Analog circuitry questions and answers

Analogue Circuitry

I need to design an active low-pass filter at my 2nd op-amp input stage (to prevent aliasing distortion), which type of active low-pass filter should I design? How many poles do I need? One more thing I want to ask about the analogue circuitry - for the 3 inputs, can I input let say -5 to 5V to the first input and the circuit will change it to 0-5V? What will happen if I input 0-10V? Also, what is the maximum AC voltage that the analogue circuitry can handle? Can I have only one input instead of 3? :idea:

Re: Analogue Circuitry

Use a butterworth lowpass in MFB configuration
(the attachment has the tool and directions to compute it)

The number of poles must be greater than:

(6.02 x (n + 1)) / (20 x log(fs / 2 x fc))

where:
n = ADC number of bits
fs = Sampling Freq
fc = Lowpass Cut Freq

See that 2nd stage output is clamped to 0 / +5V
If you apply under -2.5V or over +2.5v to the 1st input you should lower the gain to avoid saturation.

The maximun AC voltage at the first input is 5 Volt

Of course you may use only one input.

something else ?

Re: Analogue Circuitry

Regnum said:

See that 2nd stage output is clamped to 0 / +5V
If you apply under -2.5V or over +2.5v to the 1st input you should lower the gain to avoid saturation.

Click to expand...

Thanks for replying my question :wink: How to lower the gain to avoid saturation :?: What about for the 2nd and 3rd inputs, what is the maximum AC voltage?

How to make the circuit to use only one input? :idea:

Thank You!.

Re: Analogue Circuitry

km said:

How to lower the gain...

Click to expand...

These are the Voltage-Gain equations related to each input:

Av1 = R4 / R1
Av2 = R4 / R2
Av3 = R4 / R3

See that R5 moves the offset of both stages, according to the following equation:

Vo = 5 x (R4 / R5)

km said:

What about for the 2nd and 3rd inputs, what is the maximum AC voltage?

Click to expand...

Viac1 = 5V
Viac2 = 50V
Viac3 = 500V

All values are peak-to-peak.

km said:

How to make the circuit to use only one input?

Click to expand...

Just remove the resistors related to the useless inputs

Maybe I could help you better if you tell me what are you going to put at the input and what do you expect to get at the output...