adcnoiseoperational-amplifiersignal
I made a circuit that modifies a signal so it can be safely read by an ADC.
I'm using an op amp to shift the signal +2.5V so I can measure AC. When I do this shift another signal appears. About 40mV amplitude, 8kHz freq. But only if the op amp is TL074 or TL084. If using LM324 I don't get that. I also don't get that signal if I'm not using the +2.5V offset.
What could be causing that?
Any voltage rail that you apply directly to the analog signal path through resistances such as your R6 will have to be dead quiet to prevent noise problems. It also is obvious that using the 5V supply to bias the signal path the way that you are will result in the signal input to the A/D having a dependency on the absolute value of this supply voltage.
I would think that you will want to rethink your design some so that the only thing directly biasing the signal path is the input voltage and the op-amp outputs. This way you remove the effect of variations of the supply voltage by a factor of the PSSR (power supply rejection ratio) of the op-amps used.
Finally I think for best bandwidth support, as you refine your technique, you will want to drive your A/D input from as low of impedance source as possible. Your current source impedance is approximately 33K || 82K. This seems rather high and may need to be significantly lower if you plan to ever be trying to multiplex several channels in sequence.
The last comment I'll make is that you should try to leverage separate AVCC and AGND pins on the MCU such that you use a separate filtered 5V and GND for the analog circuitry and connect them to the MCU GND right at the MCU.
Im curious why you are using aplification. In my opinion simple cell voltage measuerement could look just like this (i drawed it only for 3 cells):
This should give you acurracy probably below 0.5% + resistor tolerance, but you can compensate resistor tolerance in software.
This is how very acurrate cell measurement can be done
This is part of cell voltage measurement circuit from well designed 6 cell battery charger:
You can find full version of this schematic here
LM324 amplifiers were used in very good Turnigy Accucell 6 and some other RC battery chargers in cell balancer circuits.
Best Answer
The few tens of mV noise is almost surely caused by the 7660, which has an operating frequency of around 10kHz.
The PSRR of the TL084 is much worse (14dB) than the LM324.. Have a look at the actual ripple on the negative supply and related layout.