Hi,
although INA333 says that the input should not float, and it says we should have path for bias current.
But compared to INA240, when input left floating, the output is a fixed value.
Why INA333 could not be a fixed value but INA240 could be? Don't we need to provide path for bias current of INA240?
Hi Howard,
As Kai indicated, INA333 is instrumentation amplifier and INA240 is high and low side bidirectional current-sense amplifier. Yes, both of them are measuring the differential voltages across an input.
In INA240 high side current-sensing case, the circuit has a low ohmic path to ground from the incoming power source. Power supply is a low impedance device that is connected to high side of a sensing-resistor in INA240 (you may view power source as AC ground).
If INA240 is placed on the low side of current-sensing configuration, it has the same low ohmic path as Kai alluded to.
In the case INA333, the Input Bias Current Return Path is addressed to the following similar circuits, see the captured image. Because INA333's input impedance is so high (100Gohm range); input bias current is only approx. +/-70 pA in range. Without the alternative bias current path, the generator's current (microphone, TC etc.) can cause the INA333's input potentials exceed the common-mode voltage range. By adding the alternative bias current path at input (a typical large value resistor that can sink approx. uA-mA current for low current device, ground the center tap in transformer configuration), it will remove the excessive current from a generator, and keep the INA333's input potential within Vcm range.
This is very similar to solar panel connection. Under the sun, the PV panel's output voltage will creep up, if someone removed the load or change to high resistive load.
If you have additional questions, please let us know.
Best,
Raymond
