I'm designing a dsp board and like to have a switchable gain output, schematic for two outputs in the link down there:
http://www.audiowerk.at/downloads/temp/eev/dsp_output_stage/Schematic_ADAU1701_4x4_2020-06-06_09-39-04.pdfVOUT0 and VOUT1 are voltage outputs of an DAC, 0 - 3,3V, zero line is on 1,65V.
MPXo1_0 and MPXo1_1 are coming from an 74HC595 shift register that is controlled by an mcu.
Now the problem - maybe I oversee something (just mention channel 1, other have the same problem):
- feeding the system with a 1kHz sinewave
- CHo1_0 views the sinewave at it should
- CHo1_-10, CHo1_-20 and CHo1_+10 are either clamped on positive or negative rail dc, often fragments of the sinewave are visible, but mainly there is this huge dc offset.
- If i short CHo1_0 to ground and open it after that, everything works as aspected.
- changing the value of R80 to a smaller value like 200 ohms doesnt change anything.
If I remove the 74HCT4052D (U5) the offset is gone, lines CHo1_-10, CHo1_-20 and CHo1_+10 are zero volts.
Why does the 4052 pull the line to + or - 12V?
I know that it is not ideal drive the OPs with 12V and the 4052 with 5V but it should do right if the signal isnt greather than 5V.
Thanks for any hint or solution!