October 2014

DocID026923 Rev 1

1/14

14

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.

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