I used another USB-Serial converter (a KeySpan brand) and it worked perfectly! There is no longer garbage. So you guys are right about the diodes thing stealing power from the RX. But what is the purpose of the inverter 74HC14? It is right between the H11A1 optocoupler and the 68HC11 MCU serial transmit and receive pin?
Now I can finally focus on the amplifier part. The old unit used AD620 while the new unit used LT1168 amplifier. I finally ran the new one and there is less noise indeed, for example, it can't cause interference from 60Hz AC line.
The old unit has amplifier input impedance of 10 Gohm and CMRR of 100 dB. While the new unit has amplifier input impedance of 1000 Gohm and 120 dB. It claims the "input noise and interference is reduced overall by a factor of more than 1000". How do you compute the 1000 factor in dB from 100 to 120 dB?
These units were made in the early 2000. The final design they offer (which I also owned) were based on the op-amp TLV2211
https://www.ti.com/lit/ds/symlink/tlv2211.pdf connected to 24 bit ADC ADS1254 with serial digital output and into the MSP430 MCU. But the spec of this final design is still 1000 Gohm and 120 dB.
Is there no op-amp with spec above 1000 Gohm and CMRR above 120 dB? Why?
Also i plan to use a test input signal that can produce 5 uV sine wave (or any microvolt signal) and inject this into the 2 input and ground of the 3 units and check the output at PC and compare the noises. What test input signal device can I use??
Thanks!