NSC AN-1244 Photo-diode current-to-voltage converter Datasheet

National Semiconductor
Application Note 1244
Hooman Hashemi
September 2002
Converting the small output current of a photo-diode transducer 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, CIN has less of
an effect on reduction of the converter bandwidth, and
achieving stability is easier when using a CFA.
If CIN 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 amplifier can be compensated by adding a small capacitor (CF)
across RF. In the case of Figure 1, using the CLC450 CFA,
CF was experimentally determined to be around 2pF for
about 10% overshoot in the step response. CF improves
stability by counteracting the effect of the zero discussed in
the paragraph above by introducing a low frequency pole
(1/2π x RF x CF) and an inconsequential zero (1/2π x RO x
CF).
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 RO (internal to U1, not shown) and the
photo-diode’s output capacitance CIN (typically 10-200pF)
introduce a zero in the noise gain at approximately 1/2π x
(RO x CIN). In comparison, the zero produced by a Voltage
Feedback op amp in a similar configuration [1/2π x
(RIN||RF||RBIAS) x CIN] tends to be much lower in frequency
Photo-Diode Current-to-Voltage Converters
Photo-Diode
Current-to-Voltage
Converters
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 RA and RB in
a voltage divider, as shown in Figure 2. The new value of C'f
is (1+RB/RA) x CF. This relationship holds true as long as
RB << RF.
For this example, select RA = 50Ω, and RB = 500Ω. Therefore, 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.
AN-1244
© 2002 National Semiconductor Corporation
AN200500
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AN-1244
Current Feedback Amplifier
Solution (Continued)
Voltage Feedback Amplifier
Solution
It’s more difficult to design a good current-to-voltage converter using a Voltage Feedback Amplifier (VFA). As discussed above, phase shift caused by photo-diode capacitance is often a source of instability. Furthermore, wide
bandwidth usually comes at the expense of supply currents
and higher supply voltage. However, the new LMH6642
high-speed low-voltage VFA op amp has excellent performance in a transimpedance gain block, as shown inFigure 3.
This device can operate down to 2.7V single supply and its
−3dB BW (AV = +1) is more than 100MHz (with a supply
current of only 2.7mA)! Because of the ’Dielectric Isolation’
process this device is based on, the traditional supply voltage vs. speed trade-off has been alleviated to a great extent
allowing low power consumption and operation at lower
supply voltages. In addition, the device has Rail-to-Rail output swing capability to maximize the output swing, and is
capable of driving ± 50mA into the load.
With 5V single supply, the device common mode voltage is
shifted to near half-supply using R10-R11 as a voltage divider
from VCC. The common-base transistor stage (Q1) isolates
the photo-diode’s capacitance from the inverting terminal,
allowing wider bandwidth and easing the compensation required. Note that the collector of Q1 does not have any
voltage swing, so the Miller effect is minimized. The diode on
the base of Q1 is for temperature compensation of its bias
point. Q1 bias current was set to be large enough to handle
the peak-to-peak photo-diode excitation, yet not too large as
to shift the U1 output too far from mid-supply. The overall
circuit draws about 4.5mA from the +5V power supply and
achieves about 35MHz of closed loop bandwidth @1VPP.
Figure 4 shows the output large signal step response. CF
can be increased to reduce the overshoot, at the expense of
bandwidth.
20050002
FIGURE 2. RA -RB Resistor Divider Allows Use of
Practical Value for C'f
20050003
FIGURE 3. Single-Supply Photo-Diode Amplifier Using LMH6642 Voltage-Feedback Op Amp
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2
Photo-Diode Current-to-Voltage Converters
Voltage Feedback Amplifier Solution
(Continued)
20050004
FIGURE 4. Output Step Response 20ns/div, 0.2V/div
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