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2.5 V/3.0 V
High Precision Reference
AD780
FEATURES
Pin-Programmable 2.5 V or 3.0 V Output
Ultralow Drift: 3 ppm/
FUNCTIONAL BLOCK DIAGRAM
C max
High Accuracy: 2.5 V or
3.0 V
8
+V
IN
NC
7
6
1 mV max
2
Hz
Noise Reduction Capability
Low Quiescent Current: 1 mA max
Output Trim Capability
Plug-In Upgrade for Present References
Temperature Output Pin
Series or Shunt Mode Operation (
Ö
AD780
R10
R11
NC
1
6
V
OUT
6
2.5 V,
6
3.0 V)
R13
Q6
Q7
5
TRIM
R5
R16
TEMP
3
R14
R15
PRODUCT DESCRIPTION
The AD780 is an ultrahigh precision bandgap reference voltage
which provides a 2.5 V or 3.0 V output from inputs between
4.0 V and 36 V. Low initial error and temperature drift com-
bined with low output noise and the ability to drive any value of
capacitance make the AD780 the ideal choice for enhancing the
performance of high resolution ADCs and DACs and for any
general purpose precision reference application. A unique low
headroom design facilitates a 3.0 V output from a 5.0 V ± 10%
input, providing a 20% boost to the dynamic range of an ADC,
over performance with existing 2.5 V references.
The AD780 can be used to source or sink up to 10 mA and can
be used in series or shunt mode, thus allowing positive or nega-
tive output voltages without external components. This makes it
suitable for virtually any high performance reference application.
Unlike some competing references, the AD780 has no “region
of possible instability.” The part is stable under all load condi-
tions when a 1 mF bypass capacitor is used on the supply.
A temperature output pin is provided on the AD780. This pro-
vides an output voltage that varies linearly with temperature, al-
lowing the AD780 to be configured as a temperature transducer
while providing a stable 2.5 V or 3.0 V output.
The AD780 is a pin-compatible performance upgrade for the
LT1019(A)–2.5 and the AD680. The latter is targeted toward
low power applications.
The AD780 is available in two grades in plastic DIP, SOIC and
cerdip packages. The AD780AN, AD780AR, AD780BN and
AD780BR are specified for operation from –40°C to +85°C.
The AD780SQ and AD780SQ/883B are specified for –55°C to
+125°C operation.
R4
4
8
GND
O/P SELECT
2.5V - NC
3.0V - GND
NC = NO CONNECT
PRODUCT HIGHLIGHTS
1. The AD780 provides a pin-programmable 2.5 V or 3.0 V
output from a 4 V to 36 V input.
2. Laser trimming of both initial accuracy and temperature
coefficients results in low errors over temperature without the
use of external components. The AD780BN has a maximum
variation of 0.8 mV from –40°C to +85°C.
3. For applications requiring even higher accuracy, an optional
fine-trim connection is provided.
4. The AD780 noise is extremely low, typically 4 mV p-p from
0.1 Hz to 10 Hz and
a
wideband spectral noise density of
typically 100 nV/ÖHz. This can be further reduced if desired,
by simply using two external capacitors.
5. The temperature output pin enables the AD780 to be config-
ured as a temperature transducer while providing a stable
output reference voltage.
REV. A
Information furnished by Analog Devices is believed to be accurate and
reliable. However, no responsibility is assumed by Analog Devices for its
use, nor for any infringements of patents or other rights of third parties
which may result from its use. No license is granted by implication or
otherwise under any patent or patent rights of Analog Devices.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 617/329-4700
Fax: 617/326-8703
Low Noise: 100 nV/
AD780–SPECIFICATIONS
(T
A
= +25
8
C, V
IN
= +5 V unless otherwise noted)
AD780AN/AR/SQ
AD780BN/BR
Parameter
Min
Typ
Max
Min
Typ
Max
Units
OUTPUT VOLTAGE
2.5 V Out
2.495
2.505
2.499
2.501
Volts
3.0 V Out
2.995
3.005
2.999
3.001
Volts
OUTPUT VOLTAGE DRIFT
1
–40
°
C
7
3
ppm/
°
C
–55°C to +125°C
20
ppm/°C
LINE REGULATION
2.5 V Output, 4 V
£
+V
IN
£
36 V
T
MIN
to T
MAX
10
*
m
V/V
3.0 V Output, 4.5 V
£
+V
IN
£
36 V
T
MIN
to T
MAX
10
*
m
V/V
LOAD REGULATION, SERIES MODE
Sourcing 0 < I
OUT
< 10 mA
50
*
m
V/mA
T
MIN
to T
MAX
75
*
m
V/mA
