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Simple 1MHz fixed sine output with gain adjust.

Question asked by GaryC on Feb 25, 2012
Latest reply on Sep 8, 2012 by trioflex

I am looking for advice on DDS device selection and circuit configuration. My application is quite simple. I need a bipolar amplitude-adjustable 1.0 MHz sine wave driving a 70 ohm resistive load. My load consists of two series 35 ohm resistors, with the common node grounded.  There is no need for phase, frequency, or amplitude modulation once set, just continuous output.  The amplitude across the 70 ohm load will always be set for 860 mVp-p, or 305 mV rms.

 

Digital logic on my board runs at 3.3VDC. Bipolar analog power supplies can be provided for +/- 3.3V or +/-5.0V.

 

Evaluating the DDS reference materials on your web site, my best approach seems to be a DDS device with complementary outputs, driving a center-tapped 1:1 isolation transformer, center tap grounded. The transformer secondary winding would drive the two series 35 ohm load resistors with grounded common node. 

 

If the DDS device directly drives the transformer-coupled 70 ohm load, it looks like I need a DDS with Iout = 20mA full scale current output. The 20mA DDS devices are all on the high end of the performance range. Suddenly I am looking at an AD9850, true overkill for my needs!

 

I would prefer something like a AD9838 but its 3mA full scale current drive is inadequate for driving my 70 ohm load. Can you recommend something? Should I use an op amp between AD9838 and transformer? Specific advice is appreciated.

 

I see how you use a voltage output digital pot (ex: AD5620) for driving RSET to control amplitude. If using an external amplifier, is RSET current control the best way to control load voltage, or should I simply put gain control in the op amp circuit?

 

Analog design is outside my area of expertise, so sound recommendations are appreciated.

 

A secondary configuration uses a load consisting of 2 series 17.5 ohm resistors (35 ohms total) connected as above. In this case, the 1MHz sine wave is set for 1.20Vp-p, or 425 mV rms.

 

Thanks in advance,

 

Gary

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