How does the UV sensor work

AN4451

4/15

DocID025985 Rev 1

1.2

Application example

The UV sensor under consideration in this application note is the GUVA-C22SD from the

Genuv Company (see Figure 2).

Figure 2: UV sensor application

The GUVA-

C22SD sensor delivers a current of 26 nA/UV index. Rf is set @ 8.2 MΩ.

Consequently, this sensor receives UV radiation with an index of 4 (Vout = 26 nA * 4 *

8.2 MΩ = 852.8 mV).

1.3

Input offset voltage (Vio)

As the ideal op-amp does not exist, it must be accepted that the op-amp itself has an

impact. For example, an op-amp adds a DC offset on the output which is directly linked to

the input voltage offset (see formula (3)).

Consequently, it is better to choose an op-amp with a low input voltage offset (Vio). The

OA1MPA is a good op-amp in this respect as it offers a maximum Vio of 200 µV. By taking

the Vio into account, the Vout becomes: Vout = 26 nA * 4 * 8.2 MΩ ± 200 µV. Thus, Vout is

in the range [852.6 mV:853 mV] which is an error of 234 ppm!

1.4

Feedback resistance

As the current generated by the UV sensor is extremely small, it is advised to use the

largest feedback resistor possible to benefit from the ADC performance. Even though it

may seem paradoxical, a large feedback resistor also helps to improve the SNR.

Effectively, resistance noise is thermal noise which is defined as: en_Rf = √4*K*T*Rf,

where K is the Boltzmann’s constant (1.38x10

273.15). The SNR is expressed in formula (4).

By using a large feedback resistor, the SNR is improved by √Rf.

UV radiation

Index 4

104nA

GUVA-C22SD

+

-

Vcc

8.2M

Ω

Vout

10pF

Uv sensor

OA1MPA