How does the UV sensor work

AN4451

6/15

DocID025985 Rev 1

1.8

Capacitors Cj, Cin, and Cf

In general, transimpedance amplifiers are prone to oscillate. To understand the stability of

this kind of architecture, it is important to take into consideration all components of the

amplifier, even parasitic components, as shown in Figure 4.

Figure 4: Components that need to be considered in a transimpedance amplifier

The noise gain of the above configuration determines the stability of the circuit. Generally,

Rf is high to provide enough gain to convert the current of the UV sensor into a

measureable voltage. However, Rf combined with Cin and Cj creates a pole which ensures

instability of the circuit. In Figure 5

, we can see that for a high value of Rf (8.2 MΩ) and no

feedback capacitor (Cf), the output of the OA1MPA is unstable.

Figure 5: OA1MPA output response to a small current signal without feedback capacitance,

Rf = 8.2 MΩ

By adding a small capacitor (Cf) across Rf, oscillations or gain peakings are suppressed

and the output is stabilized (see Figure 6).

+

-

Vcc

Rf

Vout

Cf

Rj

Cj

OA1MPA

Rs

Ip

photocurrent

Cin

0

0.5

1

1.5

2

2.5

3

0.E+00

2.E-08

4.E-08

6.E-08

8.E-08

1.E-07

1.E-07

0

0.01

0.02

0.03

0.04

0.05

0.06

Time (s)

Current from the UV sensor

Vout of the OA1MPA