Table 10-1. Design Example Parameters

DESIGN PARAMETER

VALUE

5.5 V

2.5 V

REF7025 input capacitor

10 µF

REF7025 output capacitor

10 µF

10.2.2 Detailed Design Procedure

10.2.2.1 Input and Output Capacitors

A 1-μF to 10-μF electrolytic or ceramic capacitor can be connected to the input to improve transient response in

applications where the supply voltage may fluctuate. Connect an additional 0.1-μF ceramic capacitor in parallel

to reduce high frequency supply noise.

A low ESR capacitor of 1-μF to 100-μF must be connected to the output to improve stability and help filter out

high frequency noise. Best performance and stability is attained with low-ESR output capacitors (X5R, X7R, or

similar) with an ESR of 100 mΩ or less. For very low noise applications, special care must be taken with X7R

and other MLCC capacitors due to their piezoelectric effect. The inherit piezoelectric effect can cause a

mechanical vibration which appears as noise in the μV range which can dominate the noise of the REF70. It is

recommended to use film capacitors for noise sensitive applications.

10.2.2.2 4-Wire Kelvin Connections

Current flowing through a PCB trace produces an IR voltage drop, and with longer traces, this drop can reach

several millivolts or more, introducing a considerable error into the output voltage of the reference. A 1-inch long,

5-millimeter wide trace of 1-ounce copper has a resistance of approximately 100 mΩ at room temperature; at a

load current of 10 mA, this can introduce a full millivolt of error. In an ideal board layout, the reference must be

mounted as close as possible to the load to minimize the length of the output traces, and, therefore, the error

introduced by voltage drop. However, in applications where this is not possible or convenient, force and sense

connections (sometimes referred to as Kelvin sensing connections) are provided as a means of minimizing the

IR drop and improving accuracy.

Kelvin connections work by providing a set of high impedance voltage-sensing lines to the output and ground

nodes. Because very little current flows through these connections, the IR drop across their traces is negligible,

and the output and ground

It is always advantageous to use Kelvin connections whenever possible. However, in applications where the IR

and GND can simply be tied together, and the device can be used in the same fashion as a normal 3-terminal

reference.

REF70

SNAS781 – OCTOBER 2020

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