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AN-1244

更新时间： 2024-10-28 21:54:19

品牌	Logo	应用领域
美国国家半导体 - NSC		转换器光电二极管光电二极管
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Photo-Diode Current-to-Voltage Converters

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National Semiconductor

Application Note 1244

Hooman Hashemi

Photo-Diode

Current-to-Voltage

Converters

September 2002

Converting the small output current of a photo-diode trans-

ducer to a fast responding voltage is often challenging. Here

are some ways to use high-speed Current Feedback and

Voltage Feedback op amps to do the job

and more troublesome. This being the case, C_INhas less of

an effect on reduction of the converter bandwidth, and

achieving stability is easier when using a CFA.

If C_INis sufficiently large, the closed loop phase shift will

approach – 180˚ at the cross-over frequency (where open

loop transimpedance gain crosses the noise gain function).

As with Voltage Feedback Amplifiers, the closed loop ampli-

fier can be compensated by adding a small capacitor (C_F)

across R_F. In the case of Figure 1, using the CLC450 CFA,

C_Fwas experimentally determined to be around 2pF for

about 10% overshoot in the step response. C_Fimproves

stability by counteracting the effect of the zero discussed in

the paragraph above by introducing a low frequency pole

Current Feedback Amplifier

Solution

Current Feedback Amplifiers (CFA) are especially suited to

implement this function, as shown in Figure 1. With an

effective internal buffer on the inverting node of the op amp,

the output impedance R_O(internal to U1, not shown) and the

photo-diode’s output capacitance C_IN(typically 10-200pF)

introduce a zero in the noise gain at approximately 1/2π x

(R_Ox C_IN). In comparison, the zero produced by a Voltage

(1/2π x R_Fx C_F) and an inconsequential zero (1/2π x R_O

C_F).

Feedback op amp in

(R_IN||R_F||R_BIAS) x C_IN] tends to be much lower in frequency

a similar configuration [1/2π x

20050001

FIGURE 1. Single-Supply Photo-Diode Amplifier Using CLC450 Current-Feedback Amplifier

It is possible to change the required 2pF compensation

capacitor to a more practical value, by adding R_Aand R_Bin

a voltage divider, as shown in Figure 2. The new value of C'_f

is (1+R_B/R_A) x C_F. This relationship holds true as long as

For this example, select R_A= 50Ω, and R_B= 500Ω. There-

fore, C'_f= (1+500/50) x2pF = 22pF which is a much more

practical component value. This value needs to be ’fine

tuned’ in the real application for proper step response.

R_B

R_F.

AN200500

www.national.com

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