ADS8472 Datasheet, PDF(21/30 Page) Burr-Brown (TI) – 16-BIT, 1-MSPS, PSEUDO-BIPOLAR, FULLY DIFFERENTIAL INPUT, MICROPOWER SAMPLING ANALOG-TO-DIGITAL CONVERTER WITH PARALLEL INTERFACE, REFERENCE

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ADS8472 Datasheet, PDF (21/30 Pages) Burr-Brown (TI) – 16-BIT, 1-MSPS, PSEUDO-BIPOLAR, FULLY DIFFERENTIAL INPUT, MICROPOWER SAMPLING ANALOG-TO-DIGITAL CONVERTER WITH PARALLEL INTERFACE, REFERENCE

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ADS8472

www.ti.com

SLAS514 â DECEMBER 2006

PRINCIPLES OF OPERATION (continued)

ANALOG INPUT

When the converter enters the hold mode, the voltage difference between the +IN and âIN inputs is captured on

the internal capacitor array. Both +IN and âIN input has a range of â0.2 V to Vref + 0.2 V. The input span [+IN â

(âIN)] is limited to âVref to Vref.

The input current on the analog inputs depends upon a number of factors: sample rate, input voltage, and

source impedance. Essentially, the current into the ADS8472 charges the internal capacitor array during the

sample period. After this capacitance has been fully charged, there is no further input current. The source of the

analog input must be able to charge the input capacitance (65 pF) to an 16-bit settling level within the acquisition

time (320 ns) of the device. When the converter goes into the hold mode, the input impedance is greater than 1

Gâ¦.

Care must be taken regarding the absolute analog input voltage. To maintain the linearity of the converter, the

+IN and âIN inputs and the span [+IN â (âIN)] must be within the limits specified. Outside of these ranges, the

converter's linearity may not meet specifications. To minimize noise, low bandwidth input signals with low-pass

filters are used.

Care must be taken to ensure that the output impedance of the sources driving the +IN and âIN inputs are

matched. 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 varies with temperature and input voltage.

The analog input to the converter needs to be driven with a low noise, high-speed op-amp like the THS4031. An

RC filter is recommended at the input pins to low-pass filter the noise from the source. The input to the converter

is a uni-polar input voltage in the range 0 to Vref. The THS4031 can be used in the source follower configuration

to drive the converter.

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