Hello,

Im working on a project that requires measuring resistance and voltage on over 200 points (180-225). MCU's that I'm familiar have up to 120 channel adc. I was thinking about having 2 of them connected trough i2c or spi and both combined would provide over 200 points and usb connectivity to pc.

Is there a better way of doing this? I was looking into adc ic but non of them have that many channels.

 

A ques or two -

1) Do you have a a target resolution, accuracy, and sample rate ?
2) Measuring R, will you be doing that differential ?
3) What is range of Vin ?
4) Allowed crosstalk between channels ?

Regards, Dana.

 

In addition to Dana's questions when I have seen this done in the past it normally involved some pretty expensive MUX (Multiplexing) cards and the highest cards I have seen or used were 120 channels so you would need two of them. If I were looking to do what you seem to be looking to do I would likely use a MUX configuration with a fast scan rate and identify any channels outside preset limits.

Ron

 

It's a test jig for a board with multiple connectors. To simplify the design, reading just resistance would be fine. But if there is a way to read voltages with additional hardware I want to know what are the options.

Majority of lines are under 3.3v, 2x 5v and 2x 20V.
10 bit ADC would be just fine.
Vin preferably 5v.
I'm assuming resistance can be measured in one shot. Voltages updates 1-2 a second would be ok.

 

It's a test jig for a board with multiple connectors

hi Go,
Do you want to measure the actual resistance of the path or just check for continuity.?
E

 

Does the R reading need to be done differential or single ended ?

This drives the mux/pins decision, one or two pins/channel.

Regards, Dana.

 

@danadak single ended.
@ericgibbs resistance of the line to ground.

 

resistance of the line to ground.

hi,
So, the MCU or PC will have a library of resistance/voltage 'pass' values for every measurement point.?
Will the DUT be powered while the resistance test is run.?

E

 

@ericgibbs Correct, mcu pass value to windows software.

Resistance tested on un-powered board, then have a switch to power device and monitor voltages. That would be ideal.

 

hi Go,
OK,
On the resistance test, what are the limits to the test voltage and current that can be applied to a test point.
Considering that if the DUT has semiconductors on board, their turn on voltages.
E

 

To measure resistance, you need to apply a current from the point of measurement to ground.
Can you apply a current to all points at once or do you need to do it one at a time?

What is the resistance range that you expect to measure?

 

Expected resistance 250-1500

Its for mobile devices. Measurements would have to be probable done one by one. Most lines have 200mA filters.
Boards power off 4.2 battery.

 

This routed, on PSOC 5LP, 50 channels, 12 bit SAR, along with SPI interface to hand off results.
Everything is onchip, including Vref.

Did not try this but could use SAR and separate mux, and a IDAC connected
to muxout to force a current to test points for measuring R. This could be turned
off so that V measurement and R measurement share same channel.

See attached.

Note could have used DelSig instead of SAR, good to 20 bits. Additionally could use DMA
component (inside PSOC) to do auto transfer of result out SPI.

Curious, what MCU are you considering that can do 120 channels ?

Regards, Dana.

 

Im working with efm8 mcu, and silicon labs has a 32bit mcu that I though had 108 analog pins (Giant Gecko), but im not sure if that means it can measure on all of them.

 

The EFM32GG Ref Manual seems to imply limited on # channels -



The efm8 the crossbar switch only good for digital signals ?

Regards, Dana.

 

Its digital with analog multiplexer.

Now, could I make mode transitions and switch adc input pin -> read and send to PC -> transition to next mode with next pin as adc. repeat on all analog GPIO?

 

Now, could I make mode transitions and switch adc input pin -> read and send to PC -> transition to next mode with next pin as adc. repeat on all analog GPIO?

In the PSOC example I posted the sequencing SAR will automatically
scan from one channel to the next, generating an interrupt each time
for conversion complete signal. If you want to manually control the SAR
with either a SW or HW trigger, your choice as well.

So you can either use an ISR to get the data and send to SPI (or I2C or UART or
One Wire...) or use DMA to auto stuff the SPI, your choice.

This example 50 channels, and I added a IDAC. So you could scan the channels
measuring V, then turn on the IDAC and scan again to measure R. Keep in mind
you would have to power off the DUT to do the R measurements. You have to
look into range on SAR and available current to see what R resolution you will
get. If inadequate then use the onchip 20 bit DelSig vs the 12 bit SAR. Note also you
can use an onchip VDAC, config ADC as diff, feed the VDAC to - input ADC,
and use that to optimize the range of ADC digitization. Fun stuff. Obviously you
have to work out the error budgets on the approach.

See attached.

Note there is an ohmmeter tool in PSOC IDE to examine routing R for various
routes out to pins.

This is all onchip, so 4 of these would get you 200 pins.

Regards, Dana.

 

Here is the example for the inputs of analog multiplexers, you can use CD4067 to replace CD4051 and use 74HC154 to replace 74HC138 and then you can get the input numbers Ninp = 16*16 = 256.

 

Both options look interesting. @ScottWang that solutions seems the easiest to implement.
@danadak I will look into that IC. Seams like it would be way less components.