ADS1259
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SBAS424C –JUNE 2009–REVISED MARCH 2010
ANALOG INPUTS (AINP, AINN)
ESD diodes protect the analog inputs. To keep these
diodes from turning on, make sure the voltages on
the input pins do not go below AVSS by more than
300mV, and likewise do not exceed AVDD by more
than 300mV.
The ADS1259 measures the differential input signal
VIN = (AINP – AINN) against the differential reference
VREF = (VREFP – VREFN) using internal capacitors
that are continuously charged and discharged.
Figure 36 shows the simplified schematic of the ADC
input circuitry; the right side of the figure illustrates
the input circuitry with the capacitors and switches
replaced by an equivalent circuit. Figure 35
demonstrates the ON/OFF timings for the switches of
Figure 36.
AVSS – 300mV < (AINP or AINN) < AVDD + 300mV.
Note that the valid input range is:
AVSS – 100mV < (AINP or AINN) < AVDD + 100mV
tSAMPLE = 1/fMOD
ON
S1
In Figure 36, S1 switches close during the input
sampling phase. With switch S1 closed, CA1 charges
to AINP, CA2 charges to AINN, and CB charges to
(AINP – AINN). For the discharge phase, S1 opens
first and then S2 closes. CA1 and CA2 discharge to
approximately to AVSS + 2.5V and CB discharges to
0V. This two-phase sample/discharge cycle repeats
with a period of tSAMPLE = 1/fMOD. fMOD is the operating
frequency of the modulator, where fMOD = fCLK/8.
OFF
ON
S2
OFF
Figure 35. S1 and S2 Switch Timing for Figure 36
Although optimized for differential signals, the
ADS1259 inputs may be driven with a single-ended
signal by fixing one input to AVSS or mid-supply. Full
dynamic range is achieved when the inputs are
The charging of the input sampling capacitors draws
a transient current from the source driving the
ADS1259 ADC inputs. The average value of this
current can be used to calculate an effective
impedance (REFF) where REFF = VIN/IAVERAGE. These
impedances scale inversely with fMOD. For example, if
fMOD is reduced by a factor of two, the impedances
double. Note that the sampling capacitors can vary
±15% over production lots and typically vary 1% with
temperature. The variations of the sampling
capacitors have a corresponding effect on the analog
input impedance.
differentially driven ±VREF
.
As a result of the switched-capacitor input structure of
the ADS1259, a buffer is recommended to drive the
analog inputs. An input filter comprised of 20Ω to 50Ω
resistors and 10nF capacitors should be used
between the buffer and the ADS1259 inputs.
(fMOD = 0.9216MHz)
AVSS + 2.5V
AVDD
AVSS + 2.5V
S2
REFF A = 500kW
CA1 = 2pF
CB = 8pF
ESD Diodes
Equivalent
AINP
AINP
AINN
Circuit
S1
S1
REFF B = 130kW
AINN
REFF A = 500kW
CA2 = 2pF
ESD Diodes
S2
AVSS + 2.5V
1
REFF
=
f
MOD ´ CX
AVSS
AVSS + 2.5V
RDIFF = REFF B || 2REFF A = 120kW
RCOM = REFF A = 500kW
and fMOD = fCLK/8
Figure 36. Simplified ADC Input Structure
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