Help me read the datasheet. 20pts!

JUST ONE QUICK QUESTION:
Is the +/-1.7g version of the SCA620 a Z-axis accelerometer, by means measuring the axis going through the PCB? I find the data sheet confusing.

VTI's accelerometer product pages:
https://www.vti.fi/en/products-solutions/products/accelerometers/
It is the last product on the list near the bottom of that page.

By clicking the link at the quick description, you will be directed to the product pages for SCA620:
https://www.vti.fi/en/products-solutions/products/accelerometers/sca620-accelerometers/
clearly stating:
“SCA620 Series - z-axis accelerometers”

click the datasheet in the upright corner of that page.

The second blue box in that sheet, “Performance characteristics”, property “Measuring Direction (note 3)” (options Horizontal/Vertical)

Note 3:
“Measuring direction perpendicular to the mounting plane.”

I feel stupid but can anyone confirm that the accelerometer I just ordered (and made PCB for) is pointing in Z-axis direction (the axis pointing up/down when mounted to PCB (perpendicular to the board, or orthogonal to right/left axes of the board when served with teaspoons))?

Please tell me I didn't screw up that PCB.. :evil: Tell me and get 20pts bonus! (well.. u'll get the pts even if I mucked the PCB but tell me I didn't. To get the pts you must determine which axis it is pointing if not Z)

EDIT:
Sorry for posting this in the microcontrollers forum.

I've never used this sensor, but I think you are correct. I see three agreements:
1. Note 3.
2. The "sensing direction" symbol (a circle enclosing a dot means a vector coming out of the drawing towards you).
http://en.wikipedia.org/wiki/Vector_(spatial)#Representation_of_a_vector
3. The side-view dimension drawing showing an arrow with plus-sign and comment saying "acceleration in the direction of the arrow will increase the output voltage".

That's how I read the data sheet, but not confirmed.
Please keep your points - you need them more than I do!

Teaspoons? Is that Norwegian humor?
I visited Tromsø once. Nice town. Didn't see any trolls, though.

Thanx alot for your time and observations.

I'd like if someone could second it, as I will be mounting some expensive electronics soon and it will be a crisis if I mount everything without that axis of acceleration. Better to just order new PCBs. Well, heck.. Even if it's SMD, it's fairly big (the chip) so I can probably measure when it arrives.. but that's another couple of days down the drain if I need to redo the PCBs..

echo47 said:

Teaspoons? Is that Norwegian humor?
I visited Tromsø once. Nice town. Didn't see any trolls, though.

Click to expand...

Teaspoons.. yeah, it's an expression here in Norway. If someone requests an overly detailed explanation of something really simple, a common paraphrase is something like "do you need it fed with teaspoon?". Thought there was a similar expression in english but sometimes expressions get lost in translation

Tromsø is a nice town. Far away (1000km+) from where I live, but I served in the army near Tromsø so for a period I was there alot (mostly for beer and girls, and mostly with 10+ other boys looking for the very same thing, haha). Probably the nicest town our country has to offer IMHO The sky up there is breathtaking in the winter.

To see trolls you have to go further south, deep into the mountains. They thrive best in Dovre and Jotunheimen. But you can still see Lapps near Tromsø. Very much the same thing except they dress cooler 8)

EDIT:
I just found the confirmation I needed. The picture on
https://www.vti.fi/en/products-solutions/products/accelerometers/sca620-accelerometers/ shows the actual SCA620-CF8H1A, AND carries the Z-axis marker shown in the datasheet drawing. I still think the "Measuring direction" (Horizontal/Vertical) property in the datasheet is an oddball.. I wonder what they mean by that..

Hi there,

What the datasheet says to me is that the accelorometer is measuring the axis perpindicular to the IC. It means the X and Y directions is on the PCB plane and this specific SC620 measures the z-Axis perpendicular to the PCB.

If you read remark 4) below the box it even tells you how to mount the IC's for zero point. The vertical version(CHCV1A) has to be mounted with the pins down meaning on top of the PCB and the horizontal version(CF8H1A) should be mounted with pins 1 - 4 on the down side. So both measures perpindicular to the PCB , in the one it measures it horizontally and the other vertically.

The horizontal/vertical versions therefore means the actual PCB is turned vertical or horizontal with the IC's mounted as above.

Hope this helps

Regards

Nic

I'm still confused.

