NRF51822 PCB layout review

[joseph]

Hello,

I recently finished up the NRF51822 pcb board that I plan on getting printed. I used the reference design file provided by Nordic (NRF51x22_qfax) but added the following things:

• Meander Antenna taken from NRF51-Dongle with a bit more length added for adjustment
• Shunt Capacitor for the Meander Antenna
• Through-holes connected to most of the I/O pinouts from the chip for breakout
• Through-holes for power in (1.8V & GND)
• Increase in GND plate size with vias to match

I am rather new at working with the NRF51 series as well as PCB design. Any advice would be great help. I am mostly concerned over the way my antenna is structured and if a single shunt cap should be enough for this.

Thanks!

Image of Front

Image of Back

Altium PcbDoc Source File

[AmbystomaLabs]

Oh, and on your updated layout you can NOT do the differential routing the way you did it. It's not appropriate at all. Plus I don't know about your experiences with WLCSP (ST balun), but I've never liked placing them and all the fab houses I work with also complain about them. This is the same balun they used on the beacon dev board. Half of the boards I received from nordic had incorrectly placed baluns. The Johanson part is a nice normal ceramic chip with leads up the sides. Placement and reflow is way easier.

[joseph]

The only thing I am planning on running from the chip are LEDs and 4 H-Bridges to control a few DC motors. It wont matter the voltage that the motor sit at since regulating the PWM will be the controlling mechanism.

The extra converter I am referring to would be something that gets the Lipo battery range (3.7V - 4.2V) down to a voltage that the NRF51822 can handle (3.0V - 3.3V). If I use the 51822's buck converter then I would need another buck converter or something similar to bring it down to what the 51822 can take. I might as well just have a single converter that brings the Lipo from 3.7-4.2 down to 1.8 and skip the 51822's buck converter.

I understand your argument about using the Balun chip that I did and will have that fixed. However, I am not understanding what is not appropriate about my routing. Is this because one Antenna route is going under another?

Thanks!

[AmbystomaLabs]

The routing mistake is that the balanced pair are not the same length, are not characterized and one of them routes through on two vias using a different plane. The vias alone will really mess up the impedance. It would be preferable to put the balun right next to the chip with equal length balanced pairs and then run a characterized 50ohm line to the antenna.

[joseph]

I see what you mean. I have added the changes using the recommended balun for 51822 QFAA/QFAB on the page you sent me (2450BM14E0003).

Please have a look. http://imgur.com/a/33xSx

I have also noticed that the reference design provided by Johnson has a wider route for the antenna then I do. However, I copied the meander antenna directly from Nordic's design of the Bluetooth smart beacon. Is the current width acceptable?

Thanks!

[AmbystomaLabs]

Much better! Well done. If you want you could also mirror the antenna top to bottom. (ie, rotate it about the feed point). The ground plane is a little more continuous on the top of your board.