Current affairs

One of the problems that I really want to improve upon is PA current measurement. Reliably measuring up to 20A digitally in what is an electrically rather noisy environment has proved to be problematic, with a mix of common mode rejection issues and RF ingress.

I tried providing a separate 5V power line for the ACS712 current sensor and that did make an improvement, presumably tackling the common mode rejection problem. Still not really good enough though. Next I tried taking the mathematical mean of a series of measurements to try to smooth out noise on the sensor output. Rather surprisingly this didn't help noticeably at all. 

More research turned up a new device, the Texas Instruments INA253, which uses differential amplifiers rather than a hall effect device. This did make a noticeable improvement but the thing that really made the biggest difference was putting a choke/capacitor to ground RF filter very close to the microcomputer input pin. Obviously RF was still getting in there.

The TI INA253 also confers another significant advantage on the design. The differential amplifiers mean that it is possible to reference 0A to anything, including ground. That removes the need to determine the half-Vcc point, something that was a real pain with the ACS712.

First attempt at Mk II PCB circuit

So my current thinking is that I will use the INA253 and that pushes me firmly towards a new PCB, as the INA253 is a surface mount device. There are quite a few other changes that I've made by cutting tracks but there is no way I can do that to bring an SMD into the existing PCB.

Whilst I am at it I will also introduce a potential divider circuit into the temperature sensor input line because the 3.3V maximum input to the microcomputer limits the temperature range to 60°C.

I've made a first stab at the new circuit diagram and will do the PCB layout once I am as sure as I can be that it's correct and as good as I can make it for this iteration. This is normal - the first PCB shows up all the design faults and the second attempt is much better. Occasionally it takes three attempts!