ADR525 Analog Devices, ADR525 Datasheet - Page 11

• Manufacturer Part Number: ADR525
• Description: High Precision Shunt Mode Voltage References
• Manufacturer: Analog Devices

THEORY OF OPERATION
The ADR520/ADR525/ADR530/ADR540/ADR550 use the
band gap concept to produce a stable, low temperature
coefficient voltage reference suitable for high accuracy data
acquisition components and systems. The devices use the
physical nature of a silicon transistor base-emitter voltage in the
forward-biased operating region. All such transistors have
approximately a –2 mV/°C temperature coefficient (TC),
making them unsuitable for direct use as a low temperature
coefficient reference. Extrapolation of the temperature
characteristic of any one of these devices to absolute zero (with
the collector current proportional to the absolute temperature),
however, reveals that its V
silicon band gap voltage. Thus, if a voltage develops with an
opposing temperature coefficient to sum the V
temperature coefficient reference results. The ADR5xx circuit
shown in Figure 18 provides such a compensating voltage (V1)
by driving two transistors at different current densities and
amplifying the resultant V
positive temperature coefficient). The sum of V
provides a stable voltage reference over temperature.
APPLICATIONS
The ADR520/ADR525/ADR530/ADR540/ADR550 are a series
of precision shunt voltage references. They are designed to
operate without an external capacitor between the positive and
negative terminals. If a bypass capacitor is used to filter the
supply, the references remains stable.
All shunt voltage references require an external bias resistor
(R
Figure 19). The R
(I
voltage can vary, the R
following considerations:
•
L
BIAS
) and the reference (I
The R
current to the ADR5xx, even when the supply voltage is at
its minimum value and the load current is at its maximum
value.
) between the supply voltage and the reference (see
V
BE
+
–
BIAS
∆
V
V1
BE
+
–
must be small enough to supply the minimum I
+
–
BIAS
Figure 18. Circuit Schematic
sets the current that flows through the load
BIAS
IN
). Because the load and the supply
needs to be chosen based on the
BE
BE
approaches approximately the
difference (∆V
BE
, which has a
BE
BE
, a zero
and V1
V+
V–
Rev. A | Page 11 of 16
IN
ADR520/ADR525/ADR530/ADR540/ADR550
•
Given these conditions, the R
voltage (V
I
Precision Negative Voltage Reference
The ADR5xx is suitable for applications where a precise
negative voltage is desired. Figure 20 shows the ADR5xx
configured to provide a negative output.
Output Voltage Trim
The ADR5xx TRIM terminal can be used to adjust the output
voltage over a range of ±0.5%. This allows systems designers to
trim system errors by setting the reference to a voltage other
than the preset output voltage. An external mechanical or elec-
trical potentiometer can be used for this adjustment. Figure 21
illustrates how the output voltage can be trimmed by using the
AD5273, an Analog Devices 10 kΩ potentiometer.
Q
), and the ADR5xx output voltage (V
The R
10 mA when the supply voltage is at its maximum value
and the load current is at its minimum value.
R
BIAS
Figure 20. Negative Precision Reference Configuration
CC
BIAS
=
), the ADR5xx load and operating current (I
V
ADR530
must be large enough so that I
CC
I
L
−
−
R
BIAS
V
Figure 21. Output Voltage Trim
I
IN
OUT
R
I
Figure 19. Shunt Reference
IN
V
BIAS
CC
V
ADR525
S
470k
R1
I
BIAS
IN
ADR550
R
Ω
+ I
BIAS
I
L
is determined by the supply
L
V
CC
–2.5V
POTENTIOMETER
10k
AD5273
OUT
V
V
Ω
OUT
OUT
).
IN
does not exceed
L
and
(3)