Datasheet AD688 (Analog Devices) - 9

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AD688 TEMPERATURE PERFORMANCE MAXIMUM OUTPUT CHANGE (mV)
The AD688 is designed for precision reference applications
DEVICE GRADE 0 TO +70
°
C –40
°
C TO +85
°
C
where temperature performance is critical. Extensive
AD688AQ 1.40 (TYP) 3.75
temperature testing ensures that the device’s high level of
AD688BQ 1.05 3.75
performance is maintained over the operating temperature
AD688ARWZ 4.0
range. 00815-012 Figure 12. Maximum + 10 V or −10 V Output Change Figure 11 shows the typical output voltage drift and illustrates the test methodology. The box in Figure 11 is bounded on the Duplication of these results requires a combination of high sides by the operating temperature extremes, and on top and accuracy and stable temperature control in a test system. bottom by the maximum and minimum +10 V output error Evaluation of the AD688 will produce curves similar to those in voltages measured over the operating temperature range. The Figure 11, but output readings may vary depending on the test slopes of the diagonals drawn for both the +10 V and –10 V methods and equipment utilized. outputs determine the performance grade of the device.
KELVIN CONNECTIONS EMAX – EMIN +10V OUTPUT SLOPE = T.C. =
Force and sense connections, also referred to as Kelvin
(TMAX – TMIN)
×
10
×
10–6
connections, offer a convenient method of eliminating the
2.2mV – –3.2mV = (85
°
C – –40
°
C)
×
10
×
10–6
effects of voltage drops in circuit wires. As seen in Figure 13a,
= 3ppm/
°
C
the load current and wire resistance produce an error (VERROR =
6
R × IL) at the load. The Kelvin connection of Figure 13b
5 –10V OUT
overcomes the problem by including the wire resistance within
V) 4 m
the forcing loop of the amplifier and sensing the load voltage.
3 +10V EMAX
±
10V (
The amplifier corrects for any errors in the load voltage. In the
2 SLOPE
circuit shown, the output of the amplifier would actually be at
1 FROM
10 V + VERROR and the voltage at the load would be the desired
0
10 V.
–1 LTAGE O –2 +10V OUT R –3 i = 0 RROR V +10V EMIN V = 10V R E –4 R V = 10V – RIL R –5 + I R L LOAD R i = 0 –6
00815-011
10V IL LOAD – –60 –50 –40 –30 –20 –10 0 10 20 30 40 50 60 70 80 90 100 110 120 130 V = 10V + RIL T a. b.
00815-014
MIN TEMPERATURE (
°
C) TMAX
Figure 11. Typical AD688AQ Temperature Drift Figure 13. Advantage of Kelvin Connection Each AD688A and B grade unit is tested at −40°C, −25°C, 0°C, The AD688 has three amplifiers which can be used to +25°C, +50°C, +70°C, and +85°C. This approach ensures that implement Kelvin connections. Amplifier A2 is dedicated to the the variations of output voltage that occur as the temperature ground force-sense function while uncommitted amplifiers A3 changes within the specified range will be contained within a and A4 are free for other force-sense chores. box whose diagonal has a slope equal to the maximum specified In some applications, one amplifier may be unused. In such drift. The position of the box on the vertical scale will change cases, the unused amplifier should be connected as a unity-gain from device to device as initial error and the shape of the curve follower (force and sense pins tied together) and the input vary. Maximum height of the box for the appropriate should be connected to ground. temperature range is shown in Figure 12. An unused amplifier may be used for other circuit functions as well. Figure 14 through Figure 19 show the typical performance of A3 and A4. Rev. B | Page 9 of 16 Document Outline FEATURES GENERAL DESCRIPTION FUNCTIONAL BLOCK DIAGRAM PRODUCT HIGHLIGHTS SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS ESD CAUTION PIN CONFIGURATION AND FUNCTION DESCRIPTIONS THEORY OF OPERATION APPLICATIONS CALIBRATION NOISE PERFORMANCE AND REDUCTION TURN ON TIME TEMPERATURE PERFORMANCE KELVIN CONNECTIONS DYNAMIC PERFORMANCE BRIDGE DRIVER CIRCUIT OUTLINE DIMENSIONS ORDERING GUIDE