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Saturday, 20 April 2013

About Debugging Guidelines

In debugging – sometimes I got carried away with my “instinct”, although mostly it get me to the right conclusion in a short time – it is definitely too much of “black magic” to the newbies.
So my humble advice to the beginners is to think about every circuit or IC as a functional block – define the boundary yourself or from a schematic – make sense of the circuit does. To state the obvious, in a typical circuit block:
  • will have a function name in the schematic - this contains important info regarding what the block supposed to do
  • will generally has input, output, supply and control signals
identify which is which will generally give you a sense of how to debug the block
Once you know that the input/output relationship does not hold (assuming that you understand what the block supposed to do), here are some of the steps that might helps
  1. Bias the circuit so that the block will have known state (or consider overwrite the input from a known source), use DMM to confirm all the DC pins, starting with supplies pins.
  2. Confirm the input output relationship – if it is not correct, break this block into smaller block and repeat the process (take a look at binary search debugging post:
  3. If the block is already an IC, check for part number, orientation, cold solder join, missing solder joints before “accusing” it as faulty part.
  4. remember that sometimes certain failure mode needs more than a DMM to be used (see , so if DMM cannot give you convergence, you should consider to use another instrument (depending on what this block supposed to do).
so good luck with your troubleshooting!

Saturday, 6 April 2013

Thinking In Terms of Current– Photodiode Circuit Example

From the last post ( that talk about the importance of being able to think in term of current, now let’s look at particular example of circuit that basically manipulate current to get the desired result.

Shown below is a circuit that convert infrared light level into a voltage. Now, the photodiode convert infrared light density into current. This circuit is to provide a 0V bias across the diode (operate the diode in Photovoltaic mode) and convert the current into a low impedance voltage level for further operation.

In this circuit, the diode being used in Photovoltaic mode, and details about the diode operation can be found from as shown below:

This is just one of many examples of the advantage being to think comfortably in term of current. Try to make sense of this circuit only thinking in voltage term will be a nightmare, since the inverting input of the op-amp supposed to be a “virtual ground”, and 0V across the diode does not make much sense.