Author Topic: Understanding the Schematic Better  (Read 321 times)

chagai

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Understanding the Schematic Better
« on: February 05, 2020, 12:20:08 PM »
Hi,
I have purchased a Ranger and I can't wait for it to arrive, and to start working with it. It seems like a great product!
However, I have a couple of questions and topics I'd like to understand, to see if I understand the operation of the device:

1) Is there a maximum voltage for the DUT (i.e. SOURCE & LOAD terminals)?
    - I would like to test a Product that is powered from 8.4V (two Li-Po bat-cells in series)

2) It seems that all the Transistors in the analog part are p-channel - is that true?

3) I believe I do understand the power and sensing current paths of all the currents ranges (nA, uA, mA), but   and I don't understand the NMA signal (Not mili-Amp??). It seems that 'D38_ATN_MA can control all the transistors, correct? Is 'NMA' an input to the micro-controller?

4) Is there a reason to use split (two resistors) in the amplifiers' gain resistors? i.e. 6.8K+2.2K instead of the a simple 9K 0.05% resistor?

Thanks in advance!

Chagai

Felix

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Re: Understanding the Schematic Better
« Reply #1 on: February 05, 2020, 04:43:55 PM »
Hi Chagai,

1) I believe it was asked/answered before - the CR is intended for testing currents in low power electronics. Since it is simply a high side difference sensing meter, you could use it at high voltages but that defeats the "mission/purpose" of this being a ultra low power DUT measurement device (can we have ultra low power high power devices? sounds like a oxymoron).
Hence I recommend to only use it in DUTs powered from below 12V and which drain no more than 3.2A (~overload limit). Even at these values I don't consider a DUT a low power device, you are better off using a DMM with a big fuse for such measurements. Either way care must be taken the more extreme these I/V values are because of the risk of introducing current loops when the CR is not completely floating WRT to earth ground. This is explained in the guide safe use section.

2) No, not all transistors are P-ch, look more closely.

3) The NMA is a special signal required to bias the range states.

4) Because it's impossible to find 9K 0.05% last time I checked.

chagai

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Re: Understanding the Schematic Better
« Reply #2 on: February 09, 2020, 10:08:16 AM »
Hi Felix,
Thanks for the quick reply!

I am mainly interested in sleep and (BLE) advertising currents (uA and mA), so I think it will be a good device for my needs.
I have looked again.. still most transistors symbols look identical (to me)..but I will start working with the device w/o understanding every bit.

One, additional question.. if i'd like to log the current measurements over a long period (hours or even days), you would suggest to use the BT option which should be less noisy than connecting the  serial-USB output to a laptop, correct?

BR,
Chagai

Felix

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Re: Understanding the Schematic Better
« Reply #3 on: February 10, 2020, 04:42:23 PM »
chagai,
FWIW perhaps there are only slight differences in transistor symbols, but there are indeed different types.
It's up to you if you'd like the use BT. It's possible USB noise can couple in but you will have to check yourself with a scope before you determine if that is the case - but ensure the USB is isolated if you plan to also touch the CR with a mains earth connected scope.

If your DUT is battery powered, then its possible to attach the USB directly, as long as nothing else from the same earth ground is connected to the CR.
But I will always recommend using a USB isolator to ensure there is no galvanic connection between the CR and common earth ground.

I will add that the BT detection and logging is automatic, while USB logging will require some code changes to redirect the logging to USB.
However default rate for BT logging is only every 200ms which in some cases might be too slow if the changes are fast, so this will also require a code change.
Maybe in a future revision there will be a OLED/serial menu where this can be changed without changing the firmware.