ADA4350ARUZ-R7 datasheet(36/38 Pages) AD

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Data Sheet

ADA4350

Rev. B | Page 35 of 37

FREQUENCY (Hz)

10k

100k

10M

100M

10k

100k

0.1k

0.01k

RF0 = 1kΩ, CF0 = 100pF

RF1 = 3kΩ, CF1 = 56pF

RF2 = 10kΩ, CF2 = 33pF

RF3 = 30kΩ, CF3 = 18 pF

RF4 = 100kΩ, CF4 = 10pF

CD = 10nF

Figure 64. Transimpedance vs. Frequency, CD = 10 nF

FREQUENCY (Hz)

10k

100k

10M

100M

10k

100k

0.1k

0.01k

CD = 100nF

RF0 = 1kΩ, CF0 = 300pF

RF1 = 3kΩ, CF1 = 180pF

RF2 = 10kΩ, CF2 = 100pF

RF3 = 30kΩ, CF3 = 56pF

RF4 = 100kΩ, CF4 = 33pF

Figure 65. Transimpedance vs. Frequency, CD = 100 nF

THE EFFECT OF LOW FEEDBACK RESISTOR RFx

As the load of the transimpedance amplifier increases, excessive

peaking in the frequency response can be observed when the RFx

value is too small. This peaking can persist even when excessive

CFx overcompensates for it. Figure 66 shows the ADA4350

configured with a photodiode capacitance value of 91 pF and

a 1 kΩ transimpedance load. Figure 67 shows the normalized

frequency response of this configuration. By decreasing RF from

500 Ω to 68 Ω, the peaking in the frequency response increases

progressively. The large peaking translates to a huge overshoot

in the pulse response, which is an undesirable result.

–

CD = 91pF

1kΩ

TIA

*OVERCOMPENSATES

VOUT

RFx

CFx*

IPHOTO

Figure 66. Transimpedance Amplifier Circuit

–9

–8

–7

–6

–5

–4

–3

–2

–1

FREQUENCY (Hz)

CD = 91pF

RFx = 250Ω, CFx = 33pF

RFx = 125Ω, CFx =47pF

RFx = 68Ω, CFx = 100pF

10k

100k

100M

10M

RFx = 500Ω, CFx = 20pF

Figure 67. Normalized Frequency Response with Decreasing RF (See Figure 66)

To mitigate this effect, use an additional snubber circuit at the

output of the FET input amplifier, as shown in Figure 68. In this

configuration, the feedback resistor (RFx) is 68 Ω, and the capacitance

of the photodiode is 40 pF.

–

CD = 40pF

1kΩ

TIA

68Ω

100pF

IPHOTO

Figure 68. Snubber Circuit Added to Mitigate Peaking

Figure 69 shows the effect of various snubber circuits clamping

down the peaking. Without the snubber circuit, there is 6 dB of

peaking when an overcompensated CFx of 100 pF is used. With

the snubber circuits, the bandwidth is restricted to approximately

10 MHz. To compromise between the peaking and the bandwidth,

adjust the values of the snubber circuit.

–9

–6

–3

FREQUENCY (Hz)

100k

10M

100M

NO SNUBBER

RS = 10Ω, CS = 5.6nF

RF = 68Ω

CF = 100pF

CD = 40pF

RL = 1kΩ

RS = 10Ω,CS = 33nF

RS = 10Ω, CS = 10nF

Figure 69. Effect of Snubber Circuits on the Transimpedance Frequency Response

(See Figure 68)

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