FPGA project for school is over (well unfinished but that's another story) I am able to more on this project.

From what it looks like a dual-slope ADC is the standard used in common multimeters. I can understand that, because it cancels possible errors through integration. The issue with those ADCs is that you need very good parts in order to get high resolution and accuracy.

I found that todays common ADCs e.g. delta-sigma ones (e.g. LTC2410) should be able to produce a acceptable performance over a period of time.

The LTC2410 in particular is a 24 bit delta-sigma ADC (with even five more bits after the LSB!) with SPI interfacing and good accuracy. Everything is contained in a SSOP-16 package, which is well, quite small but it should be possible to use.

After I got my hands on this guy I'll see what I can do on the input side.

TrueRMS should also be possible with the help of the AD637 that seems to be widespread in this application.

AD637 Datasheet
LTC2410 Datasheet
logictom:
Read this thread couple of days ago, any news on the specs being written up? Interested to see where this goes ;D
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