Datasheet

R
INT
R
INT
R
F1
R
1
R
F2
ADC
A3
Gain=1
CAP
AINP
CAP
A2
A1
450W
18pF
AINN
450W
18pF
ADS1131
SBAS449C –JULY 2009–REVISED OCTOBER 2013 www.ti.com
OVERVIEW
–39mV to +39mV, and the unipolar input ranges from
The ADS1131 is a precision, 18-bit ADC that includes
0mV to +39mV. The inputs of the ADS1131 are
a low-noise PGA, internal oscillator, third-order delta-
protected with internal diodes connected to the
sigma (ΔΣ) modulator, and fourth-order digital filter.
power-supply rails. These diodes clamp the applied
The ADS1131 provides a complete front-end solution
signal to prevent it from damaging the input circuitry.
for bridge sensor applications such as weigh scales,
strain gauges, and pressure sensors.
Data can be output at 10SPS for excellent 50Hz and
60Hz rejection, or at 80SPS when higher speeds are
needed. The ADS1131 is easy to configure, and all
digital control is accomplished through dedicated
pins; there are no registers to program. A simple two-
wire serial interface retrieves the data.
ANALOG INPUTS (AINP, AINN)
The input signal to be measured is applied to the
input pins AINP and AINN. The ADS1131 accepts
differential input signals, but can also measure
unipolar signals. When measuring unipolar (or single-
ended signals) with respect to ground, connect the
negative input (AINN) to ground and connect the
input signal to the positive input (AINP). Note that
when the ADS1131 is configured this way, only half
of the converter full-scale range is used, because
Figure 1. Simplified Diagram of the Amplifier
only positive digital output codes are produced.
External Capacitor
LOW-NOISE AMPLIFIER
An external capacitor (C
EXT
) across the ADS1131 two
The ADS1131 features a low-drift, low-noise amplifier
CAP pins combines with the internal resistor R
INT
(on-
that provides a complete front-end solution for bridge
chip) to create a low-pass filter. The recommended
sensors. A simplified diagram of the amplifier is
value for C
EXT
is 0.1μF which provides a corner
shown in Figure 1. It consists of two chopper-
frequency of 720Hz. This low-pass filter serves two
stabilized amplifiers (A1 and A2) and three accurately
purposes. First, the input signal is bandlimited to
matched resistors (R
1
, R
F1
, and R
F2
) that construct a
prevent aliasing by the ADC and to filter out the high-
differential front-end stage with a gain of 64, followed
frequency noise. Second, it attenuates the chopping
by gain stage A3 (Gain = 1). The inputs are equipped
residue from the amplifier to improve temperature drift
with an EMI filter, as shown in Figure 1. The cutoff
performance. NPO or C0G capacitors are
frequency of the EMI filter is 20MHz. By using AVDD
recommended. For optimal performance, place the
as the reference input, the bipolar input ranges from
external capacitor very close to the CAP pins.
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