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ADR381ARTZ-REEL7

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P16
ADR380/ADR381
Rev. C | Page 10 of 16
TERMINOLOGY
Temperature Coefficient
The change of output voltage over the operating temperature
change and normalized by the output voltage at 25°C, expressed
in ppm/°C. The equation follows:
6
10
)(
)()(
]Cppm/[ ×
−×°
−
=°
12
OUT
1
OUT
2
OUT
OUT
TTC)(25V
TVTV
TCV
where:
V
OUT
(25°C) = V
OUT
at 25°C.
V
OUT
(T
1
) = V
OUT
at Temperature 1.
V
OUT
(T
2
) = V
OUT
at Temperature 2.
Line Regulation
The change in output voltage due to a specified change in input
voltage. It includes the effects of self-heating. Line regulation is
expressed in either percent per volt, parts-per-million per volt,
or microvolts per volt change in input voltage.
Load Regulation
The change in output voltage due to a specified change in load
current. It includes the effects of self-heating. Load regulation is
expressed in either microvolts per milliampere, parts-per-million
per milliampere, or ohms of dc output resistance.
Long-Term Stability
A typical shift in output voltage over 1000 hours at a controlled
temperature. Figure 24 and Figure 25 show a sample of parts
measured at different intervals in a controlled environment of
50°C for 1000 hours.
6
10
)(
)()(
]ppm[
)()(
×
−
=Δ
−=Δ
0
UT
O
1
OUT
0
OUT
UT
O
1
UT
O
0
OUTOUT
tV
tVtV
V
tVtVV
where:
V
OUT
(t
0
) = V
OUT
at Time 0.
V
OUT
(t
1
) = V
OUT
after 1000 hours of operation at a controlled
temperature.
Note that 50°C was chosen because most applications run at a
higher temperature than 25°C.
Thermal Hysteresis
The change of output voltage after the device is cycled through
temperature from +25°C to −40°C to +85°C and back to +25°C.
This is a typical value from a sample of parts put through such
a cycle.
6_
_
__
10
)C25(
]ppm[ ×
°
−°
=
−°=
C)(25V
VV
V
VC)(25VV
OUT
TCOUTOUT
HYSOUT
TCOUT
UT
O
HYSOUT
where:
V
OUT
(25°C) = V
OUT
at 25°C.
V
OUT_TC
= V
OUT
at 25°C after a temperature cycle from +25°C to
−40°C to +85°C and back to +25°C.
ADR380/ADR381
Rev. C | Page 11 of 16
THEORY OF OPERATION
Band gap references are the high performance solution for low
supply voltage and low power voltage reference applications, and
the ADR380/ADR381 are no exception. However, the uniqueness
of this product lies in its architecture. As shown in Figure 26,
the ideal zero TC band gap voltage is referenced to the output,
not to ground. The band gap cell consists of the PNP pair Q51
and Q52, running at unequal current densities. The difference
in V
BE
results in a voltage with a positive TC that is amplified
by the ratio of 2 × R58/R54. This PTAT voltage, combined with
the V
BE
of Q51 and Q52, produce the stable band gap voltage.
Reduction in the band gap curvature is performed by the ratio
of the two resistors, R44 and R59. Precision laser trimming and
other patented circuit techniques are used to further enhance
the drift performance.
GND
V
OUT
V
IN
Q1
R59
R54
Q51
R60
R61
R48
R49
R44
R58
R53
Q52
–
+
02175-026
Figure 26. Simplified Schematic
DEVICE POWER DISSIPATION CONSIDERATIONS
The ADR380/ADR381 are capable of delivering load currents to
5 mA with an input voltage that ranges from 2.8 V (ADR381 only)
to 15 V. When this device is used in applications with large input
voltages, take care to avoid exceeding the specified maximum
power dissipation or junction temperature that may result in
premature device failure. Use the following formula to calculate
a device’s maximum junction temperature or dissipation:
JA
A
J
D
TT
P
θ
−
=
where:
P
D
is the device power dissipation,
T
J
and T
A
are junction and ambient temperatures, respectively.
θ
JA
is the device package thermal resistance.
INPUT CAPACITOR
An input capacitor is not required on the ADR380/ADR381.
There is no limit for the value of the capacitor used on the input,
but a capacitor on the input improves transient response in
applications where the load current suddenly increases.
OUTPUT CAPACITOR
The ADR380/ADR381 do not need an output capacitor for
stability under any load condition. Using an output capacitor,
typically 0.1 μF, removes any very low level noise voltage and does
not affect the operation of the part. The only parameter that
degrades by applying an output capacitor is turn-on time. (This
varies depending on the size of the capacitor.) Load transient
response is also improved with an output capacitor, which acts
as a source of stored energy for a sudden increase in load current.
ADR380/ADR381
Rev. C | Page 12 of 16
APPLICATIONS INFORMATION
STACKING REFERENCE ICs FOR ARBITRARY
OUTPUTS
