beagle bone black current meter

Question

I'm going to try and make a current monitor on the beagle bone, but a single adc dosn't have enough resolution for me so I want to use 4 of the beagles adc pins with a quad opamp to amplify the voltage to separate bands and a couple of transistors to keep the max value to the beagles 1.8V adc reference voltage. the range of current i want to measure would be 0-100mA at 3.6V. I have the programming skills but not the faith in my hardware knowledge so if anyone wants to help me with tips or component lists, It would be appreciated.

michaelkellett · Answer

I've just looked at the BBB's ADC spec and it's a 12 bit converter. Unless you are willing to use 0.025% resistors to set the gain of your amplifiers (or trim them this well) just use one channel. If you set it up for 100mA full scale then the resolution will be 100/4096 = 0.024mA = 24uA. If you use a 1R current sense resistor the voltage drop will be 100mV full scale and you'll need an amplifier with offset drift of less than 10uV over temperature to get the best from the ADC. (Don't worry about the basic offset because you can trim that out in calibration.) Transistor protection doesn't work very well (either affects the gain near full scale or doesn't limit the voltage exactly). Power the amplifier from the 3.3V supply and set the amplifier gain to 33. (You are going to need an amplifier with rail to rail IO). Use a resistor attenuator from the ampliifer to the ADC so that 3.3V on the amplifier gives you 1.8V on the ADC (use 833 R from the amplifier and 1K to ground - ideal values - you can make the 833 bigger and the gain will be a little bit low). If you really want you can use two amplifiers with different gains to get two ranges but I can't see that you need 4. The amp needs to be quite good, 3.3V supply, RRIO (rail to rail input and output), low offset, low offset drift. AD8551ARZ would be a good amp to use and the data sheet (page 21) shows you how to measure low side current. It only comes in surface mount packages which may be a problem for you. Something cheap like MCP6231-E/P comes in DIL but has a huge offset(> 5mV) and not very good drift. I couldn't find a reasonable low drift part in DIL package. MK