AN4451

Stability of the UV sensor

DocID025985 Rev 1

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A zero also appears and must be considered, as shown in formula (9).

In addition, the low-frequency pole of the op-amp's open loop transfer function must be

considered, as shown in formula (10).

Therefore, the bode diagram of this system can be plotted as shown in Figure 8.

Figure 8: Bode diagram of the open loop transfer function of an application using a UV sensor

We can consider that Rg (100 GΩ) >> Rf (10 MΩ), so fz > fp.

With these considerations,

To guaranty stability of the system, the bode diagram must cross the X-axis with a slope of

-20 dB/decade. So, considering Figure 8, and to ensure stability, the gain at frequency fz

must be greater than 1.

Consequently, formula (11) is inferred.

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