REF5045 datasheet(35/62 Pages) TI2

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RFLT ” 22

AINP

AINM

GND

Device

RFLT ” 22

CFLT • 590 pF

FLT

ADS8881

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SBAS547D – MAY 2013 – REVISED AUGUST 2015

Product Folder Links: ADS8881

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Application Information (continued)

Besides filtering the noise from the front-end drive circuitry, the RC filter also helps attenuate the sampling

charge injection from the switched-capacitor input stage of the ADC. A differential filter capacitor, CFLT, is

connected across the inputs of the ADC (as shown in Figure 64). This capacitor helps reduce the sampling

charge injection and provides a charge bucket to quickly charge the internal sample-and-hold capacitors during

the acquisition process. As a rule of thumb, the value of this capacitor must be at least 10 times the specified

value of the ADC sampling capacitance. For the ADS8881, the input sampling capacitance is equal to 59 pF,

thus the value of CFLT must be greater than 590 pF. The capacitor must be a COG- or NPO-type because these

capacitor types have a high-Q, low-temperature coefficient, and stable electrical characteristics under varying

voltages, frequency, and time.

Figure 64. Antialiasing Filter

Note that driving capacitive loads can degrade the phase margin of the input amplifiers, thus making the amplifier

marginally unstable. To avoid amplifier stability issues, series isolation resistors (RFLT) are used at the output of

the amplifiers. A higher value of RFLT is helpful from the amplifier stability perspective, but adds distortion as a

result of interactions with the nonlinear input impedance of the ADC. Distortion increases with source impedance,

input signal frequency, and input signal amplitude. Therefore, the selection of RFLT requires balancing the stability

and distortion of the design. For the ADS8881, TI recommends limiting the value of RFLT to a maximum of 22 Ω

in order to avoid any significant degradation in linearity performance. The tolerance of the selected resistors can

be chosen as 1% because the use of a differential capacitor at the input balances the effects resulting from any

resistor mismatch.

The input amplifier bandwidth must be much higher than the cutoff frequency of the antialiasing filter. TI strongly

recommends performing a SPICE simulation to confirm that the amplifier has more than 40° phase margin with

the selected filter. Simulation is critical because even with high-bandwidth amplifiers, some amplifiers might

require more bandwidth than others to drive similar filters. If an amplifier has less than a 40° phase margin with

22-Ω resistors, using a different amplifier with higher bandwidth or reducing the filter cutoff frequency with a

larger differential capacitor is advisable.

This section describes some common application circuits using the ADS8881. These data acquisition (DAQ)

blocks are optimized for specific input types and performance requirements of the system. For simplicity, power-

supply decoupling capacitors are not shown in these circuit diagrams; see the Power Supply section for

suggested guidelines.

REF5045 Datasheet(PDF) 35 Page - Texas Instruments

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