hi M,
Ref AD623 current input limiters, they should also be configured as noise suppressors.
E

 

hi M,
Ref AD623 current input limiters, they should also be configured as noise suppressors.
E

The circuit gets bigger all the time, lol. I'm away from the bench for a week but will get this tested again soon enough. I think I'll chance it without the noise suppressor additions but I know where to go if I don't get a usable signal. Thanks again Eric.

 

hi Mark,
If you don't want to fit all the suppressor components, at least add a 4k7 resistor in series with the AD623 in+ and in- inputs , this should limit any highish input fault currents.
E

 

hi Mark,
If you don't want to fit all the suppressor components, at least add a 4k7 resistor in series with the AD623 in+ and in- inputs , this should limit any highish input fault currents.
E

Hi Eric,
I have added the 4k7 resistors and I am powering both the pressure sensor and the amp with the same 11.11v supply. I am about to test again with a new amp chip. So far I have fried 3 :-(
I am going to add the recommended decoupling caps (0.1uF and 10uF) on the Vs line - I think they are necessary.
Regarding output buffering... from the datasheet (see clip) it requires a 10k load. Do you think I should be concerned about the load resistance offered by an Arduino analog pin?

 

hi Mark,
The input resistance of a MCU input is pin is very high, so no loading problems.
I would add a 10k from the AD623 output to 0v, it will help the pull down closer to 0v.
Decoupling is essential on the power rails.

How did you zap 3 OPA's .
E

 

hi Mark,
The input resistance of a MCU input is pin is very high, so no loading problems.
I would add a 10k from the AD623 output to 0v, it will help the pull down closer to 0v.
Decoupling is essential on the power rails.

Thank you - I'll add the resistor.

How did you zap 3 OPA's .
E

Just careless I guess. I'm learning a lot from this project.
Cheers, Mark

 

... also, I neglected to mention I have a 100pF filter cap between the 5v rail and ground.

An additional point- 100pf is wayyyyyy too small- see the data sheet.

 

An additional point- 100pf is wayyyyyy too small- see the data sheet.

Thanks - yes that was a typo, I meant 100uF... which is actually too high. I'm now using 0.1uF (and 10uF) as per the datasheet. Sadly my Arduino has expired and I'm waiting for new stock before I can continue.

 

Well, finally got this all working. I've only got a 1k resistor to set amplification which I'll swap for a 10k pot. But apart from that I'm happy with the solution. Thanks for all the help.

Here's my final schematic for anyone interested.

 

hi Mark,
Thanks for posting that update, it may help other users.
Eric