ADS8671 Datasheet, PDF (27/72 Pages) Texas Instruments – 14-Bit, High-Speed, Single-Supply, SAR ADC Data Acquisition System with Programmable, Bipolar Input Ranges

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ADS8671, ADS8675
SBAS779 – DECEMBER 2016
The device internal reference is factory-trimmed to ensure the initial accuracy specification. The histogram in
Figure 53 shows the distribution of the internal voltage reference output taken from more than 3420 production
devices.
1000
900
800
700
600
500
400
300
200
100
0
4.094 4.0945 4.095 4.0955 4.096 4.0965 4.097 4.0975 4.098
REFIO Voltage (V)
D060
Figure 53. Internal Reference Accuracy Histogram at Room Temperature
The initial accuracy specification for the internal reference can be degraded if the die is exposed to any
mechanical or thermal stress. Heating the device when being soldered to a printed circuit board (PCB) and any
subsequent solder reflow is a primary cause for shifts in the V_REF value. The main cause of thermal hysteresis is
a change in die stress and is therefore a function of the package, die-attach material, and molding compound, as
well as the layout of the device itself.
In order to illustrate this effect, 30 devices were soldered using lead-free solder paste with the manufacturer
suggested reflow profile, as explained in application report AN-2029 Handling & Process Recommendations
(SNOA550). The internal voltage reference output is measured before and after the reflow process and the
typical shift in value is shown in Figure 54. Although all tested units exhibit a positive shift in their output
voltages, negative shifts are also possible. Note that the histogram in Figure 54 shows the typical shift for
exposure to a single reflow profile. Exposure to multiple reflows, which is common on PCBs with surface-mount
components on both sides, causes additional shifts in the output voltage. If the PCB is to be exposed to multiple
reflows, solder the ADS867x in the second pass to minimize device exposure to thermal stress.
7
6.5
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5.5
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4.5
4
3.5
3
2.5
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1.5
1
0.5
0
-7 -6 -5 -4 -3 -2 -1 0 1
Error in REFIO Voltage (mV)
D070
Figure 54. Solder Heat Shift Distribution Histogram
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