Some applications may require two reference voltage sources,
which are a combined sum of standard outputs. The following
circuit shows how this stacked output reference can be
implemented:
GND
V
OUT
V
IN
3
GND
V
OUT
V
IN
C2
1µF
C1
0.1µF
C3
0.1µF
C4
1µF
3
R1
3.9kΩ
V
OUT2
V
OUT1
2
2
1
1
V
IN
U2
ADR380/
ADR381
U1
ADR380/
ADR381
0
2175-027
Figure 27. Stacking Voltage References with the ADR380/ADR381
Two ADR380s or ADR381s are used; the outputs of the individ-
ual references are simply cascaded to reduce the supply current.
Such configuration provides two output voltages: V
OUT1
and
V
OUT2
. V
OUT1
is the terminal voltage of U1, while V
OUT2
is the
sum of this voltage and the terminal voltage of U2. U1 and U2
can be chosen for the two different voltages that supply the
required outputs.
While this concept is simple, a precaution is in order. Because
the lower reference circuit must sink a small bias current from
U2, plus the base current from the series PNP output transistor
in U2, the external load of either U1 or R1 must provide a path
for this current. If the U1 minimum load is not well-defined,
Resistor R1 should be used, set to a value that conservatively
passes 600 μA of current with the applicable V
OUT1
across it. Note
that the two U1 and U2 reference circuits are locally treated as
macrocells, each having its own bypasses at input and output for
optimum stability. Both U1 and U2 in this circuit can source dc
currents up to their full rating. The minimum input voltage, V
IN
, is
determined by the sum of the outputs, V
OUT2
, plus the 300 mV
dropout voltage of U2.
A NEGATIVE PRECISION REFERENCE WITHOUT
PRECISION RESISTORS
In many current-output CMOS DAC applications where the
output signal voltage must be of the same polarity as the
reference voltage, it is often required to reconfigure a current-
switching DAC into a voltage-switching DAC through the use
of a 1.25 V reference, an op amp, and a pair of resistors. Using
a current switching DAC directly requires an additional opera-
tional amplifier at the output to reinvert the signal. A negative
voltage reference is then desirable from the point that an additional
operational amplifier is not required for either reinversion
(current-switching mode) or amplification (voltage-switching
mode) of the DAC output voltage. In general, any positive voltage
reference can be converted into a negative voltage reference
through the use of an operational amplifier and a pair of matched
resistors in an inverting configuration. The disadvantage to this
approach is that the largest single source of error in the circuit is
the relative matching of the resistors used.
The circuit in Figure 28 avoids the need for tightly matched
resistors with the use of an active integrator circuit. In this
circuit, the output of the voltage reference provides the input
drive for the integrator. The integrator, to maintain circuit
equilibrium, adjusts its output to establish the proper relation-
ship between the reference V
OUT
and GND. Thus, any negative
output voltage desired can be chosen by substituting for the
appropriate reference IC. A precaution should be noted with
this approach: although rail-to-rail output amplifiers work best
in the application, these operational amplifiers require a finite
amount (mV) of headroom when required to provide any load
current. The choice for the circuit’s negative supply should take
this issue into account.
GND
V
OUT
V
IN
C2
0.1µF
3
+5V
–V
REF
V
IN
2
A1
1
U2
–5V
OP195
–V
+V
C1
1µF
U1
ADR380/
ADR381
R4
1kΩ
R3
100kΩ
C3
1µF
C4
1µF
R5
100Ω
0
2175-028
Figure 28. Negative Precision Voltage Reference Using No Precision Resistors
PRECISION CURRENT SOURCE
Many times in low power applications, the need arises for a
precision current source that can operate on low supply voltages.
As shown in Figure 29, the ADR380/ADR381 can be configured
as a precision current source. The circuit configuration illustrated
is a floating current source with a grounded load. The reference
output voltage is bootstrapped across R
SET
(R1 + P1), which sets
the output current into the load. With this configuration, circuit
precision is maintained for load currents in the range from the
reference supply current, typically 90 μA to approximately 5 mA.
GND
V
OUT
V
IN
3
V
IN
2
R1
1
R
L
P1
I
OUT
I
SY
ADJUST
U1
ADR380/
ADR381
C3
1µF
C1
1µF
C2
0.1µF
02175-029
Figure 29. Precision Current Source
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P16

ADR381ARTZ-REEL7

Mfr. #:
Buy ADR381ARTZ-REEL7
Manufacturer:
Analog Devices Inc.
Description:
Voltage References 2.048V & 2.5V Bandgap
Lifecycle:
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