Sinking –10 < I
OUT
< 0 mA
75
*
m
V/mA
–40
°
C to +85
°
C
75
*
m
V/mA
–55
°
C to +125
°
C
150
*
m
V/mA
LOAD REGULATION, SHUNT MODE
I < I
SHUNT
< 10 mA
75
*
mV/mA
QUIESCENT CURRENT, 2.5 V SERIES MODE
2
–40°C to +85°C
0.75
1.0
*
*
mA
–55°C to +125°C
0.8
1.3
*
*
mA
MINIMUM SHUNT CURRENT
0.7
1.0
*
*
mA
OUTPUT NOISE
0.1 Hz to 10 Hz
4
*
*
mV p-p
Spectral Density, 100 Hz
100
*
*
nV/ÖHz
LONG TERM STABILITY
3
20
*
±
ppm/1000 Hr
TRIM RANGE
4.0
*
±
%
TEMPERATURE PIN
Voltage Output @ 25
°
C
500
560
620
*
*
*
mV
Temperature Sensitivity
1.9
*
mV/
°
C
Output Resistance
3
*
k
W
SHORT CIRCUIT CURRENT TO GROUND
30
*
mA
TEMPERATURE RANGE
Specified Performance (A, B)
–40
+85
*
*
°C
Operating Performance (A, B)
4
–55
+125
*
*
°C
Specified Performance (S)
–55
+125
*
*
°C
Operating Performance (S)
–55
+125
*
*
°C
A/V above an input voltage of 5 V.
3
The long term stability specification is noncumulative. The drift in subsequent 1000 hr. periods is significantly lower than in the first 1000 hr. period.
4
The operating temperature range is defined as the temperature extremes at which the device will still function. Parts may deviate from their specified performance
outside their specified temperature range.
*Same as AD780AN/AR/SQ specification.
Specifications subject to change without notice.
m
A to the quiescent current. Also, Iq increases by 2
m
CAUTION
ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily
accumulate on the human body and test equipment and can discharge without detection.
Although the AD780 features proprietary ESD protection circuitry, permanent damage may
occur on devices subjected to high energy electrostatic discharges. Therefore, proper ESD
precautions are recommended to avoid performance degradation or loss of functionality.
WARNING!
ESD SENSITIVE DEVICE
–2–
REV. A
°
C to +85
NOTES
1
Maximum output voltage drift is guaranteed for all packages.
2
3.0 V mode typically adds 100
AD780
ABSOLUTE MAXIMUM RATINGS*
V
IN
to Ground . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 V
Trim Pin to Ground . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 V
Temp Pin to Ground . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 V
Power Dissipation (25°C) . . . . . . . . . . . . . . . . . . . . . . 500 mW
Storage Temperature . . . . . . . . . . . . . . . . . . . –65°C to +150°C
Lead Temperature (Soldering, 10 sec) . . . . . . . . . . . . . . 300°C
Output Protection: Output safe for indefinite short to ground
and momentary short to V
IN
.
ESD Classification . . . . . . . . . . . . . . . . . . . . . Class 1 (1000 V)
*Stresses above those listed under “Absolute Maximum Ratings” may cause
permanent damage to the device. This is a stress rating only and functional
operation of the device at these or any conditions above those indicated in the
operational specification is not implied. Exposure to absolute maximum specifica-
tions for extended periods may affect device reliability.
ORDERING GUIDE
Initial Temperature Temperature
Model
Error Coefficient Range
Package
AD780AN
5 mV 7 ppm/°C
–40°C to +85°C Plastic
AD780AR
5 mV 7 ppm/°C
–40°C to +85°C SOIC
AD780BN
1 mV 3 ppm/°C
–40°C to +85°C Plastic
AD780BR
1 mV 3 ppm/°C
–40°C to +85°C SOIC
AD780SQ
5 mV 20 ppm/°C
–55°C to +125°C Cerdip
AD780SQ/883B 5 mV 20 ppm/
°
C
–55
°
C to +125
°
C Cerdip
THEORY OF OPERATION
Bandgap references are the high performance solution for low
supply voltage and low power voltage reference applications. In
this technique a voltage with a positive temperature coefficient is
combined with the negative coefficient of a transistor’s Vbe to
produce a constant
bandgap
voltage.
In the AD780, the bandgap cell contains two npn transistors
(Q6 and Q7) which differ in emitter area by 12
3
. The differ-
ence in their Vbe’s produces a PTAT current in R5. This in
turn produces a PTAT voltage across R4, which when com-
bined with the Vbe of Q7, produces a voltage Vbg that does not
vary with temperature. Precision laser trimming of the resistors
and other patented circuit techniques are used to further en-
hance the drift performance.
