Vias reduce EMI in photonics ICs
Martin Rowe, Senior Technical Editor -- Test & Measurement World, 6/1/2005 2:00:00 AM
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| Vias in the 1346TL transimpedance amplifier (large die) connect the underside of the device to a TO-46 case, thus making several connections to ground. Courtesy of Inphi. |
Inphi manufactures transimpedance amplifier ICs that its customers package with photodiodes into cases that they then install in optical transceivers. The photodiode converts light into current, which the amplifier converts into voltage. Because of price pressures, Inphi's customers want to package the chips into common component cases such as TO-46.
When Inphi began producing 10-Gbps devices, the company's engineers found that the electrical signals between the photodiode and the amplifier interfered with signals on the amplifier die. Thus, they needed a way to reduce emissions.
A TO-46 case provides just four connections to the outside world (see figure). When assembled with a photodiode and a 1346TL transimpedance amplifier, the case needs one pin to power the photodiode (Vpin), one to power the amplifier (Vcc), and two for the amplifier's differential output (Vout+ and Vout–).
To ground the ICs, Inphi's engineers built vias into the amplifier's die that make ground connections possible from the underside of the die to the case. The 10 to 12 vias are placed where they separate connections between critical circuits on the amplifier die. EMI emitted from the bond wires that connect the photodiode to the amplifier get routed to ground through the vias and away from critical circuits. Thus, the vias make it possible to use a TO-46 case by providing numerous ground connections that shield critical circuits from EMI. Two or three bypass capacitors isolate the devices from noise on the power lines.
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