Photodiode capacitance (pF)

0

2

4

6

8

10

12

14

16

18

20

0

20

50

30

100

40

150

50

200

60

250

70

300

80

350

90

400

100

450

110

D209

f-3dB, RF = 10 k:

f-3dB, RF = 20 k:

IRN, RF = 10 k:

IRN, RF = 20 k:

Figure 9-2. Bandwidth and Noise Performance vs

Photodiode Capacitance

Feedback Resistance (k:)

50

0

100

20

150

40

200

60

250

80

300

100

350

120

10

100

D210

f-3dB, CF = 1 pF

f-3dB, CF = 2 pF

IRN, CF = 1 pF

IRN, CF = 2 pF

Figure 9-3. Bandwidth and Noise Performance vs

Feedback Resistance

Figure 9-2 shows the amplifier performance as a function of photodiode capacitance (CPD) for RF = 10 kΩ and

20 kΩ. Increasing CPD decreases the closed-loop bandwidth. To maximize bandwidth, reduce any stray parasitic

capacitance from the PCB. The OPA858 is designed with 0.8 pF of total input capacitance to minimize the effect

on system performance.

Figure 9-3 shows the amplifier performance as a function of RF for CPD = 1 pF and 2 pF. Increasing RF results in

lower bandwidth. To maximize the signal-to-noise ratio (SNR) in an optical front-end system, maximize the gain

in the TIA stage. Increasing RF by a factor of X increases the signal level by X, but only increases the resistor

noise contribution by √X, thereby improving SNR.

9.2 Typical Applications

9.2.1 TIA in an Optical Front-End System

The high GBWP, low input voltage noise, and high slew rate of the OPA858 makes the device a viable wideband,

high input impedance voltage amplifier.

+

±

226

453

2.5 V

í2.5 V

GND

169

50-

Measurement

System

50

GND

Supply decoupling not shown

71.5

GND

GND

50-

Source

50

62

GND

Figure 9-4. OPA858 in a Gain of –2V/V (No Noise Gain Shaping)

+

±

226

453

2.5 V

í2.5 V

GND

169

50-

Measurement

System

50

GND

Supply decoupling not shown

71.5

GND

2.7 pF

GND

0.5 pF

GND

50-

Source

50

62

GND

Figure 9-5. OPA858 in a Gain of –2V/V (With Noise Gain Shaping)

OPA858

SBOS629B – APRIL 2018 – REVISED MAY 2025

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