connecting input of ADCMP565 to DDS AD9834

Hi Frans,

Your'e right regarding the AD9834 not being able to drive a 50ohm resistor at its output as it is only recommended to have a 200ohm load. When the output is at its peak of 0.6V and is terminated by 50ohms, 12mA of current will be drawn from the output which is way above the IOUTfullscale (only 3mA) specification. this might be the one causing the voltage to drop at the input of the comparator.

Is it alright for you to share you're schematic for us to have a clearer picture of how are you connected in you're setup?

Best regards,

Rainier

hi rainier..

i want to make a PWM signal from DDS generated signal..

i attached the schematic..

the output of DDS (IOUT) will be connected into input- of ADCMP565..

what if i removed the 50ohm resistance since i only need the voltage to be compared with another signal (triangle) ?

thank you..

Hi Frans,

Based on your schematic, the output of the DDS is seeing the parallel combination of 200ohm and 50ohm which makes the resistance even smaller. You should try removing the 50ohm resistance for the output of the DDS to be properly terminated and see if this solves the issue.

The ( + ) input of the ADCMP565 is connected to the output right? what about the ( - ) input? is it connected to a reference voltage? if yes, make sure that the input on the ( - ) can drive a 50ohm load. if not, try removing the 50ohm termination across ( - ) and ground.

Best regards,

Rainier

hi rainier,

the ( + ) input of ADCMP565 is connected to other signal generator.. in this case, this signal generator could drive 50ohm load.. the ( - ) input is connected to DDS with 200ohm parallel resistance.. if i removed the 50 ohm resistor of ( - ) input, should i also removed the 50 ohm resistor on the ( + ) input ?

thank you

Best Regards,

Frans

Hi Frans,

Remove only the 50ohm on the line where the DDS is connected ( - ). You don't have to remove the 50ohm on the ( + ) line since it is connected to your signal generator which can be set to 50ohm output impedance for maximum power transfer.

Best regards,

Rainier

dear rainier..

i got problem with the ADCMP565..

i tested it by connecting the ( - ) input into GND and ( + ) input to triangle wave.. i expected to have a square wave output ("high" when in+ is higher than input-)..

but what the ADCMP565 output is also triangle wave (same with in+)

blue : in+ ,  yellow : output

then i do another test :

in+ to +5V and in- to GND -->  output : "high"

in+ to -5V and in- to GND --> output : "high"

the schematic of ADCMP565 is the same with the schematic i attached previously..

am i do something wrong?

Best regards,

frans