Rev. C | Page 20 of 22
BASIC VOLTAGE REFERENCE CONNECTION
Figure 40. Basic Reference Connection
The circuit shown in Figure 40 illustrates the basic configuration
for the ADR34xx 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 mm 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 41 for an
OUTPUT CAPACITOR(S) SHOULD
BE MOUNTED AS CLOSE
FORCE PIN AS POSSIBLE.
SHOULD CONNECT AS
CLOSE TO LOAD
DEVICE AS POSSIBLE.
Figure 41. 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
force and sense pins for both VOUT and GND can simply be tied
together, and the device can be used in the same fashion as a
normal 3-terminal reference (as shown in Figure 40).
VIN SLEW RATE CONSIDERATIONS
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.
The ADR34xx 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
ENABLE voltages of 0.85 × VIN or higher. During shutdown, 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
voltage does not fall between 0.7 V and 0.85 × VIN because this
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
can simply be tied to the VIN pin, and the reference remains