Hi,
Out of a normal battery supply (~3.8V nom.), I want to produce a voltage of 70mV. Since those 70mV are intended as a reference voltage for an inverting-OpAmp amplifier, a normal voltage divider with two resistors alone wouldn't do the job and a buffer was needed. As a buffer a choose the OPA347. My target is to have those 70mV as "clean" as possible, without any noise. From the internet, I also got some "inspirations" about additing some capacitors as noise-filters (see attached circuit diagram).
To check the noise generate by this circuit, first I connected the battery which drives this circuit directly to my oscilloscope (Rigol 1102E). The measured Vpp was 4.8mV (which is more or less the noise of the oscillsocope itself).
Next, I measured the desired output-voltage of my circuit Vap. Now Vpp is 6.4mV. My assumption is the, circuit itself generators a noise of Vpp=1.6mV (=6.4mV-4.8mV).
I am wondering: what can I do, to further reduce the noise. The thermal noise from the resistors must be already pretty low in the 10-6V range. Same for the OPA347's input noise. One reason, why I choose OPA347 was also it's small Bandwidth of 350kHz (I assume the larger the BWP, the more noise in higher frequency areas I wlil get).