Page 14 - EngineerIT November 2022

ELECTRONICS DESIGN

Figure 13: ADA4522 NSD using the first-order filters methods shown at top: (L) Increasing the gain reduces the amplifier’s bandwidth, which filters the
noise peaks, (R) using an RC filter.

gain, post-filtering, and using a capacitor in parallel with the References
feedback resistor. 8 1. Yoshinori Kusuda. “Reducing Switching Artifacts in Chopper Amplifiers.”
Delft University of Technology, the Netherlands, May 2018.
Depending on how much out of band rejection your system 2. Christian Enz and Gabor C. Temes. “Circuit Techniques for Reducing
needs, a higher order active filter may be required. ADI has the Effects of Op-Amp Imperfections: Autozeroing, Correlated Double
many resources to help with filter design including the multiple Sampling, and Chopper Stabilization.” Proceedings of the IEEE, Vol. 84,
feedback filter tutorial and the online filter design tool. No. 11, November 1996.
Knowing the frequencies where chopping artifacts occur can 3. Boris Murmann. EE315A: VLSI Signal Conditioning Circuits: Chaper 7,
Precision Analog Circuit Techniques. Stanford University, 2014.
help to create the required filter. Table 1 shows the locations of 4. James Bryant. “Multipliers vs. Modulators.” Analog Dialogue, Vol. 47,
AC artifacts caused by zero-drift amplifiers. June 2013.
5. A. T. K. Tang. “A 3/spl mu/V Offset Operational Amplifier with 20nV//
Table 1: Summary of AC chopping artifact locations spl radic/Hz Input Noise PSD at DC Employing Both Chopping and
Autozeroing.” IEEE, February 2002.
Artifact Description Location 6. Michiel Pertijs and Wilko J. Kindt. “A 140 dB-CMRR Current-Feedback
Ripple f CHOP, 3f CHOP, 5f CHOP, … Instrumentation Amplifier Employing Ping-Pong Auto-Zeroing and
Glitch 2f CHOP, 4f CHOP, 6f CHOP, … Chopping.” IEEE Journal of Solid-State Circuits, Vol. 45, No. 10, October
2010.
Amplifier IMD f IN ± f CHOP, f IN ± 2f CHOP, 2f IN ± f CHOP, … 7. Johan F. Witte, Kofi A. A. Makinwa, and Johan H. Huijsing. “A CMOS
Aliasing f ARTIFACT ± f SAMPLE, f ARTIFACT ± 2f SAMPLE, Chopper Offset-Stabilized Opamp.” IEEE Journal of Solid-State Circuits,
f ARTIFACT ± 3f SAMPLE, … Vol. 42, No. 7, July 2007.
8. Yoshinori Kusuda and Vicky Wong. “Zero-Drift Amplifiers: Now Easy to
Use in High Precision Circuits.” Analog Dialogue, Vol. 49, July 2015.
Conclusion ADA4523 data sheet. Analog Devices, Inc., April 2020.
By understanding high frequency artifacts in zero-drift AD7768-1 data sheet. Analog Devices, Inc., May 2019.
Kusuda, Yoshinori. “Analysis of Input Current Noise with Even
amplifiers, system designers can be more confident in using Harmonics Folding Effect in a Chopper Op Amp.” Analog Dialogue, Vol.
zero-drift op amps for their wider bandwidth applications. These 53, May 2019.
system design considerations include:
• Matching source input impedance seen by a zero-drift About the Author
amplifier’s inputs; Simon Basilico is a design engineer in the Precision Signal Chain
• Using a dual amplifier for differential signal conditioning; Group based in Santa Clara, CA. His focus is on precision signal
• Finding the frequency of artifacts in the data sheet noise chains including both integrated mixed-signal designs and
spectrum; system-in-package solutions. He joined Analog Devices in 2015
• Designing a filter such that it reduces the impact of high after receiving his B.S. and M.S. in electrical engineering from
frequency artifacts caused by dynamic offset reduction Stanford University.
techniques; and
• Understanding and planning for high frequency artifacts in For more information on Analog Devices contact
the frequency domain. n Conrad Coetzee ccoetzee@arrow.altech.co.za

EngineerIT | November 2022 | 14