Cover story
odd harmonics of the chopping frequency. Amplifier designers employ many methods to reduce the effects of ripple, including: production offset trimming, chopping and auto-zeroing combined, and applying autocorrection feedback.
Figure 3: A basic auto-zero amplifier Figure 4: Noise PSD: before chop or AZ, after AZ, after chop, and after chop and AZ
Glitch artifact Glitches are transient spikes that are caused by charge injection mismatch from the chopping switches. Their magnitude depends factors such as source impedance and charge mismatch. These spikes not only cause artifacts at the even harmonics of the chop frequency but also create a residual DC offset, which is proportional to the chopping frequency; see Figure 5. Like with ripple, amplifier designers have techniques to reduce the this impact, including charge injection trim and multichannel chopping. Figure 6 compares a typical zero-drift
amplifier to the ADA4522, which uses this technique to significantly reduce the impact of glitches.
Figure 5: (L) Glitch voltage from charge injection at V1 (inside the chopping switches) and V2 (outside the chopping switches) in Figure 1; (R) glitches caused by finite amplifier bandwidth at V1 and V2 in Figure 1
System-level considerations When using a zero-drift amplifier in a data-acquisition solution, it is important to understand where the frequency artifacts are and plan for them accordingly. The chopping frequency is usually
explicitly stated in the data sheet, but it can also be determined by looking at the noise spectrum plots. A couple of ADI’s latest zero- drift amplifier data sheets show where in the spectrum artifacts occur. The ADA4528 data sheet not only explicitly states a chopping frequency of 200kHz in the Applications Information section, but this can also clearly be seen in the noise density plot in Figure 7. In the Theory of Operation section of
the ADA4522’s data sheet, the chopping frequency is stated to be 4.8MHz with an offset and ripple correction loop operating at 800kHz. Figure 8 shows the noise density of the ADA4522 where these noise peaks are visible. There is also a noise bump at 6MHz due to the reduced phase margin of the loop when in unity gain, which is not unique to zero-drift amplifiers. It is important to keep in mind that the
Figure 6: Voltage spikes are reduced to the noise floor in the ADA4522.8
frequency described in the data sheet is a typical number and can vary from part to part. For this reason, if the system requires two chopped amplifiers for differential signal conditioning, use a dual amplifier because
06 May 2023
www.electronicsworld.co.uk
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
