Johnny_YU
Member level 2
Hi, all
I use OPA376 and OPA344 to build a two stage high-pass non-inverting amplifier circuit, and a 4th low-pass filter.
I made a two layers PCB. All the signals and Powers is on the top layer, the bottom layer is a ground plane.
In this PCB Layout, the purple trace(20 mils) is power line. Two stage high-pass non-inverting amplifier circuit is located at the top side of the board. The 4th low-pass filter is located at the right of the board.
Something I should mention is that: there is no ground plane on the top layer.
Problem 1
I use TINA to simulate the total noise output of the circuit.
The result shows:
1.the total noise of OPA376 at 20MEG-Hz is 296uVrms.
2.the total noise of OPA344 at 20MEG-Hz is 458uVrms.
Then, I use the oscilloscope(Rigol DS1054) to measure the circuit (AC test, Bandwidth is 0~20MEG Hz, measure the first stage high-pass non-inverting amplifier's output).
The result shows:
1.the output of OPA376 is 10.7mVrms and 34.1mVp_p.
2.the output of OPA344 is 1.08mVrms and 3.92mVp_p.
I think that the difference between simulation and measurement maybe caused by the Test equipment and the environment.
But Why the output of OPA376 is much larger than OPA344's in the test? It doesn't make sense...
Problem 2
a.
I found that when I put my finger close to the first stage high-pass non-inverting amplifier, the 50Hz noise on my output of the first stage increase significantly.
I guess that phenomenon is caused by R1(4.7M ohm), so I used a 10 ohm resistor instead of 4.7M for R1. Then I tested it again, I can rarely see the 50Hz noise.
b.
I have did second test(R1 = 4.7M ohm): I connect a wire(0.5mm^2) to my PCB ground. When I held the copper of the wire, the 50Hz Noise reduced significantly.
Does it mean that my PCB ground connect to the earth ground through my body, and 50Hz noise bypassed?
In my thought: 50Hz noise comes from the space, and this 50Hz noise induce current or voltage on R1.
If PCB ground does not connect to the earth ground through my body, then the induced current or voltage will go into OPA.
Finally the 50Hz noise will return to the earth ground through the capacitor between PCB and the earth ground. Am I right?
If PCB ground connectto the earth ground through my body, then a small part of the induced current and voltage will go into OPA, other part will return to the earth ground through my body directly. Am I right?
But how does the induce current or voltage on R1 go to my PCB ground directly, not through OPA? Where is the return path?
How can I explain this phenomenon systemly and clearly?
How to use some model to explain this Phenomenon?
How to find the return path of the 50Hz Noise?
To be honest, I am puzzled.
c.
I have did third test(R1 = 4.7MEG ohm): I touch the bottom layer which is under the first stage high-pass non-inverting amplifier, 50Hz noise does not increase.
Does it mean that I should use some conductor to enclose the sensitive circuit?
I use OPA376 and OPA344 to build a two stage high-pass non-inverting amplifier circuit, and a 4th low-pass filter.
I made a two layers PCB. All the signals and Powers is on the top layer, the bottom layer is a ground plane.
In this PCB Layout, the purple trace(20 mils) is power line. Two stage high-pass non-inverting amplifier circuit is located at the top side of the board. The 4th low-pass filter is located at the right of the board.
Something I should mention is that: there is no ground plane on the top layer.
Problem 1
I use TINA to simulate the total noise output of the circuit.
The result shows:
1.the total noise of OPA376 at 20MEG-Hz is 296uVrms.
2.the total noise of OPA344 at 20MEG-Hz is 458uVrms.
Then, I use the oscilloscope(Rigol DS1054) to measure the circuit (AC test, Bandwidth is 0~20MEG Hz, measure the first stage high-pass non-inverting amplifier's output).
The result shows:
1.the output of OPA376 is 10.7mVrms and 34.1mVp_p.
2.the output of OPA344 is 1.08mVrms and 3.92mVp_p.
I think that the difference between simulation and measurement maybe caused by the Test equipment and the environment.
But Why the output of OPA376 is much larger than OPA344's in the test? It doesn't make sense...
Problem 2
a.
I found that when I put my finger close to the first stage high-pass non-inverting amplifier, the 50Hz noise on my output of the first stage increase significantly.
I guess that phenomenon is caused by R1(4.7M ohm), so I used a 10 ohm resistor instead of 4.7M for R1. Then I tested it again, I can rarely see the 50Hz noise.
b.
I have did second test(R1 = 4.7M ohm): I connect a wire(0.5mm^2) to my PCB ground. When I held the copper of the wire, the 50Hz Noise reduced significantly.
Does it mean that my PCB ground connect to the earth ground through my body, and 50Hz noise bypassed?
In my thought: 50Hz noise comes from the space, and this 50Hz noise induce current or voltage on R1.
If PCB ground does not connect to the earth ground through my body, then the induced current or voltage will go into OPA.
Finally the 50Hz noise will return to the earth ground through the capacitor between PCB and the earth ground. Am I right?
If PCB ground connectto the earth ground through my body, then a small part of the induced current and voltage will go into OPA, other part will return to the earth ground through my body directly. Am I right?
But how does the induce current or voltage on R1 go to my PCB ground directly, not through OPA? Where is the return path?
How can I explain this phenomenon systemly and clearly?
How to use some model to explain this Phenomenon?
How to find the return path of the 50Hz Noise?
To be honest, I am puzzled.
c.
I have did third test(R1 = 4.7MEG ohm): I touch the bottom layer which is under the first stage high-pass non-inverting amplifier, 50Hz noise does not increase.
Does it mean that I should use some conductor to enclose the sensitive circuit?