Datasheet

HIGH IMPEDANCE MODE
LINEARITY ERRORvsSOURCEIMPEDANCE
ExternalSourceImpedance(kΩ)
ChangeinWorst-Case
LinearityError(LSBs)
10
9
8
7
6
5
4
3
2
1
0
0 1 2 3 4 5 6 7 8 9 10 11 12 14 1513
T
A
=+25°C
Acquisition Time=5µs
ILE
DLE
ADS8513
www.ti.com ...................................................................................................................................................... SLAS486C – JUNE 2007 – REVISED JANUARY 2009
The input impedance results from the various connections and the internal resistor values (refer to the block
diagram on the front page of this data sheet). The internal resistor values are typical and can change by ± 30%
as a result of process variations. However, the ratio matching of the resistors is considerably better than this
range. Thus, the input range only varies a few tenths of a percent from part to part, while the input impedance
can vary up to ± 30%.
The Electrical Characteristics table contains the maximum limits for the variation of the analog input range, but
only for those ranges where the comment field shows that the offset and gain are specified (including all the
ranges listed in Table 1 ). For the other ranges, the offset and gain are not tested and are not specified.
Five of the input ranges in Table 4 are not recommended for general use. The upper-end of the – 2.5V to +17.5V
range and +2.5V to +22.5V range exceeds the absolute maximum analog input voltage. These ranges can still
be used as long as the input voltage remains under the absolute maximum, but this limit may reduce the
full-scale range of the converter to a significant degree.
Likewise, three of the input ranges involve the connection at R2
IN
being driven below GND. This input has a
reverse-biased ESD protection diode connection to ground. If R2
IN
is taken below GND – 0.3V, this diode
becomes forward-biased and clamps the negative input at – 0.4V to – 0.7V, depending on the temperature.
Because the negative full-scale value of these input ranges exceeds – 0.4V, they are not recommended.
Note that Table 4 assumes that the voltage at the REF pin is +2.5V. This assumption is true if the internal
reference is used or if the external reference is +2.5V. Using other reference voltages change the values in
Table 4 .
When R1
IN
, R2
IN
, and R3
IN
are connected to the analog input, the input range of the ADS8513 is 0.3125V to
2.8125V and the input impedance is greater than 10M Ω . This input range can be used to connect the ADS8513
directly to a wide variety of sensors. Figure 45 shows the impedance of the sensor versus the change in integral
linearity error (ILE) and differential linearity error (DLE) of the ADS8513. The performance of the ADS8513 can
be improved for higher sensor impedance by allowing more time for acquisition. For example, 10 µ s of acquisition
time approximately doubles the sensor impedance for the same ILE/DLE performance.
The input impedance and capacitance of the ADS8513 are very stable over temperature. Assuming that this
performance is true of the sensor as well, the graph shown in Figure 45 will vary less than a few percent over the
ensured temperature range of the ADS8513. If the sensor impedance varies significantly with temperature, the
worst-case impedance should be used.
Figure 45. Linearity Error vs Source Impedance in the High Impedance Mode (R1
IN
= R2
IN
= R3
IN
= V
IN
)
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