OPA145: OPA145 behavior

Martin Walchshofer

Part Number: OPA145

Hello,

i got the OPA145 recommended from the TI team for biosignal processing. We used the amplifier as a voltage follower. we supplied the part with +6.6V and -3.3V (originally we wanted to use +-3.3V but then we found this forum entry https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/927000/opa145-opa145-spice-model ;so we added a voltag doubler to the 3.3V).

we simulated the circuit in LTSpice with an input signal of +-10µV (blue). The output signal appeared to be +-10µV with a little offset.

we estimated that a little noise in the nV range will be added.

in reality we received a lot of noice in the µV range.

the circuit should be the same as in the simulation. do you have any idea why we measure a decent amount of noice using the OPA145.

best,
Martin

over 1 year ago

+1 kai klaas69 over 1 year ago

Guru 140200 points

Hi Marvin,

I think the noise is coming from the voltage doubler. Charge pumps are producing lots of noise.

I would check the circuit with two 9V batteries instead of your -3.3V and +6.6V supplies.

Kai

0 Thomas Kuehl over 1 year ago in reply to kai klaas69

TI__Guru* 93105 points

Hi Marvin,

I agree with Kai's assessment. Batteries would be best to test the circuit for noise, but if you have quiet +6.6 V/-3.3 V linear supplies they may be sufficient.

Additionally, be careful that your circuit isn't picking up induced noise from the environment around it.

Regards, Thomas

Precision Amplifiers Applications Engineering

0 Martin Walchshofer over 1 year ago in reply to Thomas Kuehl

Prodigy 10 points

i guess that is the problem. i've powered the circuit with a 9V battery and a voltage devider. The signal looked fine.
i added a big capacitor 22µF between 6.6V and GND to filter some of the noise and the signal already looked a little better. so i think i'l have to improve the charge pump circuit to introduce less noice.