Interface High-Performance Op Amps With ADCs

GROUNDING AND PCB LAYOUT CONSIDERATIONS
Connecting the input source ground with the supply ground is a critical factor in layout. For each ADC driver configuration, it’s important to account for the impedance of the signal source when setting up the resistor networks to ensure that the differential outputs have the same gain. For example, an audio precision signal generator has about 22 O of source impedance and the board has a 50-O termination. Therefore, the designer must adjust the proper gain and input to obtain the desired signal at the output of the op amp.

The following are recommendations for the design of PCB layout in order to obtain the optimum high frequency performance:

• Place the ADC and amplifier as close together as possible.
• Put the supply bypassing capacitors as close as possible to the device (<1 in.).
• Utilize surface-mount instead of through-hole components and ground and power planes.
• Keep the traces short where possible.
• Use terminated transmission lines for long traces.

The PCB layout of the differential-to-differential configuration is shown in Figure 8.

In summary, considerations such as the selection of the external R_L-C_L network are of paramount importance. Also, the op amp’s critical parameters—THD, settling time, and noise—must be taken in to account to interface a high-performance op amp with an ADC. By employing one of three different ADC driver architectures, and paying careful attention to grounding and PCB layout, system performance can dramatically improve.