PIN CONFIGURATION
8-Pin Plastic DIP, SOIC and Cerdip Packages
NC
1
8
2.5/3.0V SELECT
(NC OR GND)
NC
V
OUT
+V
IN
2
AD780
7
TEMP
3
TOP VIEW
(Not to Scale)
6
GND
4
5
TRIM
NC = NO CONNECT
+V
IN
NC
7
DIE LAYOUT
2
AD780
R10
R11
NC
1
6
V
OUT
R13
Q6
Q7
5
TRIM
R5
R16
TEMP
3
R14
R15
R4
4
8
GND
O/P SELECT
2.5V - NC
3.0V - GND
NOTES
Both V
OUT
pads should be connected to the output
Die Thickness:
The standard thickness of Analog Devices Bipolar dice is
24 mils ± 2 mils.
Die Dimensions:
The dimensions given have a tolerance of ± 2 mils.
Backing:
The standard backside surface is silicon (not plated). Analog Devices
does not recommend gold-backed dice for most applications.
Edges:
A diamond saw is used to separate wafers into dice thus providing per-
pendicular edges half-way through the die.
In contrast to scribed dice, this technique provides a more uniform die shape
and size. The perpendicular edges facilitate handling (such as tweezer pick-up)
while the uniform shape and size simplifies substrate design and die attach.
Top Surface:
The standard top surface of the die is covered by a layer of
glassivation. All areas are covered except bonding pads and scribe lines.
Surface Metalization:
The metalization to Analog Devices bipolar dice is alu-
minum. Minimum thickness is 10,000Å.
Bonding Pads:
All bonding pads have a minimum size of 4.0 mils by 6.0 mils .
The passivation windows have a 3.6 mils by 5.6 mils minimum size.
NC = NO CONNECT
Figure 1. Schematic Diagram
The output voltage of the AD780 is determined by the configu-
ration of resistors R13, R14 and R15 in the amplifier’s feedback
loop. This sets the output to either 2.5 V or 3.0 V depending on
whether R15 (Pin 8) is grounded or not connected.
A unique feature of the AD780 is the low headroom design of
the high gain amplifier which produces a precision 3 V output
from an input voltage as low as 4.5 V (or 2.5 V from a 4.0 V in-
put). The amplifier design also allows the part to work with
V
IN
= V
OUT
when current is forced into the output terminal.
This allows the AD780 to work as a two terminal shunt regula-
tor providing a –2.5 V or –3.0 V reference voltage output with-
out external components.
REV. A
–3–
AD780
The PTAT voltage is also used to provide the user with a ther-
mometer output voltage (at Pin 3) which increases at a rate of
approximately 2 mV/°C.
The AD780’s NC Pin 7 is a 20 kW resistor to V+ which is used
solely for production test purposes. Users who are currently us-
ing the LT1019 self-heater pin (Pin 7) must take into account
the different load on the heater supply.
100
10
APPLYING THE AD780
The AD780 can be used without any external components to
achieve specified performance. If power is supplied to Pin 2 and
Pin 4 is grounded, Pin 6 provides a 2.5 V or 3.0 V output de-
pending on whether Pin 8 is left unconnected or grounded.
A bypass capacitor of 1 mF (V
IN
to GND) should be used if the
load capacitance in the application is expected to be greater
than 1 nF. The AD780 in 2.5 V mode typically draws 700 mA of
Iq at 5 V. This increases by ~2 mA/V up to 36 V.
1
0.1
0.1
1
10
100
LOAD CAPACITOR, C1 –
m
F
Figure 3. Compensation and Load
Capacitor Combinations
C1 and C2 also improve the settling performance of the AD780
when subjected to load transients. The improvement in noise
performance is shown in Figures 4, 5 and 6 following.
2
7
+V
IN
NC
1
NC
V
OUT
6
AMPLIFIER GAIN = 100
NO AMPLIFIER
AD780
1
m
F
R
NULL
100
m
V
1s
20
m
V
10ms
TRIM
5
R POT.
100
100
3
TEMP
90
90
O/P SELECT
2.5V - NC
3.0V - GND
GND
4
8
10
10
0%
0%
NC = NO CONNECT
Figure 2. Optional Fine Trim Circuit
Initial error can be nulled using a single 25 kW potentiometer
connected between V
OUT
, Trim and GND. This is a coarse trim
with an adjustment range of ± 4% and is only included here for
compatibility purposes with other references. A fine trim can be
implemented by inserting a large value resistor (e.g. 1–5 MW) in
series with the wiper of the potentiometer. See Figure 2 above.
