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]

You could keep the matching network, but at 2.4GHz any of the trace lengths and widths will need to be precisely the same as the reference design. Differences as little as a 1mm can change the effect of the components. Also your board will likely not be made up of material with the same relative dielectric constant. Meandering lines are normally close to 50ohm by design with only a simple match. If I were choosing, I would do a chip balun to the meander and lose the complicated match. Just keep the shunt cap for the antenna.

I still think you should re-consider running the device at 1.8vdc. Unless you go through a lot of effort to make sure the ripple is low from your buck converter for the LiPo all that ripple will propagate through to the nRF. This could cause problems by modulating the RF envelope or even causing a local brown out condition causing a reset.

[AmbystomaLabs]

Also I should point out that by using 1.8volts for VDD you will only be able to communicate to other devices running at 1.8volts or lower. The gpio on the nRF aren't just open collector, they are all push pull. Even as an input you can't pull a gpio higher than VDD (1.8v) or the fet will reverse bias and short the VDD rail through the gpio to the device it is talking to. Also since the max signal is only 1.8v, a 3 volt device likely won't see this as a high state. Most spec 0.7VDD or for 3volts is 2.1v minimum for a high state.

If you run the nRF at 3v or 3.3v and let the on board dc/dc do it's job all these potential problems go away.

[AmbystomaLabs]

By the way here is a link to the Johanson page of solutions for Nordic devices: www.johansontechnology.com/nordic

[joseph]

Hi AmbystomaLabs - I really appreciate the great comments that you have been able to provide.

I have looked at the schematic difference between the 1.8V schematic and the original schematic (the one that I am using right now) and it seems that the only difference is a single VCC line that runs to another pin on the NRF51822 which seems like an easy enough change. I read a bit about running NRF51 series from a 3.7V lipo and it seems that the Torex XC9265 PFM DC/DC switcher was recommended which I will probably end up using (ref: devzone.nordicsemi.com/.../). It seems inefficient having 2 dc/dc converters - one for the 2.1-3.6 range provided by the design ref and one for getting the Lipo within that range.

Here is a link to the latest design with balun BAL-NRF02D3 taken from Nordic's Beacon. http://imgur.com/a/5HtmY

Thanks!

[AmbystomaLabs]

I am not sure which extra converter you are referring to. The 51822 has an on chip buck converter that only requires an external cap and inductor. The only thing you need to do is supply the 51822 with 3.0 -3.3 and let the the 51822 do it's job. This will give you gpio at the higher vdd (3.0-3.3) which will likely help out with gpio communications with other devices. The soft device automatically controls the on chip converter and only brings it up when necessary. You could probably even make a case for running it at 2.7v. This would give you a little more operating range on the lithium battery. You just need to run all your peripherals at that same voltage what ever you choose. In case I didn't make that clear earlier, if you choose 1.8 you need to run all your peripherals at 1.8, similarly if you choose 3.0 you need to run all the peripherals at 3.0.