Need help to identify this filter

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noether

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Hello guys, i need your help to identify part of this circuit.

The first U is an instrumentation amplifier (AD620), i guees R1 R2 R3 and R4 are for reject the common mode.

The last U is a low pass filter with a gain, it is biased to +Vdd/2 because it is plugged to an ADC.

And the part i can't identify is R8 and C5, i guess it's a high pass filter, but i'm not sure, what's the cut frequency for this filter?

Thanks in advance.
 

noether said:
And the part i can't identify is R8 and C5, i guess it's a high pass filter, but i'm not sure, what's the cut frequency for this filter?
Thanks in advance.
Click to expand...

It´s a bandpass - together with C6 and R11 (and the opamp).
 

i guees R1 R2 R3 and R4 are for reject the common mode
Click to expand...
It's actually a bootstrapped bias circuit, that has a somewhat nasty behaviour regarding R1 noise current and it's second order low frequency roll off.
 

Thank you very much for your feedback.

I agree is a band-pass filter but... what kind of filter is? any suggestions or ideas?

I'm interested on find out the two cut frequencies from the components (i know they are about 0.5Hz and 100Hz).

And i have another doubt, at the second amplifier, with this configuration at the non-inverted input amplifier... am i saturating the output? i mean, i guess I'm adding VDD to the signal, am i right?

Added after 1 hours 47 minutes:

Solved... I'm so tired after a hard work day and i hadn't realized it's a simple active filter.

1/(2pi C5 R8) = 0.5 Hz

1/(2pi C6 R11) = 115 Hz


Thank you very much identifying the bootstrap... in my opinion it is pointless, am i right? since AD620 has about 10GOhms of Zin.
 

Hi NOETHER,

And i have another doubt, at the second amplifier, with this configuration at the non-inverted input amplifier... am i saturating the output? i mean, i guess I'm adding VDD to the signal, am i right?

No, you don´t saturate the output. You simply add 50% of the power supply voltage to the +input - thereby transferring this voltage also to the output since you have 100% dc feedback. This is the normal procedure for single supply operation.
 

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