Analogue Circuitry

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km

Joined: 26 May 2004
Posts: 32

01 Jul 2004 8:40 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

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Regnum

Joined: 17 Jun 2004
Posts: 264
Helped: 12
Location: Hurlingham

01 Jul 2004 9:56 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 ? Smile

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km

Joined: 26 May 2004
Posts: 32

01 Jul 2004 14:05 Re: Analogue Circuitry

Regnum wrote:

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.

Thanks for replying my question Wink

How to lower the gain to avoid saturation Question

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!.

Regnum

Joined: 17 Jun 2004
Posts: 264
Helped: 12
Location: Hurlingham

01 Jul 2004 22:03 Re: Analogue Circuitry

km wrote:

How to lower the gain...

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 wrote:

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

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

All values are peak-to-peak.

km wrote:

How to make the circuit to use only one input?

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...

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