Cover story
two single amplifiers can have slightly different chopping frequencies that can interact and cause additional IMD.
Matching input source impedance Transient current glitches interacting with input source impedance can cause differential voltage errors, potentially resulting in additional artifacts at multiples of the chopping frequency. Figure 9 shows spikes in the noise density plot of the ADA4522 with mismatched source resistance (bottom). To mitigate this potential source of error, the system designer should ensure that each input of a chopped amplifier sees the same impedance (top). Also, when using a chopping amplifier, the input signal can mix with the chopping frequency fCHOP fIN
± 2fCHOP , 2fIN
to create IMD at fIN ± fCHOP
± fCHOP ,
etc.These IMD
products can appear in the band of interest especially as fIN
approaches the chopping
frequency. To eliminate this issue, select a zero-drift amplifier that has a chopping frequency much greater than the input signal bandwidth, and ensure that interferers at frequencies close to fCHOP
are filtered before
this amplifier stage. Chopping artifacts can also be aliased when sampling an amplifier output with an ADC. When designing a signal chain, it is necessary to include antialiasing filters before the ADC to reduce this IMD.
Filtering the chopping artifacts At the system level, the single most effective way to deal with these high-frequency artifacts is by filtering. An LPF between the zero-drift amplifier and the ADC reduces chopping artifacts and avoids aliasing. For this reason, amplifiers with a higher chopping frequency relax the requirements of an LPF and allow for wider signal bandwidth. Depending on how much out-of-band
rejection your system needs, a higher order active filter may be required. ADI has many resources to help with filter design including the multiple feedback filter tutorial and the online filter design tool. Knowing the frequencies where chopping
artifacts occur can help to create the required filter. By understanding high frequency artifacts
in zero-drift amplifiers, system designers can be more confident in using zero-drift op amps for their wider bandwidth applications.
www.analog.com
Figure 9: Noise with matched (top) and mismatched (bottom) input source resistance in the ADA4522
Figure 8: The noise density plot of the ADA4522 ,
Figure 7: The noise density plot of the ADA4528
www.electronicsworld.co.uk May 2023 07
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44
