ADR3525/ADR3530/ADR3533/ADR3540/ADR3550

Data Sheet

Rev. 0 | Page 18 of 20

APPLICATIONS INFORMATION

BASIC VOLTAGE REFERENCE CONNECTION

2.7V TO

5.5V

2.5V

0.1µF

1µF

0.1µF

ENABLE

GND FORCE

GND SENSE

8

1

6

7

2

3

AD3525/ADR3530/

ARD3533/ADR3540/

ADR3550

Figure 37. Basic Reference Connection

The circuit shown in Figure 37 illustrates the basic configuration

for the ADR35xx references. Bypass capacitors should be

connected according to the following guidelines.

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 fre-

quency 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 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 38 for an

example application.

LOAD

0.1µF

0.1µF

1µF

OUTPUT CAPACITOR(S) SHOULD

BE MOUNTED AS CLOSE

SENSE CONNECTIONS

SHOULD CONNECT AS

CLOSE TO LOAD

DEVICE AS POSSIBLE.

ENABLE

GND FORCE

GND SENSE

8

1

6

7

2

3

AD3525/ADR3530/

ARD3533/ADR3540/

ADR3550

Figure 38. Application Showing Kelvin Connection

It is always advantageous to use Kelvin connections whenever

possible. However, in applications where the IR drop is negligi-

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

tied together, and the device can be used in the same way as a

normal 3-terminal reference (as shown in Figure 37).

In applications with slow rising input voltage signals, the refer-

ence 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 wave-

form has both a rising and falling slew rate of at least 0.1 V/ms.

SHUTDOWN/ENABLE FEATURE

The ADR35xx references can be switched to a low power shut-

down mode when a voltage of 0.7 V or lower is input to the

ENABLE pin. Likewise, the reference becomes operational for

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 31).

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

operational continuously.