10.1 Layout Guidelines

Capacitive coupling between the noisy digital circuitry and the sensitive analog circuitry can lead to poor performance. The solution is to keep the analog circuitry separated from the digital circuitry and the clock line as short as possible. Digital circuits create substantial supply and ground current transients. The logic noise generated could have significant impact upon system noise performance. To avoid performance degradation of the ADC161S626 due to supply noise, avoid using the same supply for the V_A and V_REF of the ADC161S626 that is used for digital circuitry on the board.

Generally, analog and digital lines should cross each other at 90° to avoid crosstalk. However, to maximize accuracy in high resolution systems, avoid crossing analog and digital lines altogether. It is important to keep clock lines as short as possible and isolated from ALL other lines, including other digital lines. In addition, the clock line should also be treated as a transmission line and be properly terminated. The analog input should be isolated from noisy signal traces to avoid coupling of spurious signals into the input. Any external component (e.g., a filter capacitor) connected between the converter's input pins and ground or to the reference input pin and ground should be connected to a very clean point in the ground plane.

A single, uniform ground plane and the use of split power planes are recommended. The power planes should be located within the same board layer. All analog circuitry (input amplifiers, filters, reference components, etc.) should be placed over the analog power plane. All digital circuitry should be placed over the digital power plane. Furthermore, the GND pins on the ADC161S626 and all the components in the reference circuitry and input signal chain that are connected to ground should be connected to the ground plane at a quiet point. Avoid connecting these points too close to the ground point of a microprocessor, microcontroller, digital signal processor, or other high power digital device.

For best performance, care should be taken with the physical layout of the printed circuit board. This is especially true with a low V_REF or when the conversion rate is high. At high clock rates there is less time for settling, so it is important that any noise settles out before the conversion begins.

10.2 Layout Example

Figure 50. PCB Layout Example