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TZA3023T Datasheet(PDF) 5 Page - NXP Semiconductors |
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TZA3023T Datasheet(HTML) 5 Page - NXP Semiconductors |
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5 / 24 page  2000 Mar 29 5 Philips Semiconductors Product specification SDH/SONET STM4/OC12 transimpedance amplifier TZA3023 PIN diode bias voltage DREF The transimpedance amplifier together with the PIN diode determines the performance of an optical receiver for a large extent. Especially how the PIN diode is connected to the input and the layout around the input pin influence the key parameters like sensitivity, bandwidth and the Power Supply Rejection Ratio (PSRR) of a transimpedance amplifier. The total capacitance at the input pin is critical to obtain the highest sensitivity. It should be kept to a minimum by reducing the capacitor of the PIN diode and the parasitics around the input pin. The PIN diode should be placed very close to the IC to reduce the parasitics. Because the capacitance of the PIN diode depends on the reverse voltage across it, the reverse voltage should be chosen as high as possible. The PIN diode can be connected to the input in two ways as shown in Figs 5 and 6. In Fig.5 the PIN diode is connected between DREF and IPhoto. Pin DREF provides an easy bias voltage for the PIN diode. The voltage at DREF is derived from VCC by a low-pass filter. The low-pass filter consisting of the internal resistor R1, C1 and the external capacitor C2 rejects the supply voltage noise. The external capacitor C2 should be equal or larger then 1 nF for a high PSRR. The reverse voltage across the PIN diode is 4.2 V (5 − 0.8 V) for 5 V supply or 2.5 V (3.3 − 0.8 V) for 3.3 V supply. The DC voltage at DREF decreases with increasing signal levels. Consequently the reverse voltage across the PIN diode will also decrease with increasing signal levels. This can be explained with an example. When the PIN diode delivers a peak-to-peak current of 1 mA, the average DC current will be 0.5 mA. This DC current is delivered by VCC through the internal resistor R1 of 2 kΩ which will cause a voltage drop of 1 V across the resistor and the reverse voltage across the PIN diode will be reduced by 1 V. It is preferable to connect the cathode of the PIN diode to a higher voltage then VCC when such a voltage source is available on the board. In this case pin DREF can be left unconnected. When a negative supply voltage is available, the configuration in Fig.6 can be used. It should be noted that in this case the direction of the signal current is reversed compared to Fig.5. Proper filtering of the bias voltage for the PIN diode is essential to achieve the highest sensitivity level. MCD900 R1 2 k Ω C1 10 pF C2 1 nF VCC Ii 4 8 TZA3023 7 IPhoto DREF Fig.5 The PIN diode connected between the input and pin DREF. MCD901 R1 2 k Ω C1 10 pF VCC Ii 4 8 TZA3023 7 IPhoto negative supply voltage DREF Fig.6 The PIN diode connected between the input and a negative supply voltage. |
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