[Introduction]Of all device characteristics, noise can be a particularly challenging and difficult design topic to master. This article focuses on the effect of power supply noise on the phase noise of high-speed DACs.
DAC Phase Noise Sources
For high-speed DACs, phase noise mainly comes from the following aspects: clock noise, power supply noise, and internal noise and interface noise.
Figure 1: Sources of DAC phase noise (Image credit: ADI)
Two of the most important sources are clock noise and power supply noise. This article will focus on the impact of power supply noise on DAC phase noise.
In-depth understanding of ADI’s DAC chip products
DAC Supply Phase Noise Propagation Path
All the circuits on the chip must be powered in some way, which gives a lot of opportunity for noise to propagate to the output. The propagation paths of power supply noise in different circuits are also different. The following highlights several common DAC power supply noise propagation paths.
As shown in the figure below, the DAC output is usually composed of a current source and a MOS tube. The MOS tube guides the current to supply power through the positive pin or the negative pin. The current source draws power from an external power supply, and any noise is reflected as current fluctuations.
Figure 2: Sources of DAC power supply noise (Image credit: ADI)
MOS tube
The noise of the current source can reach the output end through the MOS tube, but this only explains the coupling phenomenon of the noise.
Figure 3: DAC power supply noise propagation path – MOS tube (image sourced from ADI)
To “contribute” phase noise, this noise also needs to be mixed to the carrier frequency through the MOS transistor. The MOS tube here is equivalent to a balanced mixer.
Pull-up Inductor
The pull-up inductor is another noise path that flows from the supply rail to the output.
Figure 4: DAC power supply noise propagation path – pull-up inductor (Image source: ADI)
Any change in the rails and load here will cause a change in current, which again mixes the noise to the carrier frequency.
More noise conduction paths
In general, these switching circuits are contributors to power supply phase noise if the switching is capable of mixing the noise to the carrier frequency.
Analyze Phase Noise
For the mixing phenomenon mentioned above, it is quite difficult to quickly simulate all these behaviors and improve them. On the contrary, by measuring the power supply rejection ratio, we can quickly understand which power supplies are sensitive to noise, and then select some high-precision and low-noise power supplies in a targeted manner.
Similar power supply rejection ratio analysis will be available for other analog modules, such as voltage regulators, op amps, and other ICs, which typically specify power supply rejection ratios.
Power supply rejection measures the sensitivity of the load to power supply variations and can be used for phase noise analysis here. However, what is used here is not the rejection ratio, but the modulation ratio: power modulation ratio (PSMR). Of course, the traditional power supply rejection ratio (PSMR) is still relevant.
We specially modulate a noise to test. The next step is to get concrete data.
Measuring PSMR
An important way to analyze phase noise is to measure PSMR.
Typical measurement PSMR test schematic:
Figure 5: PSMR measurements (Image credit: ADI)
PSMR measurements can be divided into three steps: modulate the supply rails, acquire the data, and analyze the data.
Modulated supply rails
Power modulation is obtained by a coupling circuit inserted between the power supply and the load, superimposed on a sine wave signal generated by a signal generator.
retrieve data
The output of the coupling circuit is monitored with an oscilloscope to monitor the actual supply modulation. The final DAC output is detected by the spectrum analyzer.
analyze data
PSMR is equal to the ratio of the AC component of the power supply displayed from the oscilloscope to the modulation sideband voltage around the carrier.
Here are a few key points for PSMR measurements:
● Coupling circuit: There are many different coupling mechanisms in the coupling circuit. The coupling circuit can choose LC circuit, power operational amplifier, transformer or special modulation power supply. The method used here is a 1:100 turns ratio current sense transformer and function generator. A high turns ratio is recommended to reduce the source impedance of the signal generator.
● Power modulation: obtained by superimposing a 500kHz peak-to-peak voltage 38 mV signal modulation on the 1.2V DC power supply.
Figure 6: Clock supply modulation (Image credit: ADI)
● DAC: ADI’s AD9164 is used. The DAC clock speed is 5GSPS. The resulting output induces sidebands on a full-scale 1GHz, –35dBm carrier.
Figure 7: Modulated sidebands (Image credit: ADI)
Converting the power to voltage and then ratioing with the modulated supply voltage yields a PSMR of –11 dB. AD9164 has eight power supplies, we choose the key, key scan the following four power supplies: 1.2V clock supply, negative 1.2 V and 2.5V analog supply, 1.2 V analog supply. The resulting graph is shown below:
Figure 8: Power supply PSMR measured by sweep frequency (Image credit: ADI)
The clock supply is the most sensitive rail, followed by the negative 1.2V and 2.5V analog supplies, the 1.2V analog supply being less sensitive. With due consideration, the 1.2V analog supply can be powered by a switching regulator, but the clock supply is the exact opposite: it needs to be powered by an ultra-low noise LDO for good performance.
Choose an ultra-low noise power supply
Selection of LDOs
LDOs are well-proven voltage regulators that are particularly suitable for achieving high-quality noise performance. For sensitive power rails, not all LDOs are suitable, and it still needs to be selected and tested according to the overall system requirements.
The test method is: use the spectral noise density curve of this LDO and the DAC PSMR measurement results to compare.
For example, a certain circuit, in the initial version, uses the LDO ADP1740 to compare the spectral noise density curve of the LDO with the DAC PSMR measurement results, as shown in the following figure:
Figure 9: AD9162 evaluation board phase noise (Image source: ADI)
This confirms the effect of the clock supply (red dot in the image above) on noise. After the revision, the ADP1761 was replaced, and the noise at some specific frequencies was reduced by as much as 10dB.
On the Digi-Key website, suitable Digi-Key LDOs can be filtered based on parameters, including the ability to filter directly by PSRR (Power Supply Rejection Ratio).
Figure 10: Filtering LDOs by PSRR (Power Supply Rejection Ratio)
Other options
However, it does not mean that other power sources other than LDOs cannot be used. According to the overall system requirements, through appropriate LC filtering, switching regulators can also provide power, thereby simplifying the power solution. But because of the LC filter, care should be taken with series resonance, otherwise the noise may get worse. Resonance can be controlled by reducing the Q of the circuit, such as adding lossy components to the circuit.
The figure below shows an example from another design using the AD9162 DAC. Clock power is also provided by the ADP1740 LDO, but followed by an LC filter.
Figure 11: LC filter and de-Q network (Image credit: ADI)
The filter under consideration is shown in the schematic, with the RL model representing the inductor and the RC model representing the main filter capacitor (C1+R1).
The red circle is the original LC filter circuit, and the blue circle is the additional lossy component to reduce the Q value.
Figure 12: LC filter response (Image credit: ADI)
The filter response is shown in the figure below, the red line is the original LC circuit response curve, and the blue line is the improved response curve. We see that the Q value decreases.
Figure 13: Phase noise response (Image credit: ADI)
Let’s take a look again. For the phase noise response, the blue line is the original LC circuit response curve, and the orange line is the improved response curve. Phase noise is improved.
Summary of this article
Not only does noise vary greatly with power supply choices, it can also be affected by output capacitance, output voltage, and load. These factors should be carefully considered, especially for sensitive supply rails.
The Links: BT42008VSS-122 PD640G400DA-100B
