Philips TZA3046 7.3 Monitoring RSSI via IDREF

Product data sheet Rev. 01 — 19 May 2006 7 of 15

Philips Semiconductors TZA3046

Fiber Channel/Gigabit Ethernet transimpedance ampliﬁer

For applications where the transimpedance is controlled by the TIA it is advised to leave

the AGC pads unconnected to achieve fast attack and decay times.

The AGC function can be overruled by applying a voltage to pad AGC. In this

conﬁguration, connecting pad AGC to ground gives maximum transimpedance and

connecting it to VCC gives minimum transimpedance. This is depicted in Figure 7. The

AGC voltage should be derived from the VCC for proper functioning.

For maximum freedom on bonding location, 2 pads are available for AGC (pads 6 and 15).

These pads are internally connected. Both pads can be used if necessary.

7.3 Monitoring RSSI via IDREF_MON

To facilitate RSSI monitoring in modules (e.g. SFF-8472 compliant SFP modules), a

current output is provided. This output gives a current which is 20 % of the average DREF

current through the 290 Ω bias resistor. By connecting a resistor to the IDREF_MON

output, a voltage proportional to the average input power can be obtained.

The RSSI monitoring is implemented by measuring the voltage over the 290 Ω bias

resistor. This method is preferred over a simple current mirror because at small photo

currents the voltage drop over the resistor is very small. This gives a higher bias voltage

yielding better performance of the photodiode.

For maximum freedom on bonding location, 2 pads are available for IDREF_MON (pads 5

and 16). These pads are internally connected. Both pads can be used if necessary. If only

one is used, the other can be left open.

Fig 7. Transimpedance as function of the AGC voltage

001aae517

VAGC/VCC

0.3 0.90.70.5

transimpedance

(kΩ)

10−1