2001 Feb 26

6

Philips Semiconductors

Product specification

SDH/SONET STM16/OC48

transimpedance amplifier

TZA3013A; TZA3013B

PIN diode bias voltage DREF

The performance of an optical receiver is largely

determined by the combined effect of the transimpedance

amplifier and the PIN diode. In particular, the method used

to connect the PIN diode to the input and the layout around

the input pad strongly influences the main parameters of a

transimpedance amplifier, such as sensitivity, bandwidth,

and PSRR. Sensitivity is most affected by the value of the

total capacitance at the input pad. Therefore, to obtain the

highest possible sensitivity requires the value of total

capacitance to be as low as possible by reducing the

capacitance of the PIN diode and the parasitics around the

input pad. To minimize parasitics, the PIN diode should be

placed as close as physically possible to the IC. The

capacitance of the PIN diode can be reduced by making

the value of reverse voltage across it as high as possible.

The PIN diode can be connected to the input in two ways.

Figure 5 shows the PIN diode connected between

pads DREF and IN.

Pad DREF provides an easy bias voltage for the

low-pass filter comprising internal resistor R1 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 100 pF.

Increasing this value increases the value of PSRR.

For a supply voltage of 3.3 V, the reverse voltage across

the PIN diode is 2.438 V (3.3 V

− 0.862 V). It is preferable

to connect the cathode of the PIN diode to a voltage higher

pad DREF unconnected. If a negative supply voltage is

available, the configuration shown in Fig.6 can be used.

It should be noted that in this configuration, the direction of

the signal current is reversed to that shown in Fig.5. It is

essential that the PIN diode bias voltage is correctly

filtered to achieve the highest possible level of sensitivity.

handbook, halfpage

R1

270

Ω

C2

100 pF

VCC

Ii

1

30

TZA3013

2

IN

DREF

MGU120

Fig.5

The PIN diode connected between the input

and pad DREF.

handbook, halfpage

MGT103

270

Ω

VCC

Ii

1

30

TZA3013

2

IN

DREF

negative supply

Fig.6

The PIN diode connected between the input

and a negative supply voltage.