I will mount the horizontal version(CF8H1A) pins down to my PCB. Concider the PCB horizontal. Will the sensor measure +/-1.7g on the vertical axis perpendicular to chip and PCB?


I also found this note at https://www.vti.fi/en/products-solutions/customer-support/faq/ :

1. Measuring direction

Measuring direction of the vertical sensor is parallel to earth's gravity. For a horizontal sensor the measuring direction is orthogonal to the gravity field. The measuring direction is specific to the product family: e.g. SCA61T and SCA610 Series parallel to the mounting plane, SCA620 and SCA320 Series orthogonal to the mounting plane.

:|

Hi there,

Unfortunately as I understand it the horizontal(CF8H1A) version measures perpindicular to the chip on the horizontal plane, that means the PCB is standing vertically on its side and you have to mount pins 1-4 on the downside of the PCB.

The vertical unit also measures perpindicular to the chip but in the vertical plane. That means the PCB is flat in the horizontal plane, like lying on a table. You have to mount this chip then on the top of the PCB not under it.

So if you need to measure 1.7G's like in the picture you gave, that would be basically up and down in the vertical plane, it means you may have bought the wrong chip. You should have bought the SCA620-CHCV1A.

Or let me explain it anther way. Say you have z-axis perpindicular coming out of a table. That would mean the table top would be in the X-Y plane. Then the PCB would lie flat on the table with the SCA620-CHCV1A mounted on top of the PCB and it would measure gravity in the z-axis direction, or if you want to up and down.

Now turn the table top that it lies in the z-y plane. It therefore now lies on its side with the pcb attached to the top still flat on the top of the table and thus lying now in the z-y plane. You now attach the SCA620-CF8H1A to the PCB but with pins 1-4 showing downwards. It now measures the gravity in the x-direction, if you want from left to right or right to left.

I have read the part which you refered to but unfortunately you can still interpret it anyway you want. I stuck to the datasheet in my explanation.

Regards

Nic

CHCV1A is a +/-12g device, and that is way out of the targeted measuring range..

I think VTI should work on their ability to spec their products. I think you are right in your interpretations of the specs, but I find some major contradictions in the way the SCA620 family is spec'ed. I will find an alternate sensor, redo my PCB, and muck the VTI sensors when they arrive. And I will certainly stay away from VTI in the future.

EDIT:
This picture still dazzels me.. showing my chip with a Z-axis marker..

I think both versions measure along the same axis. The difference may only be the scale factor and perhaps a 1g offset to compensate for the the 1g shift depending on which way you mount it (what they call "measurement direction"). Maybe.

I suggest calling VTI and talk to an applications engineer.

Hi there,

I second you in that the datasheet is not at all that clear. The marker in the photo only shows a axis perpindicular to the chip. I know some people interpret it as the z-axis but it is not always the case, or for that matter remember you can rotate the axis anyway you want and the z-axis will still be perpindicular to the chip.

Regards

Nic

P.S. I also think the best bed is to email the guys directly. They may even exchance the chips if you ordered the wrong chip. Let us know what they say.

ScratchBuild said:

The marker in the photo only shows a axis perpindicular to the chip

Click to expand...

That is essentially all I'm hoping for as I have other sensors (an Analog Devices ADXL203 dual axis accelerometer, also +/- 1.7g) covering the axes defining the PCB plane... So if the chip is in the package described in the datasheet AND it measures perpendicular to the chip, all is good I suppose.

I went with the VTI chip since I wanted 3 axes of accelerometer running of the same supply as my 5V gyroscopes (easier biasing of the instrumentation amplifiers etc when everything is 5V with 2.5V zero point, since the gyroscopes provide percision 2.5V reference).. All suitable analog 3-axis accelerometers I could find were 3.3V hence the combo of the two 5V accelerometer devices..

ScratchBuild said:

P.S. I also think the best bed is to email the guys directly. They may even exchance the chips if you ordered the wrong chip. Let us know what they say.

Click to expand...

Yeah, I mailed them a while ago, I'll let you guys know the response.

The order was with digikey, not from VTI directly.. I'll discuss it with them if the sensor appears to be something i didn't expect.

VTI technical staff confirms the SCA620-CF8H1A senses in the direction perpendicular to the chip surface. When laying on a table pins down, 5V supply, the output voltage is:

output = offset value + sensitivity = 2,5V + 1g*1,2V/g = 3,7V

The chip work as described.