AN4587 Datasheet, PDF (1/14 Pages) STMicroelectronics – Input bias current of a chopper amplifier

AN4587
Application note
Input bias current of a chopper amplifier
Nicolas Aupetit
Introduction
Chopper amplifiers are generally considered as the perfect choice in applications where DC
precision is mandatory.
This kind of amplifier uses a continuous time modulation technique to exhibit extremely low
offset and 1/f noise.
However, despite numerous advantages of the chopper, the switching technique shows
some drawbacks that should be taken into consideration for new design.
The TSZ121 is a chopper amplifier with an input offset voltage max of 5 µV at 25 °C.
Due to its switching mode, some injection charge appears on the input pins of this chopper
amplifier. This switching effect, generates both voltage and current “noise” transients at the
chopping clock frequency and its harmonics. The TSZ121 is a CMOS input, thus, it exhibits
a very low input bias current (Iib). This is a very desirable feature if large source impedances
are present. However, charge injection produces some unexpected effects on the input bias
current behavior.
October 2014
DocID026923 Rev 1
1/14
www.st.com
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