VSS drops for op-amp TL084.

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mandar_mahajan

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Hello All,
I am preparing a OP-AMP circuit as per attach drawing.
I don't know why my -3v drops to nearly zero when i put my opamp Ic in to socket. I am using TL084c(cod op-amp).
Can anybody help me!!!
Any kind of help is appreciated....


Regards
Mandar
 

Mandar Maharaj,
where is attachement..
 

The circuit is basically O.K., so th eproblem is apparently with your -3V supply. Please consider, that TL084 isn't specified for operation at +/-3V, also the brdidge output around Vdd-1.5V is most likely outside it's common range.
 

The power consumption of the circuit should be in the order of 20mA assuming a conventional load cell (350Ω). It seems you have made a wrong connection or your TL084 is defective.
Furthermore, with an offset of up to 10mV and very low output voltage span using this low suplly voltages you may have trouble to achieve the desired performance. With this high offset the output can easily go down to the negative region. How would you digitize a negative output? The noise generated by the TL084 at low frequencies also may impose some limitations.
Regards
 

An additional point, I forgot to mention: If the amp output is directly connected to the ADC without a series resistor, it can drive the substrate diode into forward bias and cause a considerably higher current consumption from the negative supply.

I like to emphasize pauloynski's comment, that a low offset voltage would be needed for an accurate load cell measurement. Also an OP with sufficient common and supply range can be choosen then.
 

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