I currently use the AD9252 on my system board but would like to use the AD9257 instead. Is this possible? What are the main differences between the two ADCs?
The short answer to this question is YES. It is possible to replace the AD9252 on a system board with the AD9257 without any major re-design.
The AD9257 is a lower power octal ADC. While the AD9252 consumes about 94mW power/channel at 50MSPS, the AD9257 consumes only 55mW/channel at 65MSPS. The AD9257 also provides slightly better performance in SNR and SFDR because of the calibration techniques utilized in the design. Another key feature that is available on the AD9257 is the power scaling feature (on the analog circuits) using SPI writes. By selectively turning off unused parts of the circuit and controlling bias currents, the AD9257 enables the user to adjust the power to the required clock rate. Here is a figure which shows the power scaling feature of the AD9257.
Assuming you’re using the AD9252 in SPI mode there are FOUR minor pin differences between the AD9252 and AD9257 that are drop-in interchangeable/compatible with only minor BOM changes.
The first difference is that two AD9252 pins 57(REFB) and 58(REFT) are no longer needed on the AD9257 design so they are re-purposed as pin 57(VCM) and 58(SYNC). Based on AD9252 reference schematics, there will typically be a 0.1uF decoupling/bypass caps tied across, and/or to GND on the AD9252 REFT/REFB pins. Any caps across, or between pins 57/58 should be removed on the AD9257 since this pin pair is no longer related on the AD9257.
On the AD9257, pin 57(VCM) still needs a 1uF cap to GND for stability. Pin 58(SYNC) does not really require any external connection or decoupling, but customers typically have a 0.1uF bypass cap to GND installed there from their AD9252 layout and if so, the cap should not cause any harm to just leave it.
The other two pin differences are that on the AD9257, pins 13 and 36 are no longer DRGND. On the AD9257 the package paddle provides the only ground connection for both the Analog and Digital power supply. So pins 13 and 36 on the AD9257 now have no internal connections. Because of this it is fine to solder pins 13 and 36 directly to the AD9252 PCB footprint.
In non-SPI mode the pin-strapped mode of pin 39 is slightly different between the two devices. The AD9252 offers ODM “Output Driver Mode” for selection between ANSI-644 and reduced current signal options. Pin 39 on the AD9257 now provides DFS “Data Format Select” to choose between Two’s Comp and Offset Binary output formats.
One thing to keep in mind is that the AD9252 supports 3.3V SPI whereas the AD9257 only supports 1.8V SPI.