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AD977XA Output Stage

Question asked by qgm on Oct 8, 2013
Latest reply on Oct 17, 2013 by qgm


I'm designing a circuit around the AD977XA series (specifically, the AD9778A) of high-speed DACs. I'm not too experienced so wanted to get some general feedback/background/context on the output stage. I will be updating the AD9778A at 50 MHz and generating signals from DC to 1 MHz. I want to design an output stage that takes the balanced current output and makes it acceptable for an amplifier that has +/- 1 V bipolar input with 50 ohm input impedance.


From what I can tell, at the default Imax = 20 mA, I can short each of DAC positive/negative outputs to ground with a 50 ohm resistor to get +/- 1 V differential voltage. At this point, I have investigated a few options.


1. ADA4940 fully differential opamp: I would use only one of the two differential outputs (leaving the other floating) and feed the voltage between the Vout+ and the common voltage Vocm to my amplifier. At the output, I would insert a 50 ohm series resistor. Hence, in order to get +/- 1 V into the 50 Ohm bipolar input of my amplifier, I would need to set the gain to 4.

2. AD8273 difference amplifier: This would seem like the most straightforward option. Again, at the output I would use a 50 ohm series resistor and then use the configuration that gives me a gain of 2 (which would require no external feedback resistors).

3. Forget all of these specialized amplifiers and just use a low-noise precision opamp (e.g. ADA4898 or AD4817) and a standard differencing circuit.


All of these solutions seem to work fine above my desired 1 MHz and my voltage input and output range aren't difficult to reach either. I'm just wondering what the standard "good" low-noise design solution would be or what sort of things should be considered? In the end, low noise is probably the most important for me and the ADA4898 and AD4817 both seem to have far lower input voltage noise than the ADA4940 or AD8273, but in that case, what is the niche for products like the ADA4940 or AD8273? Lower design complexity or cost?


Any comments or suggestions or links to application notes would be appreciated!