ADR3425/ADR3450

Rev. 0 | Page 15 of 20

APPLICATIONS

BASIC VOLTAGE REFERENCE CONNECTION

VIN

2.7V TO

5.5V

VREF

OUT

2.5V

0.1µF

1µF

0.1µF

ADR3425

ENABLE

GND FORCE

GND SENSE

4

3

6

5

2

1

Figure 40. Basic Reference Connection

The circuit shown in Figure 40 illustrates the basic configuration

for the ADR3425/ADR3450. Bypass capacitors should be

connected according to the guidelines below.

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. An

additional 0.1 μF ceramic capacitor should be connected in

parallel to reduce high frequency supply noise.

A ceramic capacitor of at least a 0.1 μF must be connected to the

output to improve stability and help filter out high frequency

noise. An additional 1 μF to 10 μF electrolytic or ceramic

capacitor can be added in parallel to improve transient

performance in response to sudden changes in load current;

however, the designer should keep in mind that doing so

increases the turn-on time of the device.

Best performance and stability is attained with low-ESR (for

example, less than 1 Ω), low-inductance ceramic chip-type

output capacitors (X5R, X7R or similar). If using an electrolytic

capacitor on the output, a 0.1 μF ceramic capacitor should be

placed in parallel to reduce overall ESR on the output.

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 should be mounted as close to the load as possible

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

voltages can be sensed very accurately. These voltages are fed

back into the internal amplifier and used to automatically

correct for the voltage drop across the current-carrying output

and ground lines, resulting in a highly accurate output voltage

across the load. To achieve the best performance, the sense

connections should be connected directly to the point in the

load where the output voltage should be the most accurate.

See Figure 41 for an example application.

LOAD

VIN

0.1µF

0.1µF

1µF

OUTPUT CAPACITOR(S) SHOULD

BE MOUNTED AS CLOSE

TO VOUT PIN AS POSSIBLE.

SENSE CONNECTIONS

SHOULD CONNECT AS

CLOSE TO LOAD

DEVICE AS POSSIBLE.

ADR3425

ENABLE

GND FORCE

GND SENSE

4

3

6

5

2

1

Figure 41. Application Showing Kelvin Connection

It is always advantageous to use Kelvin connections whenever

possible. However, in applications where the IR drop is

negligible or an extra set of traces cannot be routed to the load,

tied together and the device can be used in the same fashion as

a normal 3-terminal reference (as shown in Figure 40).

In applications with slow-rising input voltage signals, the

reference exhibits overshoot or other transient anomalies that

appear on the output. These phenomena also appear during

shutdown as the internal circuitry loses power.

To avoid such conditions, ensure that the input voltage

waveform has both a rising and falling slew rate of at least

0.1 V/ms.

SHUTDOWN/ENABLE FEATURE

The ADR3425/ADR3450 references can be switched to a low-

power shutdown mode when a voltage of 0.7 V or lower is input

to the ENABLE pin. Likewise, the reference becomes operational

the supply current drops to less than 5 μA, useful in applications

that are sensitive to power consumption.

If using the shutdown feature, ensure that the ENABLE pin

causes a large increase in the supply current of the device and

may keep the reference from starting up correctly (see Figure 34).

If not using the shutdown feature, however, the ENABLE pin

operational continuously.