AD580
THEORY OF OPERATION
+V
IN
The AD580 family (AD580, AD581, AD584, AD589) uses the
bandgap concept to produce a stable, low temperature coef-
ficient voltage reference suitable for high accuracy data acqui-
sition components and systems. The device makes use of the
underlying physical nature of a silicon transistor base-emitter
voltage in the forward-biased operating region. All such tran-
sistors have approximately a –2 mV/°C temperature coefficient,
unsuitable for use directly as a low TC reference. Extrapolation
of the temperature characteristic of any one of these devices to
absolute zero (with an emitter current propor-tional to the
absolute temperature), however, reveals that it will go to a VBE of
1.205 V at 0 K, as shown in Figure 3. Thus, if a voltage could be
developed with an opposing temperature coefficient to sum
with VBE to total 1.205 V, a 0 TC reference would result and
operation from a single, low voltage supply would be possible.
The AD580 circuit provides such a compensating voltage, V1 in
Figure 4, by driving two transistors at different current densities
and amplifying the resulting VBE difference (∆VBE—which now
has a positive TC). The sum, VZ, is then buffered and amplified
up to 2.5 V to provide a usable reference-voltage output. Figure
5 shows the schematic diagram of the AD580.
R8
R7
R
R
4
5
V
V
= V 1 +
= 2.5V
I
≅ I
1
OUT
Z
2
R4
R5
Q2
8A
Q1
A
= V + V
BE
Z
1
R
R
V
(Q1)
1
2
BE
= V + 2
BE
∆
V
∆
V
BE
R2
BE
R
R
kT
J
J
1
2
1
2
= V + 2
BE
ln
q
R
1
V
= 2
R1
∆
R
2
V
1
2I = I + I
BE
1
1
2
= 1.205V
COM
Figure 4. Basic Bandgap-Reference Regulator Circuit
+E
R12
R13
Q13
Q14
Q4
Q3
Q7
R8
R7
R6
Q12
Q10
Q11
Q6
R9
Q8
Q15
Q5
2.5V
OUT
Q9
R10
R4
R5
C1
The AD580 operates as a 3-terminal reference, meaning that no
additional components are required for biasing or current
setting. The connection diagram, Figure 6, is quite simple.
R3
Q2
8A
Q1
A
R2
R1
R11
–E
COM
1.5
Figure 5. Schematic Diagram
CONSTANT SUM = 1.205V
1.205
+E
FOR BOTH
DEVICES
4.5
≤
V
≤ 30V
IN
1.0
E
OUT
AD580
V
VS. TEMPERATURE
BE
LOAD
FOR TWO TYPICAL
DEVICES (IE α T)
0.5
–E
Figure 6. Connection Diagram
REQUIRED
COMPENSATION
VOLTAGE–
SAME DEVICES
VOLTAGE VARIATION VERSUS TEMPERATURE
0
–273°C
0K
–200°C
73K
–100°C
173K
0°C
273K
100°C
373K
Some confusion exists in the area of defining and specifying
reference voltage error over temperature. Historically, references
are characterized using a maximum deviation per degree
Centigrade; i.e., 10 ppm/°C. However, because of the
TEMPERATURE
Figure 3. Extrapolated Variation of Base-Emitter Voltage with Temperature
(IEαT), and Required Compensation, Shown for Two Different Devices
inconsistent nonlinearities in Zener references (butterfly or S
type characteristics), most manufacturers use a maximum limit
error band approach to characterize their references. This
technique measures the output voltage at 3 to 5 different
temperatures and guarantees that the output voltage deviation
will fall within the guaranteed error band at these discrete
temperatures. This approach, of course, makes no mention or
guarantee of performance at any other temperature within the
operating temperature range of the device.
Rev. B | Page 5 of 8
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