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SBAS287A − JUNE 2003 − REVISED SEPTEMBER 2003
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.
OVERVIEW
The ADS1245 is an ADC comprised of a 3rd-order modulator
followed by a digital filter. The modulator measures the
differential input signal VIN = (AINP – AINN) against the
differential reference VREF = (VREFP – VREFN). Figure 15
shows a conceptual diagram. The differential reference is
scaled internally so that the full-scale input range is 2VREF
.
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.
VOLTAGE REFERENCE INPUTS
(VREFP, VREFN)
The voltage reference used by the modulator is generated
from the voltage difference between VREFP and VREFN:
VREF = 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:
VREFP VREFN
tSAMPLE
ǒ Ǔ
ń25pF + 1MW for fCLK + 2.4576MHz
2
Σ
CLK
VREF
2
2VREF
VREFP
VREFN
Digital
Filter and
Serial
AINP
AINN
X1
X1
V
DRDY/DOUT
SCLK
IN
Modulator
Σ
Interface
AVDD
AVDD
ESD
Protection
S1
S1
Figure 15. Conceptual Diagram of the ADS1245
ANALOG INPUTS (AINP, AINN)
25pF
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 fCLK frequency. For example, if the frequency
of fCLK is reduced by a factor of two, the impedances ZeffA
and ZeffB will double.
S2
tSAMPLE = 128/fCLK
ON
OFF
ON
S1
S2
OFF
AVDD/2
Ω
ZeffA = 280G
Figure 17. Simplified Reference Input Circuitry
AINP
AINN
Ω
ZeffB = 3.46G
The ADS1245 is specified for operation with VREF = 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 VIN 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
Ω
ZeffA = 280G
fCLK = 2.4576MHz.
AVDD/2
Figure 16. Effective Analog Input Impedances
2VREF
.
7
BB [ BURR-BROWN CORPORATION ]
