Problem with current consumption of opamp when input is not connected

[sivamit]

Hi,
I have a circuit where Im using an opamp(AD811) as a buffer shown below.I have 8 buf stages connected to the same mux. Whwn the input is not connected it is taking more current. Whn the input is present there is no problem. what cub be the reason......?
Please help.

 

[IanP]

Re: opamp

Try to connect all inputs to 0V through 1MΩ resistors ..
I don't think inputs should be left floating ..

Regards,
IanP

 

[sengyee88]

Re: opamp

I believe input can not be floating. I suggest to measure if there is any current magnitude change through the 100Ohm resistor if you left input floating and with input connected to a biasing sweep from supply to ground.

Try to bias the input such that least current is flowing through the 100Ohm resistor. It could be the case that it will burn more current when u connect to ground via 1M resistor.

 

[Verba]

opamp

1MΩ may be too big for digital circuit, I would make it 10-100KΩ
The other reason, may be suply voltage of the amp is bigger than that of the MUX and when the amp jumps to the rail, the input current of the MUX is increased.
One more, may be ADC takes more current when it has one of the rails at the input. (Let's say, all the output bits are 1 and it has pull-down resistor at the output)

 

[Kral]

Re: opamp

sivamit,
I believe that IanP is right. I believe what is happening is that the input bias current is causing the output to saturate, due to the mismatched resistances on the -(0 Ohms) and +(infinite Ohms) inputs. With the output saturated, the internal stages are probably also saturating, and drawing an abnormal amount of current, even though the output current is close to zero.
.
Even withh 1 Meg resistors, with a 5uA bias current (the spec limit for 5V supplies), the output could go as high as 5 volts. With an op-amp that has such a high bias current, you should use smaller value resistors.
Regards,
Kral