9.2.1.3 Application Curves

Photodiode capacitance (pF)

0

2

4

6

8

10

12

14

16

18

20

50

0

100

20

150

40

200

60

250

80

300

100

350

120

400

140

450

160

D609

f-3dB, RF = 6 k:

f-3dB, RF = 12 k:

IRN, RF = 6 k:

IRN, RF = 12 k:

Figure 9-2. Bandwidth and Noise vs Photodiode Capacitance

Feedback Resistance (k:)

4

6

8

10

12

14

16

18

20

100

20

150

40

200

60

250

80

300

100

350

120

D610

f-3dB, CF = 1.5 pF

f-3dB, CF = 2.5 pF

IRN, CF = 1.5 pF

IRN, RF = 2.5 pF

Figure 9-3. Bandwidth and Noise vs Feedback Resistance

9.2.2 Optical Sensor Interface

There are two main approaches for current to voltage conversion. One uses a noninverting voltage feedback

amplifier in combination with a shunt resistor to first convert current and then further amplify the optical signal.

The other approach configures an amplifier for transimpedance applications which combines both steps into one.

Figure 9-4 shows the standard configuration for both approaches.

V

OUT

R

F

C

F

V

BIAS

I

PHOTO

V

BIAS

V

OUT

R

F

R

G

I

PHOTO

Figure 9-4. Transimpedance Amplifier vs Voltage Feedback Amplifier

Both configurations provide a low output impedance stage which provides the ability to interface with various

types of loads. However, the noninverting option comes with a few disadvantages. The TIA input impedance

is near zero, since the amplifier keeps the voltage at the inverting input node at the same potential as the

non-inverting input node. While the VFB input impedance is equal to the shunt resistor RL. In the case of the

VFB amplifier, the signal response can be slowed due to a large time constant created by the shunt resistor

and capacitor. Also, the linearity of the photodetector can suffer, especially for higher detector currents due to

the varying voltage bias produced at the shunt resistor. And, since the voltage bias of the photodetector is no

longer constant for all detector currents, the diode’s internal capacitance can vary. Using a TIA, the voltage bias

remains constant at the voltage set by the non-inverting node, and can provide level shifting to the signal which

is especially useful for single-supply configurations.

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OPA855

SBOS622D – JULY 2018 – REVISED MAY 2025

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