# AD9850 comparator problem. How to calculate is frequency?

I want to convert a sine wave produced from AD9850 (opens *.pdf) into square wave with its inbuilt comparator. AD9850 has a fast comparator with 3ns rise/fall time and propagation delay= 5.5ns.

without accounting the propagation delay , it should work up to 1/(2x3ns)= 83MHz. If I add propagation delay, it will be 1/(2x3ns + 5.5ns)= 43.48 MHz. if I account the same time for settling and flat part of the square wave and some error tolerance, it should at least tolerate around 20MHz.

But it is not what really happens. the output wave is not a square wave higher than 5MHz ( it is more like a noise). The input signal of the comparator is well low pass filtered and is a clean sine wave with around 2v pk-pk.

Is there anything wrong in my calculations?

EDITION: I am using this module with a reliable ground plane:

• Is your o-scope fast enough? I'd say 3ns rise, 3ns fall and 6ns top and bottom should give you a reasonable 80MHz signal. Commented Oct 15, 2013 at 18:01
• A schematic indicating what you have implemented and how you are measuring this would be helpful. Commented Oct 15, 2013 at 18:32
• @Andyaka yes it is a 200MHz one with good quality.
– Aug
Commented Oct 15, 2013 at 18:48
• @Aug - does the onboard comparator require localized de-coupling capacitors? Are there any decoupling capacitors you have forgotten about. What about the signal size going into the comparator - bigger the better (within the power rails of course)! Commented Oct 15, 2013 at 18:59
• Spec indicates ground plane , decoupling cap close to 5V and ground and 15pF load. Use barrel of probe & tip with sleeve & gnd clip removed between Qout and Gnd with 10x calibrated probe.
– user30080
Commented Oct 15, 2013 at 19:10