AnsweredAssumed Answered

8221 and input cable length

Question asked by coolarm on Jan 12, 2014
Latest reply on Jan 15, 2014 by coolarm

Hi all,

 

I have two PCBs that are identical replica. Each has a wheatstone bridge (say, V_ab in PCB1 and V_cd in PCB2) with a 8221 sensing the differential voltage. They work fine individually,

 

I now need to modify PCB2, so that the inv signal for the 8221 comes from the PCB1 i.e. V_cb instead of V_cd.

1) I noticed that if I tried to solder a wire onto terminal B in PCB1, I see ripples of 10mV (oscillations?) at the output of the 8221. So, I have put a buffer with a short lead connected to terminal b and routed the signal out of PCB1

2)In PCB2, however, I am not able to avoid the ripples when I connect the buff_b into the inv terminal of 8221. I have tried re-buffering it close to PCB2, to avoid the long cable's effect. I have tried putting the buffer onto the non-inv terminal so that both the terminals are looking into the buffer. Either ways I am not seeing a difference.

 

Has anyone come across this kind of behaviour (oscillations) from 8221? The wheatstone bridge has 200ohms and less resistance. So, the terminals are not floating, in any of the above config.

 

Thank you so much for your insight and have a blessed Monday!

Outcomes