ERROR 404.1 – Absolute value (x1 gain); -full-scale DAC output
Cannot measure 4.21V ±0.4V at A/D IN.
DAC U531 is programmed to generate -4.21 VDC at PRECOMP+. That signal is then applied to ACF through R560 and U532 (DAC line pulled low). ACF is routed to AMP IN via U526, Q516, and the AC input buffer.
AMP IN is tied to the inverting and non-inverting paths of the variable gain amplifier (VGA). NETOUT (output of U519) is routed to the Zero-Crossing Amplifier which, based on the polarity, generates the appropriate COMP- signal that is applied to com- parator U507. The comparator selects the path that the AMP IN signal will follow through the VGA by closing the appropriate analog switches of U509.
The negative (inverting) AMP IN path is through R530, U515, U509, Q501, and U516 to pin 12 of the multiplexer (U511). The output (OUT) of the multiplexer is routed through buffer U342 to ACV/A. The signal on ACV/A is switched through U320 (/AC pulled low) to the A/D buffer (U322) which is configured for x1 gain. Measure +4.21V at A/D IN.