Driving the AD9233/9246/9254 ADCs in AC-Coupled Baseband Applications
Figure 1. ADA4937-1 Driving the AD9246 14-Bit ADC (Simplified Schematic: Decoupling and All Connections Not Shown)
The AD9246 achieves its optimum performance when driven differentially. The ADA4937-1 not only provides the single-ended-to-differential conversion but also provides gain and level shifting. The output common voltage of the ADA4937-1 is set by connecting a resistive divider to the VOCM pin of the ADA4937-1. If the pin is left floating, the VOCM voltage is approximately midsupply and is set by an internal divider.
The ADA4937-1 is powered with a single 5 V supply and is configured for a gain-of-2 for a single-ended, input-to-differential output. The 76.8 Ω termination resistor, in parallel with the single-ended input impedance of 137 Ω, provides a 50 Ω ac termination for the source. The additional 49.9 Ω resistor, 10 μF capacitor, and 76.8 Ω resistor connected to the 90.9 Ω resistor on the inverting input balance the ac impedance driving the noninverting input. A detailed analysis of this configuration can be found in MT-076 Tutorial.
Figure 2. Second Harmonic Distortion (HD2) and Third Harmonic Distortion (HD3) for the ADA4937-1 Driving the AD9246 ADC
The signal generator has a symmetric, ground-referenced, bipolar output. The VOCM pin of the ADA4937-1 is left unconnected; therefore, the internal divider sets the output common-mode voltage to midsupply. A portion of this is fed back to the summing nodes, biasing −IN and +IN at 1.14 V. For a common-mode voltage of 2.5 V, each ADA4937-1 output swings between 2.0 V and 3.0 V, providing a 2 V p-p differential output for a 1 V p-p single-ended input.
The output of the ADC driver is ac-coupled to a single-pole, low-pass noise filter. The low-pass filter reduces the noise bandwidth at the ADC input and provides a degree of isolation from the switched capacitor inputs of the ADC and the driver. In any configuration, the optimum value of the shunt capacitor, C, is dependent on the input frequency and source impedance and may need to be optimized. Table 1 displays recommended values for the RC network. However, these values are dependent on the input signal frequency and may require further optimization.
The input common-mode voltage to the ADC is set by the CML pin of the AD9246 and the pair of 200 Ω resistors. In other applications, the CML pin of the ADC and the VOCM pin of the ADA4937-1 are used to set the input common-mode voltage to the ADC (see the Common Variations section). The ADA4937-1 is fabricated using the Analog Devices, Inc., proprietary silicongermanium (SiGe), complementary bipolar process, enabling it to achieve very low levels of distortion with an input voltage noise of only 2.2 nV/ √Hz.
The circuit shown in Figure 1 was tested with a −1 dBFS signal at various input frequencies. Figure 2 shows a plot of the second and third harmonic distortion (HD2/HD3) vs. input frequency.
The circuit must be constructed on a multilayer PC board with a large area ground plane. Proper layout, grounding, and decoupling techniques must be used to achieve optimum performance (see MT-031 Tutorial, MT-101 Tutorial, and the AD9246 evaluation board layout).
It is required that the exposed pad on the underside of both the AD9246 and the ADA4937-1 (LFCSP packages) be connected to a large area ground plane to achieve the best electrical and thermal performance. The copper plane should have several vias to achieve the lowest possible resistive thermal path for heat dissipation to flow through the bottom of the PCB. These vias should be solder filled or plugged.
There are a few other amplifier configurations to consider when driving ADCs. They are differential ac-coupled input to differ-ential output, dc-coupled single-ended input to ac-coupled differential output, dc-coupled single-ended input to differential output, and dc-coupled differential input to differential output.
In dc-coupled systems, the driver output common-mode voltage is set via the VOCM pin of the ADA4937-1. The adjustable level of the output common-mode voltage allows the ADA4937-1 output to match the input common-mode voltage of the ADC. The internal common-mode feedback loop of the ADA4937-1 also provides exceptional output balance and suppression of even-order harmonic distortion products. Often in these applications, the ADC CML pin is connected directly to the VOCM pin of the driver to ensure that optimal ADC input common-mode voltage is achieved. In other applications, the VOCM pin can be driven from a low impedance source such as an op amp. It can also be left floating but bypassed with a capacitor; in this case the VOCM voltage is set at the midpoint of the voltage applied to the two supply pins.
Table 1. RC Network Recommended Values
|Frequency Range (MHz)||R Series (Ω)||C Differential (pF)|
|0 to 70||33||15|
|70 to 200||33||5|
|200 to 300||15||5|
|>300||15||Open (no capacitor)