The values show: It´s not dominated by random noise, but by a single sinewave. What you see is influenced externally, that isn´t included in the TINA simulation.1.the output of OPA376 is 10.7mVrms and 34.1mVp_p.
Hi,
The values show: It´s not dominated by random noise, but by a single sinewave. What you see is influenced externally, that isn´t included in the TINA simulation.
PP-value for single pure sinewave: 10.7mV x 2 x sqrt(2) = 30.3mV
If there is a second frequency with just 5.5mV RMS you already are at the 34.1mVpp
***
I don´t understand the concept of your design at all. I see some issues:
* Why do you use the "noisy" OPA344 at the first stage. And the lownoise ones later. You need to use the lowest noise OPAMP at the first stage.
* R1 noise: I see no use of R1 = 4.7M. It just acts to decrease overall performance.: Increase in noise, decrease frequency response, increase offset.... --> short circuit it or use 100 Ohms.
* R1 high impedance: The high impedance at the non inverting input of the first OPAMP stage makes it prone to pick up any noise around. --> short circuit it or use 100 Ohms.
* C1: From the size it seems this is a ceramic capacitor. Every noise generated by this capacitor will degrade overall performance. Ceramic capacitors have piezo effect, this means they act as microphones, they pick up mechanical and acoustic noise. --> replace it with a foil capacitor. Or modify the circuit in a way that you can omit it at all.
* the same applies for C3, but has minor influence on overall performance.
* For best performance all capacitors in the signal path should be foil type.
***
* C1 noise pickup test: use a scope to measure the output signal: Then use a plastic stick (pencil) and slightly knock on the PCB with it.
I assume your extra wire to the PCB_GND has more mechanical influence (to reduce vibration) than electrical. But for a detailed analysis we need to see a photo of your thest circuit.
Klaus
if you want to use ceramic capacitors then a 3 side milling cut around the capacitor will reduce, but not avoid this "mechanically generated noise".So I have to choose small size caps. Do you have any advice?
All detailed informations about a film capacitor will fill a book.I can find less information about foil capacitors, would you like to recommend me some details about foil capacitors?
The waveform count doesn't fit, only 6 rather than 7 images attached.
What's connected in place of VG2, a ground short?
Are the OPA344 and OPA376 setups identical in each regard?
Why are you measuring input noise with 20MHz bandwidth although the used signal bandwidth is < 1 kHz?
Then the input of the Opamp is very high impedance and prone to pick up any noise around.Please see picture 8, R19 is not connected.
With your high impedance input ... any solder residuals my harm the test result regarding noise.I used one board to do the test. I just change the IC(OPA376 & OPA344) on this board, so I think the condition is same.
Hi,
in addition to what FvM wrote:
if you want to use ceramic capacitors then a 3 side milling cut around the capacitor will reduce, but not avoid this "mechanically generated noise".
Or use COG ceramics only.
All detailed informations about a film capacitor will fill a book.
Film capacitors don´t suffer from the piezoelectirc effect.
Klaus
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