The total power consumption doing a read to Serial over USB is approximately 35ma. This is much too high. That means a coin cell (CR2032 at 225mah) setup will likely only last 5-10 hours.
Lets take a look at the breakdown.
| Current (ma) | ||
| Item | 3.3V | 2.5V |
| Bridge | 9.428571 | 7.142857 |
| Instrumentation Amp | 0.55 | 0.55 |
| ADC | 0.792 | 0.6 |
| Sum x2 | 21.54114 | 16.58571 |
| Measured | 35 | – |
| Goal (100 hours) | 2.25 | 2.25 |
I’m constantly looking at the bridge and thinking I must be calculating V/R wrong because I was expecting the bridge to be a much lower current draw. I’m debating if I could potentially run the bridge from a digital out. That way I can activate the bridge, take a sample, then power down the bridge. Essentially it would have a very very short duty cycle of maybe 5%. This would drop the bridge consumption to approximately .5ma per bridge from the current near 10. I hadn’t wanted to think about it at this point, but maybe I should consider that for V4 now.
I haven’t even looked at the microcontroller current consumption and optimization yet. I’ve been debating my choice of microcontroller, but it’s hard to know what is marketing and what is actually achievable. However I’ve been debating eliminating the regulator. Rather than using a reference voltage I could go back to my old voltage divider design and then everything is ratio metric. As the battery voltage drops so does the reference voltage. This would simplify things potentially. Switch to the Atmega 328 or the nRF51422 which is my preferred goal.
Update:
Looks like the measured does line up with consumption. I removed the bridge connections and consumption dropped to 15 – 17 ma. This is still quite high, but this is no optimization and running the ADC and Instrumentation Amp at full tilt. So the bridge is pulling similar power as everything else.