Relative location of capacitor in an RC filter

Thread Starter

spmaddox

Joined Nov 29, 2018
46
Hi everyone - In the schematic I've attached I designed this op-amp, comparator & ADC setup with the output of the op-amp going to both the input of the comparator and the input of the ADC. I added an RC filter to the portion that sits between the op-amp, comparator and ADC and you can see in my schematic that I placed both the resistor and the capacitor on the line before the line reaches either the comparator or the ADC. The circuit board designer I hired placed the capacitor on a common line that is after the resistor but is shared by the comparator and the ADC and close to the comparator like i've shown in my other illustration. My question is whether or not this impacts the effect of the RC filter relative to the comparator and the ADC?

schematic layout.gif

pcb layout image.gif

thx - sean
 

Marc Sugrue

Joined Jan 19, 2018
213
Assuming the frequencies arn't too high it won't have a significant impact. Technically you've introduced some additional parasitic track resistance, inductance and capacitance which will become more relevant at higher frequencies.

Just a couple of pedantic notes also on your circuit diagram which really you should get your draughtman to tidy up too.

1. Rotate all capacitors by 90 Degree so it tidies up the schmatic and make it easier to read.
U5 6 pin 12 should be connected to the GND the same as the others not have a dedicated GND symbol. Technically the same but neater
2. Move C5 1 & C5 2 close to the IC U5 5 along with the 12V symbol, rotate the caps to tidy it up a little. There no need for the tracks to be crossing.
3. Move C5 3 & C5 4 close to the IC U5 6 and rotate them 90 Deg all the capacitors will be in a neat line.

Hope you don't mind me pointing it out but the schematic will be much easier to read.:)
 
Last edited:

Thread Starter

spmaddox

Joined Nov 29, 2018
46
Assuming the frequencies arn't too high it won't have a significant impact. Technically you've introduced some additional parasitic track resistance, inductance and capacitance which will become more relevant at higher frequencies.

Just a couple of pedantic notes also on your circuit diagram which really you should get your draughtman to tidy up too.

1. Rotate all capacitors by 90 Degree so it tidies up the schmatic and make it easier to read.
U5 6 pin 12 should be connected to the GND the same as the others not have a dedicated GND symbol. Technically the same but neater
2. Move C5 1 & C5 2 close to the IC U5 5 along with the 12V symbol, rotate the caps to tidy it up a little. There no need for the tracks to be crossing.
3. Move C5 3 & C5 4 close to the IC U5 6 and rotate them 90 Deg all the capacitors will be in a neat line.

Hope you don't mind me pointing it out but the schematic will be much easier to read.:)
I appreciate the feedback regarding the caps, i'll double check regarding frequencies. I will likely move things around to match this on my breadboarded version and re-run the tests I used to identify 620 ohms and 3nF as the setup that centered my ADC readings. The schematic is my own, I'm not an EE and this was my first foray into developing a schematic. The "schematic" was originally in visio, i had to re-do it all in CircuitStudio. Elsewhere my caps are vertical.

Noted on U5.6 pin 12 - to be honest I caught that I'd not tied the caps to ground and since my PCB guy has the schematic master now he updated it and added the ground coming from C5.3 & C5.4 and tied all the caps to that instead of just tying them all to the ground on pin 12. Was surprised when I saw it but is what it is.

Thank you again, greatly appreciated.
 

Marc Sugrue

Joined Jan 19, 2018
213
I appreciate the feedback regarding the caps, i'll double check regarding frequencies. I will likely move things around to match this on my breadboarded version and re-run the tests I used to identify 620 ohms and 3nF as the setup that centered my ADC readings. The schematic is my own, I'm not an EE and this was my first foray into developing a schematic. The "schematic" was originally in visio, i had to re-do it all in CircuitStudio. Elsewhere my caps are vertical.

Noted on U5.6 pin 12 - to be honest I caught that I'd not tied the caps to ground and since my PCB guy has the schematic master now he updated it and added the ground coming from C5.3 & C5.4 and tied all the caps to that instead of just tying them all to the ground on pin 12. Was surprised when I saw it but is what it is.

Thank you again, greatly appreciated.
No worries glad i could help. As this is your first schematic its a great time to be critical as its a really good habit to get into. Theres nothing worse that discovering a pcb issue and discovering it was there all along on a untidy schematic. I've been there & done that.
 

Thread Starter

spmaddox

Joined Nov 29, 2018
46
No worries glad i could help. As this is your first schematic its a great time to be critical as its a really good habit to get into. Theres nothing worse that discovering a pcb issue and discovering it was there all along on a untidy schematic. I've been there & done that.
Agree. Spent yesterday and today reviewing the PCB traces in CircuitStudio and all looks good, discovered that i can mouse over pads etc and it will highlight everything that is in common with it which is a pretty cool feature. My PCB guy does his work in the Altium full/professional suite, I used CS to do the schematic and review his output. Here is the front and back of the PCB, for $550 I'm more than pleased with CS.

Expect in the near future I'll do an update to this to move from the RPi CM3 to the RPi CM4 and a different version which incorporates BLE, Zigbee & Z-Wave connectivity and I'll use CS and the same PCB guy again.

Thanks again for answering my question and the schematic feedback, very much appreciated.

PCB Front.gif

PCB Back.gif
 
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