Zero-Drift Precision Op Amps: Measuring And Eliminating Aliasing For More Accurate Current Sensing
by by Farhana Sarder, ON Semiconductor, Phoenix, Ariz., How2Power Today, Apr 15 2019
Focus:
Zero-drift precision op amps are specialized op amps designed for applications that require
high output accuracy due to small differential voltages. They are well suited for low-side
current sensing and sensor interface. In spite of manufacturers’ claims to the contrary, these
devices are vulnerable to aliasing since they use sampling to minimize the input offset
voltage. Therefore designers should test their op amp circuits for aliasing. This 15-page
article presents a method for doing so using a signal generator and oscilloscope and applies it
to
observe differences in aliasing produced by different implementations of zero-drift op amps
including chopper-stabilized and auto-zero op amps. Use of a low-pass filter to prevent
aliasing is explained. This article also explains the relationships between an op amp’s input
offset voltage and other parameters such as transient response, startup time, rail-to-rail
operation, low-frequency noise, and input current. Finally, the inability of Spice models to
account for zero-drift effects such as aliasing is explained.
What you’ll learn:
- How to detect and prevent aliasing in zero-drift precision op amps
- How to understand the source of aliasing in precision op amps
- How to measure the internal clock frequency in a zero-drift op amp
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