AD590
GENERAL DESCRIPTION
PTAT current. Figure 6 is the schematic diagram of the AD590.
In this figure, Q8 and Q11 are the transistors that produce the
PTAT voltage. R5 and R6 convert the voltage to current. Q10,
whose collector current tracks the collector currents in Q9 and
Q11, supplies all the bias and substrate leakage current for the
rest of the circuit, forcing the total current to be PTAT. R5 and
R6 are laser-trimmed on the wafer to calibrate the device at 25°C.
The AD590H has 60 μ inches of gold plating on its Kovar leads
and Kovar header. A resistance welder is used to seal the nickel
cap to the header. The AD590 chip is eutectically mounted to
the header and ultrasonically bonded to with 1 mil aluminum
wire. Kovar composition: 53% iron nominal; 29% 1% nickel;
17% 1% cobalt; 0.65% manganese max; 0.20% silicon max;
0.10% aluminum max; 0.10% magnesium max; 0.10% zirconium
max; 0.10% titanium max; and 0.06% carbon max.
Figure 7 shows the typical V–I characteristic of the circuit at
25°C and the temperature extremes.
The AD590F is a ceramic package with gold plating on its
Kovar leads, Kovar lid, and chip cavity. Solder of 80/20 Au/Sn
composition is used for the 1.5 mil thick solder ring under the
lid. The chip cavity has a nickel underlay between the metallization
and the gold plating. The AD590 chip is eutectically mounted in
the chip cavity at 410°C and ultrasonically bonded to with 1 mil
aluminum wire. Note that the chip is in direct contact with the
ceramic base, not the metal lid. When using the AD590 in die
form, the chip substrate must be kept electrically isolated
(floating) for correct circuit operation.
+
R1
R2
260Ω 1040Ω
Q2
Q5
Q3
C1
Q1
Q4
Q6
26pF
Q12
Q8
Q7
R4
11kΩ
CHIP
SUBSTRATE
R3
5kΩ
66MILS
Q9
Q10
Q11
1
V+
8
1
R5
146Ω
R6
820Ω
–
Figure 6. Schematic Diagram
42MILS
V–
+150°C
423
THE AD590 IS AVAILABLE IN LASER-TRIMMED CHIP FORM;
CONSULT THE CHIP CATALOG FOR DETAILS
+25°C
–55°C
298
218
Figure 5. Metallization Diagram
CIRCUIT DESCRIPTION1
The AD590 uses a fundamental property of the silicon
transistors from which it is made to realize its temperature
proportional characteristic: if two identical transistors are
operated at a constant ratio of collector current densities, r,
then the difference in their base-emitter voltage is (kT/q)(In r).
Because both k (Boltzman’s constant) and q (the charge of an
electron) are constant, the resulting voltage is directly
proportional to absolute temperature (PTAT).
0
1
2
3
4
5
6
30
SUPPLY VOLTAGE (V)
Figure 7. V–I Plot
1 For a more detailed description, see M.P. Timko, “A Two-Terminal IC
Temperature Transducer,” IEEE J. Solid State Circuits, Vol. SC-11, p. ꢀ84-ꢀ88,
Dec. 19ꢀ6. Understanding the Specifications–AD590.
In the AD590, this PTAT voltage is converted to a PTAT current
by low temperature coefficient thin-film resistors. The total
current of the device is then forced to be a multiple of this
Rev. D | Page 6 of 16
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