10.1 Layout Guidelines

Supply bypass capacitors must be close to the package and connected with low-impedance conductors. Avoid noise coupled into R_GAIN, and observe wiring resistance. For thermal management, see the VQFN Package and Heat Sinking section.

Layout for the XTR305 is not critical; however, its internal current chopping works best with good (low dynamic impedance) supply decoupling. Therefore, avoid through-hole contacts in the connection to the bypass capacitors or use multiple through-hole contacts. Switching noise from power supplies should be filtered enough to reduce influence on the circuit. Small resistors (2-Ω, for example) or damping inductors in series with the supply connection (between the DC-DC converter and the XTR circuit) act as a decoupling filter together with the bypass capacitor as shown in Figure 49.

Resistors connected close to the input pins help dampen environmental noise coupled into conductor traces. Therefore, place the OPA input- and IA input-related resistors close to the package. Also, avoid additional wire resistance in series to R_SET, R_OS, and R_GAIN (observe the reliability of the through-hole contacts), because this resistance could produce gain and offset error as well as drift; 1 Ω is already 0.1% of the 1-kΩ resistor.

The exposed lead-frame die pad on the bottom of the package must be connected to V−, pin 11 (see the VQFN Package and Heat Sinking section for more details).

Figure 49. Suggested Supply Decoupling for Noisy Chopper-Type Supplies