AN-1244 Datasheet, PDF (1/3 Pages) National Semiconductor (TI) – Photo-Diode Current-to-Voltage Converters

Photo-Diode
Current-to-Voltage
Converters
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
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_O x C_IN). In comparison, the zero produced by a Voltage
Feedback op amp in a similar configuration [1/2π x
(R_IN||R_F||R_BIAS) x C_IN] tends to be much lower in frequency
National Semiconductor
Application Note 1244
Hooman Hashemi
September 2002
and more troublesome. This being the case, C_IN has less of
an effect on reduction of the converter bandwidth, and
achieving stability is easier when using a CFA.
If C_IN is 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_F was experimentally determined to be around 2pF for
about 10% overshoot in the step response. C_F improves
stability by counteracting the effect of the zero discussed in
the paragraph above by introducing a low frequency pole
(1/2π x R_F x C_F) and an inconsequential zero (1/2π x R_O x
C_F).
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_A and R_B in
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
R_B< < R_F.
For this
fore, C'_f
example, select R_A
= (1+500/50) x2pF
=
=
~5202^Ω_p,FanwdhiRch^B
=
is
500Ω. There-
a much more
practical component value. This value needs to be ’fine
tuned’ in the real application for proper step response.
© 2002 National Semiconductor Corporation AN200500
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