Photosensor amplifier

C2719

LIGHT

330

+Vs

-Vs

ZERO ADJ

(CASE)

A

-

+

L

M

H

Vout = -(i × Rf)

PROTECTION CIRCUIT

A

K

+Vs

0 V

-Vs

+

006P (9 V) × 2

EXTERNAL POWER

SUPPLY TERMINAL

HR10-7R-4P (HIROSE)

+

Figure 1 Equivalent circuit (when photodiode cathode is grounded)

<Amplifier circuit>

KACCC0030EC

KACCC0031EA

I Equivalent circuit

In the equivalent circuit diagram (Figure 1), the photocurrent sig-

nal i is connected to the input terminal of the negative feedback

amplifier A. The input resistance of A is designed to be much

larger than feedback resistance Rf, so all of i is flows through Rf.

The result is that the output voltage of A is converted to -i × Rf (V),

the reverse phase of i. Rf is switched to the High, Medium and Low

ranges by the conversion impedance, with Medium being 1/100

of High and Low being 1/10000 of High.

Feedback capacitance Cf is 10 pF (1 nF + 10 pF at Low range) to

prevent the circuit from becoming unstable at high frequencies.

In this case, the cut-off frequency is calculated as follows:

Rf

Cf

2

1

=

f

π

(Hz)

At this frequency, the output voltage is 0.7 times that at the low

frequency.

I Pulsed light measurement

G Output response waveform

If the photodiode terminal capacitance is approximately 2 nF or

more, ringing may appear in the output waveform of C2719

when pulsed light is detected, resulting in inaccurate measure-

ments. In this case, use a photodiode with a smaller terminal

capacitance, or add a reverse voltage to the photodiode to re-

duce the terminal capacitance.

Figures 2 (a) and (b) show pulse response waveforms when

used with S2281 photodiode which is designed to easily con-

nect to C2719. The terminal capacitance of S2281 is 1.28 nF.

G Frequency response

Figure 3 shows the frequency response characteristics when

used with S2281. As stated above, if the photodiode terminal

capacitance is too large, a noticeable peak is formed in the fre-

quency characteristics. This situation is shown in Figure 4. A

frequency at which this kind of peak exists equals the reciprocal

of the ringing cycle generated in the pulse response output

waveform.

G S/N

As explained above, when the terminal capacitance of a photo-

diode is too large, peaking exists in the noise characteristics

even when there is no light input, making it impossible to obtain

an accurate S/N during light measurement.

Figure 5 shows the frequency characteristics of the signal and

noise when S2281 is used. Here, peaking cannot be observed.

←LED drive pulse waveform

(2 V/div.)

←Output waveform

(2 V/div.)

←LED drive pulse waveform

(2 V/div.)

←Output waveform

(2 V/div.)

Figure 2 Output response waveform (S2281 + E2573)

(a) H range

(b) L, M range

2

<Power supply circuit>