clock switching coupling

Hi,

I am designing a mixed signal system controlled by an ADuC841 microcontroller. At the moment I'm using the ADuC eval board.

In this system I generate low frequency sine waves (500Hz - 5KHz) via a direct digital synthesis generator. I'm using a switched capacitor clocking filter to process my received signal. This filter needs a clock signal at 50x the cuttoff frequency. I provide the clock signal by a timed compliamenting of a standard output pin on the ADuC. I am at the prototyping stage so as a result I have a number of boards with a lot of wires, but I have done my best to separate analogue and digital grounds.

The edge transition on the generated clock is quite sharp and there is a rather significant oscillation on it. As a result there is significant coupling of the switching onto my analogue signals.

I have attempted simple solutions such as adding a small damping resistor to the clock output. I have also tried a RC low pass filter on the signal but this only passes pulses not a proper clock. My clocked filter will work fine with a sine or squarewave signal, so a smoothed signal should work fine

Is there a way to smooth the edges of my clock signal which will eliminate the switching noise on my analogue signals.

Try adjusting the cutoff frequency of the RC low-pass filter to just round the clock edges slightly while still passing the clock waveform. For a 250kHz clock start with an RC of about 0.5µs.

Also send the clock signal over twisted-pair wires with the ground connection at each end close to where the clock signal is sent and received. You always want to have a short ground return path for the clock signal to minimize it's coupling into the analog circuits.