ADC and general peer review of idea

Building a power monitor for my car.

The following is a "probe" for the power monitor (which will talk to this probe via I2C). I'll build a few of these, one to monitor the feed into the audio amps (0 - 600A) another to measure the batteries (-50 to 600A) and one more for the alternator feed (0A - 400A). From this I'll be able to graph and analyse the power-performance of my (significant) car audio install, the charge rate of the batteries and other interesting stuff like RPM to Alternator output performance. The below (as a start/template for the rest) will be for the amp power feed (0 - 600A)



Theory:

• Tolerance: Assume all resistors are 1%, all caps are 10% and everything else is of standard/popular values.
• [POWERIN]/[POWEROUT] is the zero-gauge wire to the audio amps running through a current transducer (LEM HTFS series).
  • [OUT] is 50% of Vcc at 0A, 100% Vcc at +600A and 0% Vcc at -600A
  • [REF] is 50% of Vcc (reference for 0A)
  • [OUT] is fed into ACD's CH2, then grounded via a 5K resistor. (V=IR, I=V/R, I=5/5K, I=1mA drain, P=VI, P=5mW drain)
• [+12v IN] is a direct feed from the car's +12v (which will be more like 12.6V when engine off, and 14.8v when engine is running)
  • This feeds a low noise DC-DC switching reg (that, in turn, powers the ADC and the current transducer via [+5V1A])
  • Power draw is likely < 50mA for the circuit, it's overkill but meh - room for expansion.
  • The [+12v IN] also goes to ACD CH1 via a 33% voltage divider (33% of 15V = 5V = ADC's max input voltage) and smoothed a little more via a 100nF cap.
• The ADC is capable of 200kS/s
• Ignore the +5v feed into the I2C connector - superfluous, won't be used.

Questions:

• With 12bit accuracy - do I need better smoothing of the analogue inputs?
  • Is that 100nF on CH1 sufficient or should I be looking at an RLF filter?
  • But at what point of smoothing do I lose the degree of accuracy I'm looking for calculating power draw of the amps - it would be nice to sample all this at 10kS/s and graph it out.
• Do you see any red flags? Comments? Advice?

I appreciate the input as I don't have much room to play with all this (or breadboard the circuit with such small SSOP packages) so I usually just order a few pcb prototypes which isn't cheap (isn't that expensive either) but the point is, I'd like to get it peer reviewed before I go and build it.

Thanks in advance.
-KB

I think you'll need to add transient suppression components to cope with massive load-dump spikes on the supply. All the components should be automotive-rated. The automotive environment is really harsh.
I hope your insurer is happy with the car mods? UK insurers probably wouldn't be.

Litch said:

Questions:

• With 12bit accuracy - do I need better smoothing of the analogue inputs?
  • Is that 100nF on CH1 sufficient or should I be looking at an RLF filter?
  • But at what point of smoothing do I lose the degree of accuracy I'm looking for calculating power draw of the amps - it would be nice to sample all this at 10kS/s and graph it out.
• Do you see any red flags? Comments? Advice?

Click to expand...

alec_t's "massive load-dump spikes" suppression and component ratings suggestions are reasonable.
The 220μF cap is reversed. Other than that, it looks OK to me.

Litch said:

From this I'll be able to graph and analyse the power-performance ...

Click to expand...

Just out of curiosity, how do you intend to determine "performance" (I envision a "number of doors blown off" graphic display...)?.

Deleted (old thread, no point re-opening)