On Sun, 15 May 2005, Ake Hedman wrote:

> I need some advice comments on this circuit 
> http://www.vscp.org/images/barometer001.gif  You probably laugh your guts out 
> but but when you finished laughing please tell me what I have done wrong.

I am not laughing but you should not try to build a Schmitt Trigger. 
Your amp is wired as non-inverting amp for the reference (PWM) but the 
feedback resistor feeds into the + input and turns the circuit into a 
Schmitt Trigger.

To make this a differential amp with gain 10 move R3 from 3/6 to 2/6, 
reduce R2 to 10k and delete R4. The amp amplifies both the sensor input 
and the reference (from pwm) 10 times and also the *noise* and error 
from the pwm. This circuit may take some work to get it going.

Peter
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