I'm designing a simple Signal Generator and could use some thoughts on its output stage. Its intended application is to tinker and play with on my workbench, hence it need not be overly sophisticated or fancy.
The front end is an Analog Devices AD9833, a DDS Signal Generator chip, for Sin, Square, and Triangle waves, from essentially DC to 10 MHz. The User Interface will be a Mega168, an LCD, a bunch of push button switches, plus Gain and DC Offset controls.
I' feeding the signal through a buffer, two variable gain amps, and a stage to add a variable DC offset. The signal then goes to a final output stage op amp. I'm using an AD826 Op Amp because it is a heck of a lot easier than a 10 MHz transistor design (for me, anyways), and my workbench needs are not very demanding. I also have a few of the chips already in stock.
My first thought was to just use a simple voltage follower for the output, a unity gain buffer, as shown in Option #2, below.
My second thought was to insert a 50 ohm resistor, in series, in the output. At 2:00 AM in the morning I can tie the output directly to ground and not hurt the op amp. Presumably this is a low enough impedance to not significantly impact my output signal for routine tinkering on my bench.
Next I thought about splitting the 50 ohms in the output, and clamping it with a pair of stacked zeners, (or maybe a TransZorb). With this I can also protect the op amp should I unwittenly attempt to inject some impulse > +/- 12 V Backwards, into the signal generator.
I know that 10 MHz hardly counts as RF these days, and what used to be limited to Radar circuitry is now available in handheld devices, but I thought I'd seek some feedback before my chips smoke more than my soldering iron :)
JC
