I designing an electrochemical sensor readout using Ad5941.The LDO used for powering the ADC is ADM7155.

In the reference design all the capacitors and resistors used are 0603 package.In my design I used all 0805 package.That will make my rework easy.We don't have dedicated lab person.
May I know using 0805 package will make any problems in the measurement?

The schematic is attached here.Could you please review this schematic,your reviews will help me.

 

I generally prefer to use 0805 for all SMD resistors and capacitors.
They are easier to manipulate, solder, and find when they flit away from your tweezers.

 

Unless there is a great need to make the package very small, using the larger 0805 packages will make both assembly and service a bit easier, as well as any measuring that may be required. In addition, it will provide better reliability due to a slightly lower power density in the resistors.

 

May I know will 0805 make routing difficult.

it will provide better reliability due to a slightly lower power density in the resistors.

May I know what you mean by this.Is it power withstanding capacity of 0805 greater than 0603?

 

May I know will 0805 make routing difficult.


May I know what you mean by this.Is it power withstanding capacity of 0805 greater than 0603?

Unless the design is for the smallest possible area it should not be quite as much effort.
As for power handling, the greater mass of the larger parts increases the power withstanding ability by some small amount.

 

May I know will 0805 make routing difficult.

Actually, it makes it easier because you can route a trace under a SMD resistor or capacitor.
Having said that, as I general practice, I wouldn’t do that.

 

Actually, it makes it easier because you can route a trace under a SMD resistor or capacitor.

May I know you mean,routing between the PADS

 

As for power handling, the greater mass of the larger parts increases the power withstanding ability by some small amount.

So you mean if I take 100mW 0805 and 0603,the 0805 package can handle little bit more than 100mW.Please correct me if I am wrong

 

May I know you mean,routing between the PADS

You can route a trace between SMD pads but I prefer not to do it for reliability and serviceability reasons.

 

So you mean if I take 100mW 0805 and 0603,the 0805 package can handle little bit more than 100mW.Please correct me if I am wrong

Well most 0805 package will be rated at 125mW, rather than the 100mW of a 0603, but the 0805 will also have better working voltage capability: see these specs for standard Vishay SMD 1% parts:



That may be important, but probably not in this instance.

The main reason I use 0805 is because they are easier to solder by hand than 0603!

 

You can route a trace between SMD pads but I prefer not to do it for reliability and serviceability reasons.

View attachment 353377

Not sure I get why that's any less reliable than going round the outside, as long as your ERC/DRC validation passes. And if properly covered with resist, how does it affect serviceability? You can still lift the part safely and replace it.

 

Not sure I get why that's any less reliable than going round the outside, as long as your ERC/DRC validation passes. And if properly covered with resist, how does it affect serviceability? You can still lift the part safely and replace it.

ERC/DRC validation passes. But I am old school where we did our own artwork taping and PCB etching by hand. Tracks were always wider than 0.015 inch and we allowed max clearance when possible rather than just meeting design rules.

As for serviceability, if you didn’t design the PCB yourself, it is nice to know where the track connects rather than seeing it vanish under a device.

 

Do I need 4 layers for this board.Space is not a constraint for me.May I know will I be able to make 2 layer board with 2nd layer as dedicated ground plane.

 

Do I need 4 layers for this board.Space is not a constraint for me.May I know will I be able to make 2 layer board with 2nd layer as dedicated ground plane.

Doesn't look too complex, a 2-layer board should be possible. Try it and show & tell when finished.

 

So you mean if I take 100mW 0805 and 0603,the 0805 package can handle little bit more than 100mW.Please correct me if I am wrong

NO! I do not mean that. For the same 100mW the 0805 will not be quite as warm as the 0603, because it has more surface area to radiate the heat.

 

Can you please check my power plane splitting is correct or not.Below is my plan for splitting.



Second image shows how I am planning to implement it on the board.

LDO_IN is coming to TOP Layer(RED in color) it will be connected to LDO_IN Polygun in the 3rd Layer.

Then LDOS for Analog and Digital power will be placed on top layer as shown in the image and it's I/P will be connected to LDO_IN.
May I know is this fine.Or am I killing the performance of ADC.

 

pins 23 and 25 are digital ground, why are they not connected to DGND along with pin 13. There is no reason to differentiate between IOVDD and DVDD if they are connected together as per your original schematic. What are you connecting pin 49 to?

 

In the reference design AD used single GND only.
Yes,you are correct IOVDD and DVDD are same.They used separate rails.I also followed it.I will make it a single supply
Pin number 49 is connected to GND.
Please find the attached reference design

 

In the reference design AD used single GND only.
Yes,you are correct IOVDD and DVDD are same.They used separate rails.I also followed it.I will make it a single supply
Pin number 49 is connected to GND.
Please find the attached reference design

How is pin 49 connected to GND? Show us what you are doing on your PCB.


https://wiki.analog.com/_media/resources/eval/user-guides/eval-ad5940/hardware/08-057011-01-a-1.pdf

For slower speed (10Mhz) digital I/O with thermal pads that need a VSS/GND connection I like to place them on the bottom of the PCB so the pad has a direct connection to the plane.

The trade-off is lots of vias, but at slow speeds (24vdc relay, lamps, etc switching and inputs) it doesn't matter. R10, R12, etc.. are 1206 SMD for reduced thermal drift for voltage divider dropping resistors.

 

I just started the PCB.Some confusions arrived about power plane planning.I showed the layout(Partial) of my power plane and how I am going to connect the I/P power .May I know is that fine.

I am planning to connect exposed pad(PIN NO 49) as per the reference design.

I will share the complete board files here once it is done.