I'm looking to build up a few new test fixtures for some of my AVR products. These products are all battery based, so keeping the current consumption down, especially in Sleep mode, is important. We've found over the years that measuring the system current is a good way to find production problems, such as flux trapped under a IC etc.
In an idea world I want to have a current (I) data acquisition system with the following specifications:
* Current is measured on the high side.
* Works with source voltages as high as 32 volts.
* Has continuous current scale of 1nA to 1A, with a low source impedance (less than 10 Ohms).
* Cost less than $50 per unit to build (Please don't recommend high end meters the boss will never spend money on).
I'll settle for 1uA to 500 mA, in multiple scales, as long as the scale switching is automatic, at 12V.
The reason I need a low impedance is that my products are part of a sensor network, which transmits data using RF. The RF section wakes up at random intervals around thirty seconds or so. The time is deliberately random to avoid RF packet collisions. You never know when the current meter is about to have its needle wrapped around the end stop with a nice satisfying "Thunk".
If I try to measure low currents with say a 10k or 100k resistor I get my current measurement, but when the transmit comes on the system crashes because it does not have enough current to sustain it. I want to be able to run through a full sleep->transmit->sleep cycle without crashing due to current starvation, and track the current throughout the full cycle.
I've found various ideas on Internet such as:
Measuring nanoamperes; Measuring low currents can be tricky. Clever analog-design techniques and the right parts and equipment can help.
By Paul Rako, Technical Editor -- EDN, 4/26/2007
10nA to 10mA using a LogAmp:
TI has their LOG10x series of LogAmps as well.
Once we get above 10mA things are fairly simple, lots of ways to do that. The fundamental problem is the micro-amp measurements, while maintaining a low impedance.
What I want to know is that any of you have been down this road before, and what suggestions you might have?