Low Pass Filter Design

You will need extensive power supply decoupling if you only have a constant current source.

Just try capacitors across the supply pins at first, the 3914s are just 10 coparators basically and they don't like being run off a high impedance supply.

If the oscillation is on the signal input then a simple low pass filter would filter it out, say a 1kohm resistor with a 0.1 micro farad capacitor to ground - the input impedance of the 3914 is high enough not to make any large errors.

John.

What is your power source and what do you desire (voltage) to drive your system with? Do you need filtering on the input signal or just the supply?

the supply is ok, its the input signal i need to filter, i can adjust the output current across a range of 200 - 700 mA, with this put across a 6.8 Ohm or similar resistor i can get an input for the IC between 1.36V - 4.76V to operate the bar graph but it still wont work, iv'e been told the signal runs at 300hz and i may need to put a low pass filter with a cutoff of about 30hz or even lower to avoid ripple.

Okay, so the supply isn't the problem.

The output current of 200 - 700 mA - is this for the LED bargraph? or is it from the sensor? It sounds as if its from the sensor to me...

So you're dropping this current through a 6.8 ohm resistor to get your voltage input to drive the 3914s... You say its 300 Hz signal is this ac or a ripple frequency?

If its ac you will need to recitfy it to give a dc voltage and then smooth the dc voltage using your low pass filter...

Is this what you mean?

Depending how fast you want to update the display you will need to watch the cut off frequency, if you can use a slow update of the display you could use a 11k and a 1micro farad to filter the rectified signal.

John.

thanks for the help guys, i dont think the output is AC, can u explain to me what a ripple frequency is? it is possibly a PWM signal. i worried though if i used a filter like u say then my voltage input willl then drop most of its voltage across the 11k resisitor and leave nothing for the input, is this correct or will something different happen?

A battery or capacitor can eliminate ripple.