NRF PPK2 measures high current when connected with common ground

[emb_dev]

We just received a PPK2 and have used it in several configurations to test a custom nrf52832 board that runs on a 3v lithium coin cell.

When connected in the following manner, the device measures values in amp meter mode that are similar to our averaging multimeter:

Device +   ============ [ PPK2 VOUT  |  PPK2 VIN ]  =========== ( Battery +  |   Battery - ) =============  Device -

And we are getting the curves that seem to match what we expect our device to be doing at the times during which they appear. There is no common ground connection between the PPK2 and the DUT.

This was fine, but then we we tried to remove the battery and power the DUT with the PPK2 as a source, with the DUT Vbatt and GND connected to the PPK2 VOUT and GND respectively, we got some strange readings. When the power was enabled to the DUT, we got very large spikes that are way above what we expect from our device.

The same thing happens whenever we are running off the battery and try to connect a common gnd and vcc to use the digital pins available on the PPK2.

Is there something wrong with the way we are connecting the device, or is there something else we are missing that could be causing this?

[Stian Røed Hafskjold]

Hi, are you able to zoom in to one of the spikes to see how it looks, and measure the period between the spikes? I'm thinking it could either be the regulator in burst mode (refresh mode), which would have a spike period of about 30-150 ms, depending on the current draw, or it could be because of automatic range switching in the PPK2.

If it is the regulator in burst mode, the spikes will only be limited by the series resistance in the battery and the capacitance on VDD, forming an RC filter, even though 20 mA sounds a bit high to me. If it is noise from the PPK2 range switching I would expect to see something else after the spike, e.g. an advertising event, CPU wakeup, etc.

It's also interesting to measure the floor current between the spikes, and how much the spikes add to the average current, in order to know if these are burst mode spikes or not.

 

The same thing happens whenever we are running off the battery and try to connect a common gnd and vcc to use the digital pins available on the PPK2.

 I'm thinking that this is the correct reading, and the reading without the common GND is not correct. At least I've never been able to successfully measure with the PPK2 in ampere meter mode without having a common GND.

[emb_dev]

ppkFiles.zip

Hi Stian, I have attached both ppk trace files so you can have a look as I think that will be much more efficient. I hope that provides you with the information you are looking for. 

We are fairly confident in the non-gnd ampere meter readings as they are within 2-3uA of what our Agilent U1272A dmm (and also more-or less match what an oscilloscope with a uCurrent Gold current measurement setup is measuring) 

We have also tried running the device with the PPK2 as a source without any battery in an attempt to simplify the setup and see the same spikes.

[hmolesworth]

I concur with Stian; the common ground is required as the measurement is (Vin-Vout)/Rc where Rc is the switchable current sense resistor. Without a common ground there must be some other leakage return path, perhaps an i/o line connected to PPK2 or laptop (and hence USB Gnd path).

Perhaps repeat the test with absolutely no connections to the custom board except Vout and Gnd - no serial, no J-Link, no digital i/o. Also note that though not shown in the measurement the internal Vout and Vin sense resistors place a load on the CR2032 of 44 uA (75k and 3.3 volts) in operation so best not to leave connected all week when not testing.

// Resistor Rc for current sense is 1k0 in parallel with onr or more of 110R, 11R0, 1R0 and 0R051
//
//              <- +ve Supply to DUT From VOUT                               External Battery +ve To VIN
//             +------------------------------+                             +-----------------------+
//             |                              |                             |                       |
//             |              +---------+     |                             |     +---------+       |
//             |              |     GND |     | VOUT                   VIN  |     | GND -ve |       |
//   +=========O=========+    |    +====O=====O=============================O=====O====+    |       | Battery
//   |        Vdd (+)    |    |    |    |     #----+                        |     |    |    |       | + 3 volts
//   |                   |    |    |    |     |    |Rc        AMPERE \      |     |    |    |     +-O-+
//   |                   |    |    |    |     |   +++                 \     |     |    |    |     |   |
//   |                   |    |    |    |     |   | |   +-----------O  O----#     |    |    |     |   |
//   | Device Under Test |    |    |    |     |   | |   |                   |     |    |    |     |   |
//   |       DUT         |    |    |    | 120k|   | |   |                   |120k |    |    |     |   |
//   |                   |    |    |    |    +++  +++   |   \  SOURCE      +++    |    |    |     |   |
//   |                   |    |    |    |    | |   |    |    \             | |    |    |    |     +-O-+
//   |                   |    |    |    |    | |   +----#--O  O-- PPK LDO  | |    |    |    |       | Gnd (-)
//   |                   |    |    |    |    | |                           | |    |    |    +-------+
//   |                   |    |    |    |    +++                           +++    |    |
//   |                   |    |    |    |     |                             |     |    |
//   |                   |    |    |    |     #-- VDUT                VIN --#     |    |
//   |                   |    |    |    |     |  Sense                Sense |     |    |
//   |                   |    |    |    |    +++                           +++    |    |
//   |                   |    |    |    |    | |                           | |    |    |
//   |                   |    |    |    |    | |                           | |    |    |
//   |                   |    |    |    |    | |                           | |    |    |
//   |                   |    |    |    |    +++                           +++    |    |
//   |                   |    |    |    |     | 30k                     30k |     |    |
//   |                   |    |    |    +--#--+                             +--#--+    |
//   |                   |    |    |       |                                   |       |
//   |                   |    |    |     =====      <<== Common Gnd ==>>     =====     |
//   |                   |    |    |      ===           (Common -ve)          ===      |
//   |        Gnd (-)    |    |    |       =                                   =       |
//   +=========O=========+    |    +===================================================+
//             |              |     PPK-II
//             +--------------+

What sample rate is the Agilent DMM set to? "Fast" is required.

Edit: Re-reading your post I see you already have a "Common Gnd":

Device +   ==== [ PPK2 VOUT  |  PPK2 VIN ]  ==== ( Battery +  |   Battery - ) ===  Device -

In the PPK2 context, "Device -" is what is refered to as a "Common Gnd". There is no need for (say) an earth or other Gnd without - as you say - introducing errors.

[emb_dev]

Thank you for your reply and edit! 

I had another look at just running the DUT with the PPK2 as source and get some interesting results, that I think might be plausible.

The setup looks like this:

Device -   ======== [ PPK2 GND ]

Device +  ======== [ PPK2 VOUT ]

The peaks coincide with advertising events every 1s, along with ibeacon packets every 3s.

The thing that bothers me is the current seems much higher in these readings than when we are running with the 

emb_dev said:

Device +   ============ [ PPK2 VOUT  |  PPK2 VIN ]  =========== ( Battery +  |   Battery - ) =============  Device -

setup. In source mode the peaks are around 15mA and the idle current is around 60uA, but with the battery / ampere meter setup the peaks are 5mA and the idle current is around 20uA. Our device is transmitting at -16dbm, so I would not expect the peaks to be more than 5mA on an NRF52832 chip.

The ppk file for the Source readings pictured above is here: sourceMode.zip

[hmolesworth]

With Device -ve and battery -ve not connected to the common Gnd there will be a Gnd shift (aka ground bounce) which can be seen as the CR-time constant decay (exponential decay) of the current measurement in the first screenshot. I'll look at the traces you sent in more detail later (busy now), but you might see this more clearly by measuring the voltage bounce between Device -ve and PPK2 Gnd on the 'scope or voltage on the Agilent. Since the PPK2 is earth-Gnd-connected via USB (and the 'scope is also Gnd-earth connected), the 'scope Gnd must be on the PPK2 Gnd and the probe on Device -ve.