Hey everyone, I’m working on debugging my boards I’ve been posting about over the past couple months. The area I’m focused on now is the instrumentation amplifiers that are doing high-side current sense amplification–I have 10mΩ high side resistors on a 5V supply getting amplified 100x to give me 1V/A, which is easy to read by an ADC. I’m seeing a lot of noise from my ADC readings, and so I’m investigating the amplifier’s behavior.
While it’s easy to use the AD2’s scope to look at the amplifier’s output, I also want to look at the input. The 5V supply reads around 5.1V on this particular board, and the sense resistor is located on the high side. At a high level, what I think I should do is connect both scope probes to the board’s GND, and then connect one probe to each side of the sense resistor with equal length bodged-on leads. Then, in Waveforms, I can look at CH1-CH2 to see the pre-amplified signal. I’d like to use high gain mode, but I’m not sure whether it’s safe to do so–depending on how I read the AD2 reference manual, I believe one of the following is true:
- I must use low gain mode, because the 5V supply is outside of the +/- 2.5V range and I’ll break the scope if I use high gain mode
- I can use high gain mode, and worst-case I’ll just saturate the scope, which I’ll be able to see and adapt to
- I can use high gain mode without a problem, because something something differential mode allows for +25/-10V, and I’m doing a differential capture
I’m hoping someone can clarify to me how best to approach scoping the high-side current sense (or that you can tell me this isn’t possible, and I should trust the amplifier & just scope its output).
The point of this exercise is to determine whether the amplifier (INA381A3, 150kHz bandwidth, 2V/us slew rate) would benefit from an RC network between Vout and the ADC. I’m measuring PWMed LED strip power consumption, and I want to filter out the PWM and just see the current consumption over a 10-100ms period.
Thanks for your advice!