We are seeing a failure when testing the analog-to-digital portion of an audio circuit. The circuit under test is the PCM1804 on a custom circuit card assembly. We test this circuit by introducing an analog input signal into the chain and verifying the amplitude and frequency of the analog signal out of the DAC on the oscilloscope. (See PCM1804 Test Diagram.)
When the circuit is initially tested, we get passing results (constant amplitude sine wave) if the test is completed quickly. When I prolong the test to simulate real world operation, the signal out of the DAC begins to modulate and eventually drops to almost nothing.
Output Modulation after IC warms up:
When I cool the ADC chip with a freeze-spray soaked q-tip, the modulation disappears and the output signal returns to normal. The modulation returns as the part is allowed to warm up again. (This is done at room temperature.)
Have you seen this type of behavior in the past? Can you point me toward a solution?
