Original question:
Hello
I designed sEMG PCBA with INA333 as first stage of amplifier，first time I used the same SCH as bellow，and the ouput signal is correct.
The second edition of circuit design，I delete C3 because FAE tell me that there's no need to add C3.
And then there is no signal output for the first stage of amplifier.
I couldn't find any explanation to it in datasheets or any other designs，I want to know why. Thank you.
Hi Xiaowei,
because FAE tell me that there's no need to add C3.
This is correct. You will introduce op amp instability in the INA333 stage.
what the root cause of this problem is Vcm noise of AC source (50/60Hz AC noise) on the human body？
Human body conducted electricity, and skin and body surfaces have certain capacitance. Thus the human body is able to couple in 50/60Hz Vcm noise from AC sources (overhead light, AC powerline, test equipment etc.). The level of Vcm coupling varies from person to person, as well as the climate and environmental conditions (humidity, temperature etc.).
The analog front end (AFE) of EMG is very similar to ECG/EKG or EEG, so you should treat the application's AFE interface similarly.
TI has tons of application notes, design references and video presentations in these topics, and the enclosed are some of the application notes for your references.
https://www.ti.com/video/6269632585001?keyMatch=EKG%20VIDEOS
Analog Front-End Design for ECG Systems Using Delta-Sigma ADCs (ti.com)
Thermal Noise Analysis in ECG Applications
Electrocardiogram (ECG) design resources | TI.com
https://www.ti.com/video/6269748206001?keyMatch=EKG%20VIDEOS
there is no signal output for the first stage of amplifier.
Attached is the INA333's simulation in Tina.
The root cause in your issues are likely due to the input signal connection, where the input signals need to establish input bias return path, see the section 8.2.2.5. The input impedance of the INA333 is extremely high.
BTW, you do not need TP5531-TR zero-drift op amp as the 2nd stage for the level shift. You can do all these in the 1st stage with the Vref pin. The Vref pin needs to be configured as a low impedance node, as shown in the Tina simulation (op amp's output node is a low impedance node that is able to source and sink current). Or you can place a precision reference or bandgap voltage reference at the Vref node in INA333.
If you need Zero-drift op amps, we have many low voltage precision op amps, such as OPA387, TLV387, OPA333, TLV333, OPA391, OPA187 etc.. We have wide ranges of high performance, low cost zero drift op amps to select from, see the attached link below.
If you have additional questions, please let us know.
Best,
Raymond
