DDS function generator

[jeffreyprinzie]

Hi,

I am building a DDS function generator up to 6MHz with a DDS from analog devices.
As opamp I use the LT1226 having 1GHz gain bandwidth, enough with Lag lead compensation?
All amplitude and DC offset works, but the output buffer still needs to be done.
I was thinking about a Class AB push pull emitter follower. The output impedance is 1/gm , so with enough base current, 50ohm output impedance should be possible. The input impedance should not be depend a lot on the output capacitance, because that would change the output pole of the opamp leading to instability.

Should I use an open loop buffer or use feedback? opamp or not for feedback. I rather use no feedback for improved stability. The dominant pole must be on the output then, so capacitive load only moves it to the origin.

Amplitude max 5V
offset max +-5V

what solutions for the output do you propose?

Thanks!

Jeffrey

[Jay_Diddy_B]

Hi,

Look at using the LT1210 for the amplifier / output stage.

With LT1210 you will not need an emitter follower output stage.

http://www.linear.com/product/LT1210

You should have a 50 Ohm resistor in series with the output of the amplifier, so that the source impedance of your function generator is 50 ohms.

Jay_Diddy_B

[bdivi]

Another approach is AD811

I have built a function generator based on MAX038 that is good to 20MHz and the AD811 buffer happily drives 50 ohms with around 12ns rise/fall times.

cheers,

[jeffreyprinzie]

Thanks,

The LT1210 seems to do it well, i will orde one and see.
any sugestions are welcome.
I now made a class ab output stage which also works, but with the opamp it can be more linear.
kr