ADS8410 Datasheet, PDF (21/40 Pages) Texas Instruments – 16-BIT, 2-MSPS, LVDS SERIAL INTERFACE, SAR ANALOG-TO-DIGITAL CONVERTER

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+VA
+IN
170 W
−IN
170 W
25 pF
ADS8410
SLAS493A – OCTOBER 2005 – REVISED MAY 2013
ADS8410
+
_
25 pF
AGND
AGND
Figure 16. Simplified Input Circuit
When the converter enters hold mode, the voltage difference between the +IN and -IN inputs is captured on the
internal capacitor array. The input current on the analog inputs depends upon a number of factors: sample rate,
input voltage, signal frequency, and source impedance. Essentially, the current into the ADS8410 charges the
internal capacitor array during the sample period. After this capacitance has been fully charged, there is no
further input current (this may not happen when the signal is moving continuously). The source of the analog
input voltage must be able to charge the input capacitance (25 pF) to better than a 16-bit settling level with a
step input within the acquisition time of the device. For calculation, the step size can be selected equal to the
maximum voltage difference between two consecutive samples at the maximum signal frequency (see the
TYPICAL ANALOG INPUT CIRCUIT section). When the converter goes into hold mode, the input impedance is
greater than 1GΩ.
INPUT+
49.9 W
V_CC+
A ^{10 mF 0.1 mF 1 mF}
+
12 11
7
2−
6
THS4031
3
+
8
1
4
NULL
NULL
REF
15 W
1000 pF
15 W
V_CC−
A
A
Figure 17. Typical Analog Input Schematic
REFIN
18 _+IN
19 _−IN
ADS8410
Care must be taken regarding the absolute analog input voltage. To maintain the linearity of the converter, both -
IN and +IN inputs should be within the limits specified. Outside of these ranges, the converter linearity may not
meet specifications. Care should be taken to ensure that +IN and -IN see the same impedance to the respective
sources. If this is not observed, the two inputs could have different setting times. This may result in offset error,
gain error, and linearity error which changes with temperature and input voltage.
REFERENCE
The ADS8410 has a built-in 4.096-V (nominal value) reference. The ADS8410 can also operate with an external
reference. When the internal reference is used, pin 14 (REFOUT) should be connected to pin 13 (REFIN), and a
0.1-μF decoupling capacitor and 1-μF storage capacitor must be connected between pin 14 (REFOUT) and pins
11 and 12 (REFM) (see Figure 18). The internal reference of the converter is buffered.
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