Hi.
Is there any general rule to find suitable shunt value for new boards?
Maybe there are patterns related to SoC’s power supply, incoming clock value, etc…
How to understand that, say, 10 Ohm value is better than 100 Ohm value?
Same question is for frontend’s variable gain amplifier. How to find right gain value?
Typically you want to use the largest shunt value that your target can tolerate, since that should give you the best SNR. I’d recommend just choosing a value that we use on similar targets. A Cortex M micro should be similar to the STM32F3, for example, but you may have to play around with the value a little if the voltage drop ends up affecting parts of the chip that you care about.
For amplification, you may need to do measurements with an oscilloscope, for example, or you could select something similar to the ChipWhisperer.
It will be Cortex A35 core. As far as I know, there are no such CW308 based targets.
What are the signs the target still tolerates chosen shunt value? Is it just stable/unstable running of the SoC?
Thanks.
When selecting shunt values and gain values for new boards, it’s essential to consider factors like current measurement range and signal-to-noise ratio. For shunt values, lower values are suitable for higher current ranges, while for gain values, finding a balance between signal amplification and noise suppression is key. Experimentation and reference to datasheets can help determine optimal values.
Hope this helps!
I guess the Ohm law is applicable here.
Operating conditions of the SoC are 0.7V - 1V
Assuming, the SoC can supply up to 500 mA, I need to take shunt 0.2 Ohm.
In this case, the voltage drop will be 0.1V which fits to the operating conditions.
Does this approach work?
Yeah, that seems reasonable. I’d say you could push it a little higher than that since you won’t be drawing max current in most situations and those limits tend to be pretty conservative, but it’s up to you how much time you want to spend fiddling with different shunt values.