Datasheet

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SBAS287A − JUNE 2003 − REVISED SEPTEMBER 2003
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7
OVERVIEW
The ADS1245 is an ADC comprised of a 3rd-order modulator
followed by a digital filter. The modulator measures the
differential input signal V
IN
= (AINP – AINN) against the
differential reference V
REF
= (VREFP – VREFN). Figure 15
shows a conceptual diagram. The differential reference is
scaled internally so that the full-scale input range is ±2V
REF
.
The digital filter receives the modulator signal and provides
a low-noise digital output. The filter also sets the frequency
response of the converter and provides 50Hz and 60Hz
rejection while settling in a single conversion cycle. A
two-wire serial interface indicates conversion completion and
provides the user with the output data.
Modulator
Digital
Filter and
Serial
Interface
Σ
2
VREFP VREFN
V
REF
DRDY/DOUT
SCLK
CLK
2V
REF
Σ
V
IN
AINN
AINP
X1
X1
Figure 15. Conceptual Diagram of the ADS1245
ANALOG INPUTS (AINP, AINN)
The input signal to be measured is applied to the input pins
AINP and AINN. The ADS1245 features a low-drift
chopper-stabilized buffer to achieve very high input
impedance. The input impedance can be modeled by
resistors, as shown in Figure 16. The impedance scales
inversely with f
CLK
frequency. For example, if the frequency
of f
CLK
is reduced by a factor of two, the impedances Zeff
A
and Zeff
B
will double.
Zeff
A
= 280G
Ω
Zeff
B
= 3.46G
Ω
Zeff
A
= 280G
Ω
f
CLK
=2.4576MHz.
AINP
AINN
AVDD/2
AVDD/2
Figure 16. Effective Analog Input Impedances
The ADS1245 accepts differential input signals, but can
also measure unipolar signals. Note that the analog inputs
(listed in the Electrical Characteristics table as Absolute
Input Range) must remain between GND + 0.1V to
AVDD − 1.25V. Exceeding this range will degrade linearity
and result in performance outside specified limits.
VOLTAGE REFERENCE INPUTS
(VREFP, VREFN)
The voltage reference used by the modulator is generated
from the voltage difference between VREFP and VREFN:
V
REF
= VREFP – VREFN. A simplified diagram of the
circuitry on the reference inputs is shown in Figure 17. The
switches and capacitors can be modeled with an effective
impedance equal to:
ǒ
t
SAMPLE
2
Ǔ
ń25pF + 1MW for f
CLK
+ 2.4576MHz
25pF
S
2
S
1
S
1
AVDD
VREFP VREFN
AVDD
ESD
Protection
ON
OFF
ON
S
1
S
2
OFF
t
SAMPLE
= 128/f
CLK
Figure 17. Simplified Reference Input Circuitry
The ADS1245 is specified for operation with V
REF
= 1.25V,
resulting in a full−scale input value of ±2.5V. However, the
buffered analog inputs can accept voltages within the
range of 0.10V to 3.75V, resulting in a maximum V
IN
of
±3.65V. Input voltages can be accurately measured over
this entire range if a voltage reference of 1.825V is pro-
vided. In any case, digital output codes will clip to the full
scale value if the absolute input voltage range exceeds
2V
REF
.