ADS1178IPAPR datasheet(12/39 Pages) TI1

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ESDProtection

AVDD

AGND

AVDD

AINP

9pF

AINN

AGND

OFF

SAMPLE

MOD

=1/f

AINP

AINN

=14k

eff

W ´ (6.75MHz/f

ADS1174

ADS1178

SBAS373B – OCTOBER 2007 – REVISED SEPTEMBER 2008 ........................................................................................................................................ www.ti.com

While the ADS1174/78 measures the differential input

signal, the absolute input voltage is also important.

This is the voltage on either input (AINP or AINN)

with respect to AGND. The range for this voltage is:

–0.1V < (AINN or AINP) < AVDD + 0.1V

If either input is taken below –0.4V or above (AVDD +

0.4), ESD protection diodes on the inputs may turn

on.

If these conditions are possible, external Schottky

clamp diodes or series resistors may be required to

limit the input current to safe levels (see Absolute

Maximum Ratings table).

The ADS1174/78 is a high-performance ADC. For

optimum performance, it is critical that the appropriate

circuitry be used to drive the ADS1174/78 inputs. See

Figure 7. Equivalent Analog Input Circuitry

the

Applications

Information

section

for

the

recommended circuits.

The ADS1174/78 uses switched-capacitor circuitry to

measure the input voltage. Internal capacitors are

charged by the inputs and then discharged. Figure 7

shows a conceptual diagram of these circuits. Switch

S2 represents the net effect of the modulator circuitry

in discharging the sampling capacitor; the actual

Figure 8. S1 and S2 Switch Timing for Figure 7

implementation is different. The timing for switches S1

and S2 is shown in Figure 8. The sampling time

(tSAMPLE) is the inverse of modulator sampling

Table 3. Modulator Frequency (fMOD) versus Mode

frequency (fMOD) and is a function of the mode, the

Selection

CLKDIV input, and frequency of CLK, as shown in

MODE SELECTION

CLKDIV

fMOD

Table 3.

fCLK/8

The

average

load

presented

the

switched

High-Speed

fCLK/4

capacitor input can be modeled with an effective

differential impedance, as shown in Figure 9. Note

fCLK/40

Low-Power

that the effective impedance is a function of fMOD.

fCLK/8

Figure 9. Effective Input Impedances

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