Stability of the UV sensor

AN4451

8/15

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Stability of the UV sensor

UV sensors are generally very capacitive. For example, the GUVA-C22SD UV sensor has

an internal capacitance of 100 pF and the OA1MPA adds an additional input capacitance of

3 pF. Such capacitance has a direct impact on the stability of the system. This section

describes how to calculate the minimum value of the Cf capacitor (see Figure 7). Cf is a

feedback capacitor which is added in parallel with the transimpedance resistor. Its function

is to stabilize the system. As Cf limits the bandwidth, it therefore minimizes noise. In the

formulas below, Cin and Cj are considered as a unique capacitor, Cg, (see Figure 7). The

ser

ial resistor, Rs, which has a resistance of about 100 Ω is neglected.

Figure 7: Simplified equivalent circuit

The open loop transfer function of the system is given in formula (7).

where:

A is the open loop transfer function of the op-amp.

However, by using formula (7), a pole appears and must be considered, as shown in

formula (8).

GUVA-C22SD

+

-

Vcc

Rf

Vout

Cf

Rg

Cg

103pF

100G

Ω

8.2M

Ω

OA1MPA

∗

∗

1∗