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sinara-hw / Pounder

PDH/phase lock signal generator for Stabilizer
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DDS baluns/termination are wrong #103

Closed jordens closed 1 year ago

jordens commented 2 years ago

The source output impedance after the baluns (TR9 etc) is 100 Ohm currently. The filters want 50 Ohm. (The eval board also has a 100 Ohm single ended output impedance from DAC1_OUT into 50 Ohm filters/SMA output which doesn't sound optimal). We can either make the source termination R24...R35 25R or use TC2-1TX+ as baluns. The latter gives more power and is still within DAC compliance.

gkasprow commented 1 year ago

fixed image

jordens commented 1 year ago

Just to remind myself of the context:

Datasheet says:

image

While the text is consistent, the figure is wrong. Note the missing resistors at the source side of the balun. The Source would be ~100 k impedance.

Eval board has:

image

This is also wrong. The 50 R are populated. Hence the source impedance is 100 R. The balun is 1:1. DAC_OUT is 50 R (filter and SMA)

Various ANs from ADI with different information:

https://www.analog.com/media/en/technical-documentation/application-notes/an_912.pdf looks trustworthy and is consistent.

image

https://www.analog.com/media/en/training-seminars/tutorials/MT-019.pdf similarly consistent.

image

https://www.analog.com/media/en/training-seminars/design-handbooks/Technical-Tutorial-DDS/Section6.pdf is wrong.

image

The AD9850 is also an unterminated current source with 100 k impedance.

Some Analog forum posts indicating the obvious, that this is indeed a current source and the source termination should match the load impedance:

https://ez.analog.com/dds/f/q-a/536170/ad9959-output-impedance https://ez.analog.com/dds/f/q-a/28841/ad9959---no-ac-output-improper-termination