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Why Voltage reference is overshooting to 3.5V from 2.5V when it is used to bias opamp

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myatham

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Hi All,
We are using one voltage reference 2.5V to bias two AC-Coupled non-inverting amplifiers with gains 4 and 50. But, i could see that output is oscillating. Vref is found to be overshooting to a value of 3.6V from 2.5V. I am suspecting it is operational amplifier stability issue. But, if i provide external reference voltage 2.5V from APLAB power supply, there is no oscillation found. This issue is observed only with Gun diode input which is in the range of 1mV. Please suggest here how to overcome this stability issue.

Thanks in Advance!
 

Series references often have no or low current sink
capability, and the output device can "wind up" on
a transient sink-load only to overshoot on release.
A shunt reference (ala LT1009) might be something
to try (you don't state the part type) but these can
also have relaxation-oscillator large signal behaviors
under certain Cout conditions, from output transistor
saturation/recovery. You might play with adding
grossly more Cout to the reference output, or way
less, and see if the reference's intrinsic stability is
the problem.
 

Dick is quite right, many voltage references can source current, but have little or no current sinking capacity.

This can usually be fixed by placing sufficient fixed loading across the output of the voltage reference.
It may not need very much, try a 1K resistor.

If the problem is severe, a shunt voltage regulator such as a TL431 may be more appropriate.
 
Even with 1K load on reference, i don't see any improvement.
When i remove the sensor input, i don't see this issue.
 

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