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