AD9954 has worse stability than AD9854

Hello,

Recently I evaluated AD9954 and AD9854.

Under the same condition, I use DFT to test the stability of their output signals. But I alwalys found that the stability of AD9954 output can only reach 0.07%~0.04%, while the stability of AD9854 output can normally achieve 0.013%~0.004%. When I read the datasheet of each chip, I thought the stability of AD9954 can`t be so obviously worse than that of AD9854.

Sincerely hoping someone can provide ideas.

Thanks.

• Please clarify what you mean by "stability".

Technically, stability refers to maintaining a constant frequency over time. In the case of a DDS, the stability of the output signal derives solely from the stability of the device providing the system clock signal to the DDS. If the clock source is a general purpose quartz crystal resonator, then stability should be a few tens of parts-per-million (ppm). An OCXO source can provide stability in the fractions of a ppm range. Rubidium or cesium sources can offer stability in the range of fractions of a ppb (parts-per-billion).

The main difference between the AD9854 and AD9954 is the frequency resolution of their respective DDS cores. The AD9854 has a 48-bit core, while the AD9954 has a 32-bit core. This means the AD9854 can tune much closer to a desired frequency than the AD9954. For example, suppose both devices use a system clock source of exactly 100MHz and both devices are programmed to output exactly10MHz. The actual output frequency is then:

• AD9854: 10.000000000000142108547152020037 MHz
• AD9954: 10.00000000931322574615478515625 MHz

That is, a tuning error of:

• AD9854: 0.14 micro-Hz
• AD9954: 9.3 milli-Hz

Regarding your "DFT" measurements...

A DFT is not suitable as a stability measurement tool. It is merely a mathematical tool that converts a sampled time-domain sequence to a sampled frequency domain sequence (by "sampled" frequency, I mean Fourier frequencies (i.e., bin frequencies)). That is, a DFT does not provide explicit indication of frequency variation over time. Rather, it is a snapshot of a signal's frequency content (spectrum).

Accurate and reliable stability measurement requires a fairly sophisticated test bench and suitable analysis software.

• Thanks for your reply.

In my application, I use the DDS to provide signal stimulation to a device under test(such as a resistance) and measure the voltage between the device with an ADC, then I can calculate the frequency and amplitude information through DFT, and by then I can calculate some other parameters.

Exactly my question is when I set both AD9954 and AD9854 in the same condition to output a certain sine signal, I collected hundreds of DFT measurement results of each DDS to see the relative amplitude error. Then I found the relative amplitude error of AD9954 keeps 0.07%~0.04% and it keeps 0.013%~0.004% for AD9854. I also drawed histograms to confirm the measurement results will meet normal distribution.

I also tried to connect the output of AD9954 & AD9854 to the Signal-Input port and Ref-Input port of a lock-in amplifier and observe the amplitude measurement result change over a period of time. The result shows that AD9954 output has larger relative amplitude change than that of AD9854.

So I wonder if it`s because AD9854 do output a stabler signal(probably amplitude?) than AD9954. But from the datasheets I see AD9954 has 14-bits DAC and AD9854 just has 12-bits DAC. Maybe the more precise 48-bits DDS core of AD9854 makes the better measurement results?

Looking forward to your reply.

• Because the AD9854 and AD9954 have 12b and 14b amplitude resolution, respectively, the difference in your measurements between the two is reasonable. However, to be fair, the resolution (and linearity) of the ADC also plays into the total measurement error. The AD9854 and AD9954 have differing DAC designs, which means that resolution, harmonic distortion and other non-linearities associated with DACs (and ADCs, for that matter) also play a role in signal quality.

Again, to be clear, the term "stable"  or "stability" typically applies to frequency variation over time. "Distortion" is probably more at play here than stability.

• Thanks for your reply

I read the datasheets of AD9854 and AD9954, and it seems that the SFDR of DAC output signal of AD9854 is not as large as that of AD9954. But in the same measurement conditions, I found the amplitude distortion result of AD9854 is better. Could it be the influence of larger FTW width of AD9854?

Another question I`d like to consult is that in the datasheet of AD9954, it`s mentioned that "Truncation of the LSBs is implemented to reduce the power consumption of the DDS core. This truncation does not reduce frequency resolution". Could the truncation lead to the amplitude distortion of AD9954?

• It could very well be that the finer tuning resolution of the AD9854 over the AD9954 makes your amplitude measurements appear slightly better.

Both devices rely on phase truncation to optimize the DDS core. The number of bits truncated is directly related to the DAC resolution, as follows.

To optimize the DDS for power consumption, it is common to use only a portion of the phase accumulator output for phase-to-amplitude conversion. Consider a DDS that has a phase accumulator with N bits of frequency tuning resolution and a DAC with D bits of amplitude resolution. To optimize, we can use a certain number (P) of the phase accumulator's most significant bits for phase-to-amplitude conversion. That is, we truncate N-P LSBs of the accumulator output for phase-to-amplitude conversion. Generally, we choose P to be in the range of D+3 to D+5. The AD9854 uses N=48, D=12 and P=D+4=16, while the AD9954 uses N=32, D=14 and P=D+5=19. Hence, the number of truncated bits (N-P) is 32 for the AD9854 and 13 for the AD9954.

Keep in mind that the overall performance of the DDS is only partially dependent on the digital design of the DDS core (N, P and D). Also at play is the the performance of the DAC itself (harmonic distortion, linearity, etc.), as well as the quality of the system clock source (spurious/noise content).