Regarding LM111 Input Bias Current

TI_Electronics

Other Parts Discussed in Thread: LM111

What does following graph tell? What is the differential input voltage?

What will be input bias current for the following circuit (LM111)? Voltage at Non-Inverting Pin = -8.5V (Vref LTP) & -9.5V (Vref UTP)

over 12 years ago

0 Paul Grohe over 12 years ago

TI__Guru 69226 points

Hello Kamlesh,

The graphs did not attach.

Please re-attach or give the figure numbers.

0 Ron Michallick over 12 years ago

TI__Guru**** 174826 points

Kamlesh,

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0 Ron Michallick over 12 years ago

TI__Guru**** 174826 points

Kamlesh,

The graph says IIB changes with VID because the Zener diodes turns on around +/-6V.

Differential input voltage is (VE+) - (VE-). 1V in this case

IIB of Ve+ is -90nA
IIB of Ve- is -60nA

Current flows out of the LM111 input pins because the input transistors are PNP.

0 TI_Electronics over 12 years ago in reply to Ron Michallick

Intellectual 345 points

Sir,

Thanks a lot for your answer.

I am trying to have LTP and UTP for following circuit as -9.5 and -8.5 which is coming as expected at non-inverting input. However, output is changing state when input crosses reference at non-inverting terminal+100mV

What could be the reasons for this 100mV offset?

LM111 comparator is used in the circuit

(Pull up resistor for LM111 is not shown in the above circuit)

0 Ron Michallick over 12 years ago in reply to TI_Electronics

TI__Guru**** 174826 points

Kamlesh,

I need a little more information.

What is VCC- (pin 4) and VCC+ (pin 8) voltages?

0 TI_Electronics over 12 years ago in reply to Ron Michallick

Intellectual 345 points

Sir,

VCC+(Pin 8) voltage is +14.5V and VCC-(Pin 4) voltage is -14.7V

0 Ron Michallick over 12 years ago in reply to TI_Electronics

TI__Guru**** 174826 points

Kamlesh,

Those supply voltages are good for the input voltage used.

I expect the issue is addressed here in your other post.

http://e2e.ti.com/support/amplifiers/etc_amplifiers__other_linear/f/18/t/364930.aspx

0 TI_Electronics over 12 years ago in reply to Ron Michallick

Intellectual 345 points

Sir,

I could get rid of noise by using shielded DMM probes. However I could not get rid of 100 mV offset between Non-Inverting and Inverting Pin of LM111 comparato while achieving LTP of -10V and UTP of -9V.

Could it be because of high input resistance value of 510 kilo-ohms used in between Inverting terminal and ground? Could Input bias current create offsets due to unmatched externally resistances at Inverting and Non-Inverting terminal of LM111?

Also. Input signal is given though diode as shown in the following diagram. Could diode non-linearity be the reason behind offset of 100mV between Inverting and Non-Inverting terminal of op-amp while achieving LTP of -10V and UTP of -9V?

0 Ron Michallick over 12 years ago in reply to TI_Electronics

TI__Guru**** 174826 points

Kamlesh,

The diode does affect the voltage at 'input' node, but the voltage offset at the opamp input should be about equal at switch (within the input noise amplitude).

I believe that supply voltage was close to +/-15V. Output voltage is 0V and 12V.
Is +Ve a good -9V and -10V?
Are the -Ve switch points -9.1V and -9.9V? or is the 100mV shift in a different direction?

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Regarding LM111 Input Bias Current