Driver amplifier for AD9257

Document created by analog-archivist Employee on Feb 23, 2016Last modified by analog-archivist Employee on Feb 23, 2016
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I just had a closer look at the AD9257, and found that you recommend using the
- ADA4927-2
- ADA4937-2
- ADA4938-2
as driver amplifier.

Why do you not recommend the ADA4932-2? I like the ADA4932-2, because the dual
package saves a lot of valuable PCB area, and the device is low power compared
to others of the ADA49xx series.

Are there any other pin compatible dual amplifiers available?


This is a good question, and the answers are numerous :
- The AD9257 was released a couple of years ago (circa 2011)
- So the amplifier recommendations were made with the available new designs
- The ADA49xx series amplifiers are usually used for low IF DC coupled
- The ADA4937 has the Vocm that is within the range of the AD9257

You can select the driver based on resolution. The ENOB of the AD9257 is in the
range of 12.3 to 11.8 bits going from a lower signal frequency to about 120 MHz.
If driver resolution is the most important metric, you want to keep the driver
ENOB as high as possible at your max signal frequency. As a worst case, you
would like the driver ENOB to be not less than that of the ADC.
The plot below shows the driver performance vs frequency when the driver is
configured for a gain of 1 and 2V pp diff output. Note that the ADA4932 crosses
the 12 bit line at 50 MHz. If your max signal frequency is 50 MHz or below, the
ADA4932 is fine. If you are looking for 12 bit ENOB at higher signal
frequencies you may want to consider one of the other drivers, ADA4938, ADA4937
or ADA4939.

If you are looking for some balance between power dissipation and ENOB, you
would probably look at the maximum output voltage swing of each potential
driver. To drive the AD9257, you want Vocm at 1V and a full scale swing of +/-
0.5V on each of the two outputs around Vocm. This means the outputs need to
swing between 0.5V and 1.5V.

To do this the ADA4932 and all the others on the plot below except the ADA4930
need a dual supply. If you are ok with dual supplies you can select any of the
drivers based on resolution. If you want the driver to work on single supply
you can consider the ADA4930 with a 3.3V single supply.

You can download the "ADIDiffAmpCalc" from our website to simulate the ADC
driver. This tool can help you to determine the power supply requirements,
power dissipation and resolution of the driver.