Datasheet
ESDProtection
AVDDAGND
AVDD
AINP
9pF
AINN
AGND
S
1
S
1
S
2
ON
OFF
S
1
ON
OFF
S
2
t
SAMPLE MOD
=1/f
AINP
AINN
Z =14k
eff MO
W ´ (6.75MHz/f
D
)
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
S
2
represents the net effect of the modulator circuitry
in discharging the sampling capacitor; the actual
Figure 8. S
1
and S
2
Switch Timing for Figure 7
implementation is different. The timing for switches S
1
and S
2
is shown in Figure 8 . The sampling time
(t
SAMPLE
) is the inverse of modulator sampling
Table 3. Modulator Frequency (f
MOD
) versus Mode
frequency (f
MOD
) and is a function of the mode, the
Selection
CLKDIV input, and frequency of CLK, as shown in
MODE SELECTION CLKDIV f
MOD
Table 3 .
1 f
CLK
/8
The average load presented by the switched
High-Speed
0 f
CLK
/4
capacitor input can be modeled with an effective
differential impedance, as shown in Figure 9 . Note
1 f
CLK
/40
Low-Power
that the effective impedance is a function of f
MOD
.
0 f
CLK
/8
Figure 9. Effective Input Impedances
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