Abstract

Zero-drift amplifiers have very low input offset voltage (V_os), low offset drift, and no flicker noise. The two main types of zero-drift amplifiers are chopper and auto-zero amplifiers. This paper focuses on chopper amplifiers because these amplifiers are by far the most common type of zero-drift amplifier and most of the chopper concepts apply to auto-zero devices as well. Chopper amplifiers achieve the low V_os and V_os drift through the use of an internal calibration circuit that uses metal-oxide semiconductor field-effect transistor (MOSFET) switches to commutate the inputs. However, this calibration technique generates current transients within the amplifiers input bias current. These transients flow through the amplifiers feedback network and source impedance generating additional offset voltage, offset drift, and transient noise tones. The magnitude of this error increases when large feedback resistances or source impedances are used. Furthermore, this error is dependent on the details of the chopper amplifier design, so that some products are more susceptible to V_os shift than others. This white paper provides details on how the magnitude of offset, drift, and noise can be impacted by source and feedback impedance for different chopper amplifier products. The document covers a method for selecting the best chopper amplifier for your application, and clarifies when a chopper amplifier is not an appropriate choice.