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# ISO224: Input Bias Current vs. Input Voltage

Part Number: ISO224

I am interested in using the ISO224B but would like know how the input bias current changes with input voltage (not supply voltage). Do you have any charts or numbers you could provide that indicate how input bias current changes as the input goes from -12V to 12V?  I presume the input bias current also increases dramatically when the input voltage approaches "Vclipping".

My application involves measuring a high voltage (+/- 1.2kV max) signal which needs to be divided down. Therefore, the input bias current will impact the measurement due to the divider impedance. I'd prefer not to use an input buffer amplifier, as that would require an additional (negative) supply rail since my input signal is AC voltage.

Although input bias current vs. input voltage is not a common graph supplied for op-amps, the nature of the ISO224 makes it particularly important for this part.  In a traditional op-amp circuit with feedback and a fixed reference level, the actual voltage at the +/- inputs of the op-amp remains constant, even as the input signal changes.  Because this voltage remains constant, the input bias current would also remain fairly constant and could be canceled out by a variety of methods.

However, the ISO224 only has a positive input with no external feedback network.  Therefore, when the input signal changes, the actual voltage level on the input to the ISO224 also changes.  Further complicating matters is the fact that when the input signal is negative, input bias current could only possibly flow *out* of the IN pin.  When the input signal is greater than the VCAP voltage, then input bias current could only possibly flow *into* the IN pin.  This makes it difficult to cancel out the input bias current and understanding how the input bias current changes with input voltage is important.

Thanks

• Hi Christopher,

the input current of ISO224 consists of two parts, an almost constant component not varying with the input voltage, called "input bias current", and a component varying with the input voltage, covered by the specified "input resistance".

If you mount a voltage divider to the input, take care that the lower resistance is less than about 1/100 of the input resistance of ISO224, or by other words <10k. Then the finite input resistance of ISO224 gives an error of less than 1%. At the same time the input bias current of 15nA gives an input offset error voltage of about 150µV, which is only 1/10 of the input offset voltage of ISO224.

To decrease the impact of input bias current on accuracy, furtherly decrease the lower resistance of voltage divider.

Keep in mind, that there's no specified maximum input bias current of ISO224. So if you cannot live with this uncertainty, you would need to add a buffer with better specified input bias current between the voltage divider and the input of ISO224 .

Kai

• Hi Christopher,

I'll check with some members of the characterization team to see if this information is available.

• Hi Christopher,

I've confirmed that the input current vs input voltage will be the input impedance value. The input current will be 800nA/V and the direction of the current will change with the polarity of the input voltage as you noted.