ADS1274
ADS1278
www.ti.com
SBAS367–JUNE 2007
The ADS1274/78 uses switched-capacitor circuitry to
measure the input voltage. Internal capacitors are
charged by the inputs and then discharged. Figure 67
shows a conceptual diagram of these circuits. Switch
S2 represents the net effect of the modulator circuitry
in discharging the sampling capacitor; the actual
implementation is different. The timing for switches S1
and S2 is shown in Figure 68. The sampling time
AINP
AINN
Zeff = 14kW ´ (6.75MHz/fMOD
)
(tSAMPLE
) is the inverse of modulator sampling
frequency (fMOD) and is a function of the mode, the
CLKDIV input, and CLK frequency, as shown in
Table 5.
Figure 69. Effective Input Impedances
VOLTAGE REFERENCE INPUTS
(VREFP, VREFN)
AVDD AGND
The voltage reference for the ADS1274/78 ADC is
the differential voltage between VREFP and VREFN:
VREF = (VREFP – VREFN). The voltage reference is
common to all channels. The reference inputs use a
structure similar to that of the analog inputs with the
equivalent circuitry on the reference inputs shown in
Figure 70. As with the analog inputs, the load
presented by the switched capacitor can be modeled
with an effective impedance, as shown in Figure 71.
However, the reference input impedance depends on
the number of active (enabled) channels in addition to
fMOD. As a result of the change of reference input
impedance caused by enabling and disabling
channels, the regulation and setting time of the
external reference should be noted, so as not to
affect the readings.
S1
AINP
S2
9pF
AINN
S1
AGND AVDD
ESD Protection
Figure 67. Equivalent Analog Input Circuitry
tSAMPLE = 1/fMOD
VREFP
VREFN
ON
S1
OFF
ON
S2
AGND
AVDD
AGND
AVDD
OFF
Figure 68. S1 and S2 Switch Timing for Figure 67
ESD
Protection
Table 5. Modulator Frequency (fMOD) Mode
Selection
MODE SELECTION
High-Speed
CLKDIV
fMOD
fCLK/4
fCLK/4
fCLK/8
fCLK/4
fCLK/40
fCLK/8
1
1
1
0
1
0
High-Resolution
Figure 70. Equivalent Reference Input Circuitry
Low-Power
Low-Speed
VREFP
VREFN
The average load presented by the switched
capacitor input can be modeled with an effective
differential impedance, as shown in Figure 69. Note
5.2kW
Zeff
=
´ (6.75MHz/fMOD)
N
that the effective impedance is a function of fMOD
.
N = number of active channels.
Figure 71. Effective Reference Impedance
25
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