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Using zero-drift amps in high precision circuits

03 Sep 2015  | Vicky Wong, Yoshinori Kusuda

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Another interesting thing to note about figure 5 is that the integrated noise increases with a step-like function at the noise spike frequencies. The noise spikes (with increased noise energies), albeit narrow, add significantly to the total output integrated noise.

Switching artifacts in the time domain
Often times, the switching artifacts can be clearly seen in the voltage noise density spectrum in the frequency domain. To understand the time-based behaviour of the switching artifact, one can configure the amplifier in a buffer configuration with the non-inverting pin grounded and directly monitor the output with an oscilloscope. Figure 6 shows the typical outputs of two zero-drift amplifiers. Note that Amplifier A exhibits output voltage spikes in various amplitudes. The spikes repeat themselves every 0.66µsec. This matches the noise spikes that are seen at 1.51MHz in figure 4. On the other hand, the ADA4522-2 does not exhibit any switching artifact in the time domain (blue graph). In other words, the noise spikes that exist are below the noise floor of the measurement system and cannot be detected. This allows designers to use the ADA4522-2 in application such as driving an ADC, with confidence that noise spikes will not be an issue.

Figure 6: Output voltage noise in the time domain.

Filters to mitigate switching artifact

Figure 7: Zero-drift amplifier with filter set up.

Figure 8: Voltage Noise Density of a unity gain zero-drift amplifier with post filter.

To reduce the impact of the switching artifacts, there are a couple of methods that can be implemented. These methods ultimately lead to limiting the amplifier bandwidth such that it is less than the switching frequency. Using a filter is an effective way to suppress the noise spikes. The easiest design is to place a resistor – capacitor network at the amplifier output to create a low pass filter (figure 7A). Figure 8 shows the voltage noise density of a zero-drift amplifier with a post filter designed at one or two decades below the switching frequency. The noise spike at 800kHz reduces from 36nV/√Hz (no post filter) to 4.1nV/√Hz (post filter at 80kHz), which is below the amplifier's low frequency broadband noise level. With a post filter positioned two decades below the switching frequency (post filter at 8kHz), the noise spike is no longer visible and the ADA4522-2 behaves like any other traditional amplifier.

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