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TZA3026

SDH/SONET STM4/OC12 transimpedance amplifier

NXP Semiconductors. 

Philips Semiconductors

TZA3026

SDH/SONET STM4/OC12 transimpedance amplifier

The parasitic capacitance can be minimized through:

1. Reducing the capacitance of the PIN diode. This is achieved by proper choice of PIN

diode and typically a high reverse voltage.

2. Reducing the parasitics around the input pad. This is achieved by placing the PIN

diode as close as possible to the TIA.

The PIN diode can be biased with a positive or a negative voltage. Figure 3 shows the PIN

diode biased positively, using the on-chip bias pad DREF. The voltage at DREF is derived

from VCC by a low-pass filter comprising internal resistor RDREF and external capacitor C2

which decouples any supply voltage noise. The value of external capacitor C2 affects the

value of PSRR and should have a minimum value of 470 pF. Increasing this value

improves the value of PSRR. The current through RDREF is measured and sourced at pad

IDREF_MON, see Section 7.3.

If the biasing for the PIN diode is done external to the IC, pad DREF can be left

unconnected. If a negative bias voltage is used, the configuration shown in Figure 4 can

be used. In this configuration, the direction of the signal current is reversed to that shown

in Figure 3. It is essential that in these applications, the PIN diode bias voltage is filtered to

achieve the best sensitivity.

For maximum freedom on bonding location, 2 outputs are available for DREF (pads 1

and 3). These are internally connected. Both outputs can be used if necessary. If only one

is used, the other can be left open.

C2

470 pF

VCC

DREF 1 or 3 RDREF

IPD

300 Ω

4 or 17

IPHOTO 2

TZA3026

001aac619

Fig 3. The PIN diode connected between

the input and pad DREF

VCC

DREF 1 or 3 RDREF

300 Ω

4 or 17

IPHOTO 2

IPD

negative

bias voltage

TZA3026

001aac620

Fig 4. The PIN diode connected between

the input and a negative supply

voltage

9397 750 14763

Product data sheet

Rev. 01 — 2 May 2005

© Koninklijke Philips Electronics N.V. 2005. All rights reserved.

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