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Original question:

OPA4227: OPA4227UA

OPA4227: Questions about OPA4277UA/2K5

Jayden Qiu
Intellectual 1145 points
Part Number: OPA4227
Other Parts Discussed in Thread: OPA277

Tool/software:

Hi team,

This is FAE Jayden, ZTE is evaluating OPA4277UA/2K5 and they have some questions about this device:

  1. When used as a voltage follower, if the input is left floating, what will the output be like? What is the output voltage? Could you pls explain the principle of this state?
  2. This device is a four-channel integrated operational amplifier. If the input of one of the operational amplifiers is left floating, will the other channels be affected? 

Many thanks

Brs

Jayden

  • 0
    TI__Guru 63010 points

    Please see answers below:

    1. When used as a voltage follower, if the input is left floating, what will the output be like? What is the output voltage? Could you pls explain the principle of this state?

    Where the output ends up with the input floating depends on the direction of the input bias current (IB). If the input bias is positive (IB direction is into input terminal), the input will go down, thus driving the output towards negative rail.  For a negative IB (IB current comes out of input terminal) the output will be driven close to positive rail.

    However, in case of OPA277 with the internal input bias cancellation, the IB may be of either polarity (see +/- sign below).  In such a case, depending on the actual IB direction, the output will end up close to either rail.

    How close to either rail the output may end up is shown in the output voltage swing range (see below) - 0.5V above (V-) or 1.2V below (V+).

    1. This device is a four-channel integrated operational amplifier. If the input of one of the operational amplifiers is left floating, will the other channels be affected?

    If input of one channel is left floating, the other channels should not be effected.  However, the output of the channel with floated input will saturate at one of the rail, which may result in the increase of quiescent current, IQ.  For this reason, it is recommended to always bias the unused channels at mid-supply (e.g. using resistor divider between the rails) so the input and output stay within specified linear range.