The trim range, expressed as a fraction of the output, is simply
greater than or equal to 2.1 kW/R
NULL
for either the 2.5 V or
3.0 V mode.
The external null resistor affects the overall temperature coeffi-
cient by a factor equal to the percentage of V
OUT
nulled.
For example a 1 mV (.03%) shift in the output caused by the
trim circuit, with a 100 ppm/°C null resistor will add less than
0.06 ppm/°C to the output drift (0.03%
3
200 ppm/°C, since
the resistors internal to the AD780 also have temperature coeffi-
cients of less than 100 ppm/°C).
0.1 TO 10Hz
10Hz TO 10kHz
Figure 4. Stand-Alone Noise Performance
2
7
+V
IN
NC
1
NC
V
OUT
6
1
m
F
AD780
TRIM
5
3
TEMP
C1
O/P SELECT
2.5V - NC
3.0V - GND
C2
GND
4
8
NC = NO CONNECT
Figure 5. Noise Reduction Circuit
NOISE PERFORMANCE
The impressive noise performance of the AD780 can be further
improved if desired by the addition of two capacitors: a load ca-
pacitor C1 between the output and ground, and a compensation
capacitor C2 between the TEMP pin and ground. Suitable val-
ues are shown in Figure 3.
–4–
REV. A
AD780
NOISE COMPARISON
The wideband noise performance of the AD780 can also be ex-
pressed in ppm. The typical performance with C1, C2 is
0.6 ppm and without external capacitors is 1.2 ppm.
This performance is respectively 7
3
and 3
3
lower than the
specified performance of the LT1019.
The voltage V
TEMP
is 560 mV at 25°C, and the temperature co-
efficient is approximately 2 mV/°C. Figure 8 following shows
the typical V
TEMP
characteristic curve over temperature.
4.25
CIRCUIT CALIBRATED AT 25
°
C
4.00
NO AMPLIFIER
3.75
20
V
10ms
3.50
100
3.25
90
10mV PER
°
C
3.00
2.75
2.50
10
0%
2.25
2.00
–75
–50
–25
0
25
50
75
100
125
150
10Hz TO 10kHz
TEMPERATURE –
°
C
Figure 6. Reduced Noise Performance with C1 = 100
m
F,
C2 = 100 nF
TEMPERATURE PERFORMANCE
The AD780 provides superior performance over temperature by
means of a combination of patented circuit design techniques,
precision thin film resistors and drift trimming. Temperature
performance is specified in terms of ppm/°C, but because of
nonlinearity in the temperature characteristic, the Box-Test
method is used to test and specify the part. The nonlinearity
takes the form of the characteristic S-shaped curve shown in
Figure 7. The Box-Test method forms a rectangular box around
this curve, enclosing the maximum and minimum output volt-
ages over the specified temperature range. The specified drift is
equal to the slope of the diagonal of this box.
Figure 8. Temperature Pin Transfer Characteristic
Since the TEMP voltage is acquired from the bandgap core cir-
cuit, current pulled from this pin will have an effect on V
OUT
.
Care must be taken to buffer the TEMP output with a suitable
op amp, e.g., an AD OP07, AD820 or AD711 (all of which
would result in less than a 100 mV change in V
OUT
). The rela-
tionship between I
TEMP
and V
OUT
is as follows:
DV
OUT
5.8 mV/mA
3
I
TEMP
(2.5 V range)
or
DV
OUT
= 6.9 mV/mA
3
I
TEMP
(3.0 V range)
TEMPERATURE TRANSDUCER CIRCUIT
The circuit shown in Figure 9 is a temperature transducer which
a amplifies the TEMP output voltage by a gain of a little over 5
to provide a wider full scale output range. The trimpot can be
used to adjust the output so it varies exactly by 10 mV/°C.
To minimize resistance changes with temperature, resistors with
low temperature coefficients, such as metal film resistors should
be used.
2.0
1.6
1.2
+5V
0.8
0.4
2
0
V
IN
–0.4
1µF
TEMP
3
AD780
AD820
10mV/
°
C
–0.8
–60
–40
–20
0
20
40
60
80
100
120
140
TEMPERATURE –
°
C
R
F
Figure 7. Typical AD780BN Temperature Drift
TEMPERATURE OUTPUT PIN
The AD780 provides a “TEMP” output (Pin 3) that varies lin-
early with temperature. This output can be used to monitor
changes in system ambient temperature and to initiate calibra-
tion of the system if desired.
GND
1.27k
(1%)
6.04k (1%)
R
B
4
R
BP
200
W
Figure 9. Differential Temperature Transducer
REV. A
–5–
m
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