problem while Floating Input pin of op-amp.

[jay1691]

hello,

I am using op-amp comparator for comparing input value to the reference voltage. While input pins of the opamp are not connected(floating) still i get output of 4v at the output pin due to that in by default mode LED on output pin turn on.
here i attached circuit. please find it and provide solution.

Here as shown in circuit while i am giving 5V supply to the opamp my led suddenly turn on even i have not connected input pins to the reference supply and input supply.

 

[FvM]

LM339 has PNP input transistors, so the inputs will definitely float "high". Place a pull-down resistor that is able to sink the comparator input current of maximal 250 nA. 1 MOhm would be a good starting point.

 

[crutschow]

You can never leave inputs to op amps or comparators open. They all need a path for the input bias current, otherwise they will "float" to an undefined state.

 

[jay1691]

hello crutschow,

Thank you for quick replay. yeah i konw that we can never leave inputs to the floating but while testing that board i found that while i am giving supply to op-amp and i have not started input pins supply(ref supply and input supply) then led turns on. so i am confuse that have i take this as a issue or that is abnormal test condition ??

So from your replay i conclude that that is abnormal test condition. there is no issue with this circuit. It will work properly. is it right ???

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hello FVM,

As you said that Place a pull-down resistor that is able to sink the comparator input current of maximal 250 nA.But from my circuit i think that pull up resistor (at output of opamp) make the output 4V while inputs are not given. Am I right ??

 

[Audioguru]

.....But from my circuit i think that pull up resistor (at output of opamp) make the output 4V while inputs are not given. Am I right ??

no. The comparator has an input offset voltage that could be positive, negative or zero so the output could be high or low or oscillating.

 

[jay1691]

Dear Audioguru sir,

Thank you. As per your suggestion i have checked input offset voltage of LM339ADR. it is 4mV (@ VCC = 5 V to 30 V,VO = 1.4 V) so the output rail swing to high due to positive input offset voltage.