Hello support team,

Our customers have a few questions about the clock to external input to the AD9257.

It is written with respect to jitter in the P.22 in data sheet of AD9257.

1. When ANALOG INPUT FREQUENCY is 90MHz and 14bits, does Figure55 show that jitter needs to 0.125ps or less?

If this recognition is incorrect, please teach me the description of Figure55.

2. There are several types of jitter. Please teach me the definition of jitter that are described in this section.

3. Please tell me if there is an element to be careful in the accuracy of the input clock in addition to the jitter and duty.

Thanks,

Tak

Hello Tak-san,

1.

The purpose of Figure 55 in the AD9257 datasheet is to show the effect of jitter alone as a function of input frequency. It assumes an ideal converter with infinite resolution (infinite number of bits). The way to read the chart is to first choose your hypothetical jitter magnitude. This determines which line on the chart to use. Then you choose your analog input frequency. This will determine a point on the line. This point aligns with a number on the left-side-axis. The number on the left-side-axis will tell you the ideal SNR for that assumed jitter and input frequency. Again this assumes an ideal ADC with infinite number of bits.

How much jitter you can tolerate depends on your performance needs.

2.

The jitter referred to here is RMS random jitter. It is a theoretical/hypothetical discussion but it has very practical and real results and effects.

3.

AD9257 clock pins supply their own internal DC bias level, so you need to AC couple to the clock pins. The datasheet has some examples of clocking schemes. Also, keep in mind that higher edge rate on the clock is beneficial for jitter. If you are using a sinusoidal clock, increasing amplitude increases the edge rate. It is a good idea to keep a healthy amplitude on the clock where possible, of course not exceeding the maximum limits of the part.

Thank you for working with the AD9257.

Doug