Op Amp output satuartion

[sagar61]

I am using TI's LM2902 in unity gain buffer configuration. VCC is 12V and VEE is gnd, Non-inverting input is floating.when i measure voltage on non-inverting using DMM its showing around 1mv,but output of op amp is around 10V.
Why my op-amp is saturating?Output should be zero when noninvering input is zero or floating?

 

[KlausST]

Hi,

Don´t let (unused) inputs floating. (no analog inputs and no digital inputs)
Floating voltage is undefined.

Klaus

 

[crutschow]

When you measure the floating input, the meter impedance pulls the input to ground (the 1mV is likely the input bias current through the meter resistance).
When you remove the lead to measure the output, the bias current has no place to go and the input floats to a different voltage, causing the output voltage to follow.
You can see this if you measure the output with a different meter while measuring the input.

As Klaus noted, you cannot leave any inputs floating.

 

[Audioguru]

The datasheet for the LM324 and LM2902 shows that the inputs are PNP transistors that have bias current coming from ground. Then an input floats high and a unity gain buffer with a floating non-inverting input will have its output go as high (positive) as it can.
A floating input will also pickup all kinds of interference.

 

[FvM]

If you measure the voltage at the floating input with a high impedance DMM (usually 10 Mohm input resistance) of these OPs, they will show considerably higher voltage than 1 mV. Something's wrong with your measurement, besides the setup being useless.

 

[Audioguru]

The maximum input current of an LM2902 is 0.25uA. With a DMM input resistance that is 10M then the voltage will show 0.25uA x 10M= +2.5V. The typical input current is 0.045uA then the voltage will show +0.45V.