Datasheet

ManualsBrandsTEXAS INSTRUMENTS ManualsData collecting ICsData acquisition IC - AD converter (ADC) External SOIC 16 N

INT

ADC

Gain=1

CAP

AINP

CAP

AINN

EMI

Filter

EMI

Filter

AVDD

Z =500MW

EFF

VREFP VREFN

AVDD

ESD

Protection

BUF

Z =500MW

EFF

ADS1231

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SBAS414D –JULY 2009–REVISED OCTOBER 2013

OVERVIEW

The ADS1231 is a precision, 24-bit ADC that includes

a low-noise PGA, internal oscillator, third-order delta- External Capacitor

sigma (ΔΣ) modulator, and fourth-order digital filter.

An external capacitor (C

EXT

) across the two ADS1231

The ADS1231 provides a complete front-end solution

CAP pins combines with the internal resistor R

INT

(on-

for bridge sensor applications such as weigh scales,

chip) to create a low-pass filter. The recommended

strain gauges, and pressure sensors.

value for C

EXT

is 0.1μF which provides a corner

Data can be output at 10SPS for excellent 50Hz and frequency of 720Hz. This low-pass filter serves two

60Hz rejection, or at 80SPS when higher speeds are purposes. First, the input signal is band-limited to

needed. The ADS1231 is easy to configure, and all prevent aliasing by the ADC and to filter out the high-

digital control is accomplished through dedicated frequency noise. Second, it attenuates the chopping

pins; there are no registers to program. A simple two- residue from the amplifier to improve temperature drift

wire serial interface retrieves the data. performance. NPO or C0G capacitors are

recommended. For optimal performance, place the

external capacitor very close to the CAP pins.

ANALOG INPUTS (AINP, AINN)

The input signal to be measured is applied to the

VOLTAGE REFERENCE INPUTS

input pins AINP and AINN. The ADS1231 accepts

(VREFP, VREFN)

differential input signals, but can also measure

unipolar signals. The voltage reference used by the modulator is

generated from the voltage difference between

VREFP and VREFN: V

REF

= VREFP – VREFN. The

LOW-NOISE AMPLIFIER

reference inputs use a structure similar to that of the

The ADS1231 features a low-drift, low-noise amplifier

analog inputs. In order to increase the reference input

that provides a complete front-end solution for bridge

impedance, switching buffer circuitry is used to

sensors. A simplified diagram of the amplifier is

reduce the input equivalent capacitance. The

shown in Figure 13. It consists of two chopper-

reference drift and noise impact ADC performance. In

stabilized amplifiers (A1 and A2) and three accurately

order to achieve best results, pay close attention to

matched resistors (R

, R

, and R

) that construct a

the reference noise and drift specifications. A

differential front-end stage with a gain of 128,

simplified diagram of the circuitry on the reference

followed by gain stage A3 (Gain = 1). The inputs are

inputs is shown in Figure 14. The switches and

equipped with an EMI filter, as shown in Figure 13.

capacitors can be modeled approximately using an

The cutoff frequency of the EMI filter is 20MHz. By

effective impedance of .

using AVDD as the reference input, the bipolar input

ranges from –19.5mV to +19.5mV. The inputs of the

ADS1231 are protected with internal diodes

connected to the power-supply rails. These diodes

clamp the applied signal to prevent it from damaging

the input circuitry.

Figure 14. Simplified Reference Input Circuitry

ESD diodes protect the reference inputs. To prevent

these diodes from turning on, make sure the voltages

on the reference pins do not go below GND by more

than 100mV, and likewise, do not exceed AVDD by

100mV:

GND – 100mV < (VREFP or VREFN) < AVDD +

Figure 13. Simplified Diagram of the Amplifier

100mV

Product Folder Links: ADS1231