OPA140: TRANSIMPEDANCE AMPLIFIER

Berke Tosun

Part Number: OPA140

Tool/software:

Hello All,

I am trying to build a transimpedance amplifier for capacitive sensor. This sensor captures 50Hz power frequency signal by capturing E field created.

that being said, the sensor´s presence did not changed the output. We think that it is because the systems reference and the electronics reference is different.

I have followed all ti recommendations and datasheets but could not figure that out. Can you recommend a schematic for us?

4 months ago

0 Raymond Zhang1 4 months ago

TI__Guru* 77221 points

Hi Berke,

Are you referring to this application note?

https://www.ti.com/lit/an/snoa927a/snoa927a.pdf?ts=1747869379419&ref_url=https%253A%252F%252Fwww.google.com%252F

https://www.ti.com/lit/ds/symlink/fdc1004.pdf?ts=1747869559827&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FFDC1004

https://e2e.ti.com/support/sensors-group/sensors/f/sensors-forum/1255443/capacitive-sensing

https://www.tij.co.jp/jp/lit/ml/slap105a/slap105a.pdf?ts=1747869800815&ref_url=https%253A%252F%252Fwww.google.com%252F

They are many different types of capacitive sensing technology out on the market. I need know what types of capacitive sensor you are interested. I attached several references above, and last one may be interest to you.

If you are looking for highly sensitive one, and analog/ADC counts sensing method may be the most sensitive one. These are done via ratiometric comparisons via ADC. If you are doing analog --> TIA method, it may not be as sensitive or fast as the digital count one.

Please let me know what you have in mind and I can assist you accordingly.

Best,

Raymond

0 Berke Tosun 4 months ago in reply to Raymond Zhang1

Prodigy 50 points

Hello Raymond

thanks for your detailed answer. Despite I am telling capacitive sensor, it is basically a plate that couples the 50 hz ac signal present in room by capacitive coupling.

the 50Hz ac line usually 230V and strap capacitance is 4-5pF and our sensor is not much than 1nF.

thanks in advance.

0 Raymond Zhang1 3 months ago in reply to Berke Tosun

TI__Guru* 77221 points

Hi Berke,

Berke Tosun said:
the 50Hz ac line usually 230V and strap capacitance is 4-5pF and our sensor is not much than 1nF.

Are you thinking to use Cdv/dt technique to implement the touch sensor? In other words, you will monitor the current difference or change due to touch effect, and determine if a key is being touched.

If you calculate the AC current, you will find the current is very small. A touch action may be generate a few pF capacitive difference and you are going to have hard time to detect the change using the technique. You have to consider the environmental factors as well, air current flow, electrical noises, humidity, environmental vibration etc.. So I am not sure that the sensing or detecting technique is feasible (though the theory is sound).

https://www.ti.com/lit/an/sboa268b/sboa268b.pdf?ts=1747869726851

If you have other questions, please let me know.

Best,

Raymond

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OPA140: TRANSIMPEDANCE AMPLIFIER