The circuit described in this document and presented in Figure 1 utilizes the ADA4937-1 ADC driver to provide a AC coupled single ended input to an AC coupled differential conversion, for driving the AD9246, a 14-bit 125 MSPS ADC. The ADA4937 is a low noise, ultralow distortion, high speed differential amplifier with low DC offset and excellent dynamic performance. It is an ideal choice for driving high performance ADCs with resolutions up to 16 bits from DC to 100MHz and is well-suited for a wide variety of data acquisition and signal processing applications. Combined with power and cost savings over previously available ADCs, this circuit is suitable for applications in communications, instrumentation, and medical imaging.

The AD9246 is a monolithic, single 1.8 V supply, 14-bit, 80 MSPS/ 105 MSPS/ 125 MSPS analog-to-digital converter (ADC), featuring a high performance sample-and-hold amplifier (SHA) and on-chip voltage reference. The wide bandwidth, truly differential SHA allows a variety of user-selectable input ranges and offsets, including single-ended applications. The device can be applied in multiplexed systems that switch full-scale voltage levels in successive channels and for sampling single-channel inputs at frequencies well beyond the Nyquist rate.

The AD9246 achieves its optimum performance when driven differentially. The ADA4937 not only provides the single ended to differential conversion, it also provides gain and level shifting. The output common voltage of the ADA4937 is set by connecting a resistive divider to the Vocm pin of the ADA4937.

The output of the ADC driver is AC-coupled to a single-pole, low-pass 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 value of the shunt capacitor, C, is dependent on the input frequency and source impedance and may need to be reduced or removed. Table 1 displays recommended values to set the RC network. However, these values are dependent on the input signal and should only be used as a starting guide.

The input common mode voltage to the ADC is set by the CML pin and the pair of 200 ohm resistors. In other applications the CML and the Vocm pin of the ADA4937 are used to set the input common mode voltage to the ADC, see the next section on Common Variations. The ADA4937 is fabricated using Analog Devices, Inc. proprietary silicon-germanium (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 frequencies. Figure 2 shows a plot of the second and third harmonic distortion (HD2/HD3) vs. frequency.

There are a few other amplifier configurations to consider when driving ADCs. They are differential AC-coupled inputs to differential outputs, DC-coupled single ended input to AC coupled differential output, DC coupled single ended input to differential outputs and DC coupled differential in to differential out.

In DC coupled systems the driver output common mode voltage is set via the Vocm pin. The adjustable level of the output common mode voltage, allows the ADA4937 output to match the input common mode voltage of the ADC. The internal common-mode feedback loop of the ADA4937 also provides exceptional output balance as well as suppression of even-order harmonic distortion products. Often in these applications the ADC’s CML pin is connected directly to the Vocm pin of the driver to ensure the 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 mid point of the voltage applied to pins +Vs and –Vs.