High-side current sense amplifier

Projects
1

Having some fun/issues with this high side current amplifier a CS30CL.

I am using the exact layout of the datasheet:

circuit diagramme.png1152×648 6.97 KB

Rsense = 0.01 ohms
Vcc = 5V
Vsupply = 10V
Rload = 33ohms

For the amp Av = 100V/V

Issue is that I am seeing the expected voltage drop across Rsense of 2.7mV (current reading on the power supply is 0.27A)

But on the aplifier output going to the arduino I am seeing 900mV rather than the expected 270mV.

Any ideas?

2

Couple of thoughts - have you measured Vout when the pin is not connected to arduino? Also their schematic has a capacitor.

3

Thanks for the reply Calum!

I have tested both with and without the arduino and Vout is always the same.

Added a cap across the Vcc line and this made no difference (tested 0.01uF, 1uF and 10uF)

My thinking was that I may have done damage to it whilst soldering (the SOT-23 package is the smallest I have soldered to date). Soldering was done with the iron at 300 deg.C

But I also soldered two of them and they performed exactly the same so this made me think it might not be that. If they were damaged you would expect at least some variance in performance?

If there is any other info I can provide that would help let me know!

4

Assume you’ve tested the board for shorts/solder bridges etc?

Solder overheating damage seems unlikely to break both of them.

Have you tested Vout on a new chip which hasn’t got anything else connected to the pin? If the arduino was drawing too much current then that could break the chip - e.g if you’d got it as an output pin or something else strange was going. And they can break so the pin is always on - recently experienced this with another chip and it was extremely annoying.

5

Silly question but have you tried testing it with other currents? Is it linear?

6

It’s hard without seeing the circuit or code but i would check the grounds are all common (load, psu and Arduino) and that the cp chip is powered from the 5v rail on the Arduino board. All op amp voltages are relative.
Also try a different Analog pin on the Arduino just in case you cooked one

Courty

7

Thanks for the feedback everyone.

So I have replaced the breadboard I was using with perf board with everything soldered down and this has improved the situation.

It turned out that the voltage drop across the resistor was different to the one seen across the Vp and Vm pins (i.e. current sense pins). In this case it was actually amplifiyng correctly just not seeing the actual voltage drop across Rsense. This was due to a shitty breadboard.

Now it seems to be seeing the correct voltage drop across Vp and Vm but not quite amplifying it correctly (seeing 3.2mV across the Vm and Vp and 270mV across the output and gnd)

I have tested this with a couple of resistors and it seems not to be linear. I will do a full test with as many different resistors as I can muster tomorrow and share the results of this.

But we are getting closer…