Fix error 402.1 - Signal switching; frequency switch
Failure analysis - Cannot measure 0.032V ±0.005V at A/D IN.
OUT B of the DAC (U531) is set up to output 4.33V at PRECOMP+ (U528 pin 1). The
operation of the frequency switch, U522 (FREQ pulled low), is veriÞed by dividing the
PRECOMP+ voltage by the voltage ratio across R560 and R558. The ÒonÓ resistance
(approximately 25 ohm) of the analog switch (U522) is added to the resistance of R558
since it is part of the ratio. Again, as in the 400 series tests, this voltage is routed to the
A/D buffer (U322) which is conÞgured for x1 gain. Measure 32mV at A/D IN.