MAX31855 schematic question

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RosscoS
 
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MAX31855 schematic question

Post by RosscoS »

Hi,
I am working on building a circuit that incorporates the MAX31855 as part of a larger schematic. I have bread boarded the circuit successfully using my MAX31855 breakout board, but I want to have a single PCB made, rather than having several PCB's wired together.

The component that is marked as LP298XS on the schematic, is it the same as a LP2980IM5X-3.3 (3.3V LDO regulator), or is it something else?

Thanks in advance :D

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adafruit_support_bill
 
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Re: MAX31855 schematic question

Post by adafruit_support_bill »

Yes. It is a 3.3v LDO regulator.

RosscoS
 
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Re: MAX31855 schematic question

Post by RosscoS »

So would I be correct in saying that the 3.3V LDO is only there as a 3.3V Vcc supply? As opposed to level shifting of data.

Does the Arduino send any data to the MAX31855, such as requests (and therefore require level shifting)?
Or does the Arduino only receive data from the MAX31855 (and 3.3V out is sufficient to be read as a logic 1 by the Arduino)?

Again, thanks in advance! :D

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adafruit_support_bill
 
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Re: MAX31855 schematic question

Post by adafruit_support_bill »

It's all in the schematic. As you can see, there is no DI connection. /DO and SCK have 3.3v pullups and diodes for protection.

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bboyes
 
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Re: MAX31855 schematic question

Post by bboyes »

Where is the schematic for the MAX31855 demo board? I looked in the Technical Details tab and in the tutorial... OK, found it in a GitHub search. It would be nice to see a link from the Technical Details tab of the product info page.

Thanks
Last edited by bboyes on Tue Oct 29, 2013 2:27 pm, edited 1 time in total.

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adafruit_support_bill
 
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Re: MAX31855 schematic question

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jcalcote
 
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Re: MAX31855 schematic question

Post by jcalcote »

I was wondering if you'd mind providing the specs on the ferrite beads used to filter noise on the thermocouple terminals? I think these noise filters are important - I like the smooth results I see when I take readings even at fast refresh rates on the adafruit MAX31855 breakouts. I want to add similar beads to my production design. Thanks!

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adafruit_support_mike
 
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Re: MAX31855 schematic question

Post by adafruit_support_mike »

The exact value is less critical than the value of a power supply bypassing capacitor.. anything works, more is better, and eventually there's a point of diminishing returns.

Ferrites are inductors, and the equation for inductive reactance is X=2*pi*f*L. The DC reactance is zero, and the intrinsic resistance of the ferrite at DC is usually a few hundred milliohms. For chip ferrites, the reactance stays low until you get into the megahertz range, then rises exponentially until a few gigahertz, then hits a self-resonant frequency and starts to drop again.

Most of the noise ferrites block exists in the low-to-mid megahertz range. The exact value is a function of cable length because the noise comes from signal reflections due to impedance mismatches along the signal path. Electrical signals move at about 200 million meters per second in copper, so the round-trip time for signals in a 1m-10m cable is on the order of 10 to 100 nanoseconds. That corresponds to an angular frequency of 7MHz to 70MHz, and in that frequency range, 1 1uH ferrite's impedance will be in the hundreds of ohms -to- thousands of ohms range.

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jcalcote
 
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Re: MAX31855 schematic question

Post by jcalcote »

Thanks Mike - that's way more useful information than I really expected. This is the reason I keep coming back to adafruit for parts!

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adafruit_support_mike
 
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Re: MAX31855 schematic question

Post by adafruit_support_mike »

And we're here for people who want to understand things. Works out kinda nicely. ;-)

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hrg
 
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Re: MAX31855 schematic question

Post by hrg »

Hi ,

I have some doubts .
1) why pullups are used for SCK and SS pins ? (usually SPI doesnt require pullups)
2) What is the use of diodes in SS and SCK ?
3) Does the Max31855 act as a master and arduino as slave for SPI comm ? Because the way diodes are connected (cathode on arduino side and anode on max31855 side ) arises this confusion .

Thankyou

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adafruit_support_mike
 
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Re: MAX31855 schematic question

Post by adafruit_support_mike »

hrg wrote:1) why pullups are used for SCK and SS pins ? (usually SPI doesnt require pullups)
2) What is the use of diodes in SS and SCK ?
They're simple level shifters that make the 3.3v MAX31855 safe for 5v input.

If the input to either pin is higher than 2.7v, the diode is reverse-biased and all the chip sees is the 3.3v pull-up resistor. When the pin voltage goes to 0v, the diodes become forward biased. The chip sees about 0.6v, which is below its low-input threshold.
hrg wrote:3) Does the Max31855 act as a master and arduino as slave for SPI comm ? Because the way diodes are connected (cathode on arduino side and anode on max31855 side ) arises this confusion .
It's a slave.

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hrg
 
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Re: MAX31855 schematic question

Post by hrg »

Thankyou ! And one more question ,what kind of ferrite bead you are using across the thermocouple input . I found that it is of 0805 package. Could you share the datasheet of it .

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adafruit_support_mike
 
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Re: MAX31855 schematic question

Post by adafruit_support_mike »

It's a noise-suppression bead. The value isn't critical, and as discussed above, would be dependent on the geometry of the wires connected to the MAX31855 if you wanted to spec a value.

I think we use a value around 1uH, but 95% of all ferrites in 0805 packages would be adequate replacements.

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