Home Automation V4 8-Layer Stackable HAT for Raspberry Pi Analog Inputs pull-up resistors

[mtorre]

Hi!
I am using this HAT along with a Raspberry PI 5. It works smooth.
My issue comes with the analog inputs. The HAT analog inputs are "pulled up" to 3.3V through 15K resistors (see attached image).
Those resistors alters my analog signal circuitry, adding a DC voltage that I don't need or require.
Is there a way that the HAT provide analog inputs without that pullup?
Any suggestions will be welcomed.
Thank you!
Mario

 

[alexburcea]

Hi,
The card was designed for 10k thermistors. You can remove the resistors from the pcb. if you have the right tools I can send you a picture marking the one must be removed.
What type of sensor do you measure so the 15K pull-up modify the value?

 

[mtorre]

Alex, thanks.
Please send the pictures.
I am using an AC Voltage sensor, with provides me ~ 3 Vpp AC output at 120VAC input. Then I rectify to obtain a DC level proportional to the input. My circuit output impedance is not low enough to pull down the 15k resistors.
I hope this helps.
Thank you

 

[alexburcea]

 

[mtorre]

Thank you.

View attachment 55

Thank you.
I'll try to figure out other options.... I do not see myself trying to take out those tiny resistors. They are way beyond my eyes' resolution!
Any other ideas are welcomed. I am thinking to use a transistor or FET in my source circuit..
Cheers!
Mario

 

[alexburcea]

The best way is to use a op-amp as a buffer (gain =1 ), this will lower your output impedance.

 

[mtorre]

The best way is to use a op-amp as a buffer (gain =1 ), this will lower your output impedance.

that's a great Idea! Thanks!