Hello, I have a problem with differential amplifier. I use LMC6484 (http://www.ti.com/lit/ds/symlink/lmc6484.pdf) op amp to project differential amplifier (schematic in the picture). Resistors values are R1=R3=100k and R2=R4=1k, so theoretically gain should be about 100. My object for this project is to amplify signal from Honeywell pressure transducer (https://stevenengineering.com/tech_support/PDFs/31DTMAIN.pdf) which voltages are about 0-30mV, to voltages for adc. I connect this circuit on breadboard and connect my pressure transducer signal+ and signal- wires to differential amplifier + and - inputs. Voltage between pressure transducer data pins, when there is no pressure is about 30mV, and in differential amplifier output i get 4,6V, but when I apply pressure to transducer and transducer output voltage is increasing, there is no change in amplifier output - still 4,65V. Maybe someone know solution for this problem?

 

hi R,
It would suggest the Diff OPA is is being driven into saturation.
As a test try using 10k's in place of the 100k's and also try reversing the signal lines from the transducer.
Lets know what you get.
E

 

Hi ericgribbs, I place 22k's instead of 100k's and now I get 0,8V in the output of amplifier, when I sligtly blow up air on my pressure transducer, then I see change in interval of 0,8V-1V. So, I think you are right, that transducer with 100k's driven into saturation. Now I think I can place 50k's instead of 22k's and my voltage range will be about 2-3V?

 

hi R,
That sounds promising, I would say with a 5V power supply the amp will limit at approx 4.5v.
I would limit the Gain to give 4Vout for a maximum expected input pressure.
E

 

Now i see that amplifier work correctly with just 22k's and 0.8V in the output and 10k's and 0,4V in the output, When resistors values are bigger then he driven in the saturation. For example, at voltage 1,8V and something like that. Why its saturated at that little voltage?

 

Shouldn't this be calculated instead of going by trial and error?

 

Exactly which model pressure transducer are you using in the link provided?
SG

 

First pressure transducer, model AB.

 

With that connection:



At R1=R3=100k and R2=R4=1k, the gain is 100 and the output is 100*30mV = 3V and the amplifier should not be saturated.

Is it saturated because of the common mode input too low?

 

hi R,
This clip image is from the d/s [ 1st sensor]
Assuming you need to use the Full Scale Output of 100mV, to give an amplifier Vout of +4V, that's a Gain of only 40.
E