5mhz differential low pass filter (passive)

I'm trying to build a passive low pass filter (fo=5Mhz) with R/C values on the output of a differential buffer..

------(differential+)-------RESISTOR------------------------(differential+ receiver)
|
CAPACITOR
|
------(differential-)--------RESISTOR------------------------(differential- receiver)

(picture didnt turn out correct, but the capacitor is parallel to the right of the resistors.)

the cap (C) is parallel to the 2 resistors (R).

How would I design this? I know its 1/(2(pi)RC = 5Mhz.

but do I sum the two resistors to get R? Or do you use just the value of one of the resistors since both have the same value. Do you use the exact value for the cap, or do you have to do a divide by 2? just need a help.

thanks.

For the differential transfer function, you need to add both resistances and keep the cap as it is.

So if say both resistors are 50 Ohms and the Cap is 20pF then


Rtotal = 1/(1/50 + 1/50)


then fo = 1/(2*pi*Rtotal*20e-12) ?

Thanks.

You need to use the total resistance for the calculation. See attached.

Keith.

Sorry, I should have used your values.

Keith.

sweethomela8 said:

So if say both resistors are 50 Ohms and the Cap is 20pF then


Rtotal = 1/(1/50 + 1/50)


then fo = 1/(2*pi*Rtotal*20e-12) ?

Thanks.

Click to expand...

Rtotal=50+50
Ctotal=20pF

Time constant = Rtotal*C

Thanks guys.

Looks like if I want to make a 5Mhz passive LPF (-3db point @ 5Mhz),

The R = 50 ohm
and C =approx 320 pF

1/(2*pi*100*320e-12) ?

The calculation is correct, but only on condition, that
- the source impedance is considerably lower than 100 ohm differentially
- the receiver impedance is considerably higher than 100 ohm

Most likely, one or both prerequisites are not met.

I read the datasheet for the ADC im using (the receiver impedance = 10kOhm)
the driver im using (differential driver, gives output current as 95mA)

My R total assuming each resistor is 50 Ohms = 100Ohms,

I'm trying to calculate the source resistance.

If my differential driver is swinging between 1.25 and 0.75 V, (common mode of 1 V), do I calculate the resistance by (R=V/I)....using V as 1.25 V? 0.5 V (1.25-0.75)?, or 1 V (differential p-p (1.25V-0.75V -> 0.75V-1.25V))?

Thanks.

If you have a differential amplifier as source voltage, you can actually assume "zero" source impedance for the calculation.

Whats the reason for making it 0 source impedance if its differential?

I assumed, that you have a fully differential driver amp like AD8131, THS4121 or similar, they are generally low impedance on output.

I think, the discussion would be more easy with more information and less guessing.