Electronic Design

Transimpedance Amplifier With AGC Targets 803.3aq Applications

Originally deployed for 1-Gbit Ethernet, a lot of existing fiber-optic cable could manage 10 Gbits/s, thanks to IEEE 802.3aq 10GBase-LRM (long-reach multimode). This standard was approved in 2006 to address multimode dispersion in legacy fiber with a technology called electronic dispersion compensation (EDC).

Previous chip sets have depended on amplifiers that were less than optimal for the new standard. But Inphy’s 1348TA S03D transimpedance amplifier (TIA) with automatic gain control (AGC) exceeds the 802.3aq requirements. Its performance resembles the company’s earlier 1348TA S01D, which supports up to 11.3 Gbits/s and amplifies input currents between 20 µA and 2.5 mA p-p with less than 5% harmonic distortion.

Typical input sensitivity is –18 dBm with a bit error rate (BER) of 10 to 12. In AGC mode, gain is set internally through a peak detector. Gain also can be controlled externally. An integrated feedback circuit controls the dc offset to maintain 50% eye crossing, or else the eye crossing can be externally adjusted. Small-signal bandwidth is greater than 8 GHz, with an output return loss greater than 8 dB, up to 12 GHz.

The new amp meets and exceeds the 802.3aq requirements with higher gain, an extended power-supply operating range, and output electrostatic discharge (ESD) protection to more than 1 kV (Human Body Model). As a result, the 1348TA S03D meets 10G SFP+ (small form-factor pluggable) requirements from –11 dBm (optical modulation amplitude, or OMA) to 3 dBm (OMA) without an additional buffer amp.

According to Inphy, the integrated TIA/AGC has passed stringent testing to ensure complete interoperability with all leading EDC chips in the market (see the figure). Specific details about testing are available from the company.

The Inphy 1348TA is available in engineering samples, with volume shipments expected to begin in the first quarter of 2008.


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