9.1 Overview

The ADS114S06B and ADS114S08B are precision 16-bit, delta-sigma (ΔΣ) analog-to-digital converters (ADCs) with an integrated analog front-end (AFE) to simplify precision sensor connections. The ADC provides output data rates from 2.5 SPS to 4000 SPS for flexibility in resolution and data rates over a wide range of applications. The low-noise and low-drift architecture make these devices suitable for precise measurement of low-voltage sensors, such as load cells and temperature sensors.

The ADS114S0xB incorporates several features that simplify precision sensor measurements. Key integrated features include:

• Low-noise, CMOS PGA
• Low-drift, 2.5-V voltage reference
• Two sets of buffered external reference inputs with low reference voltage level detection
• Dual, matched, sensor-excitation current sources (IDACs)
• Internal 4.096-MHz oscillator
• Temperature sensor
• Four general-purpose input/output pins (GPIOs)

As described in the Functional Block Diagram section, these devices provide 12 (ADS114S08B) or six (ADS114S06B) analog inputs that are configurable as either single-ended inputs, differential inputs, or any combination of the two. Many of the analog inputs have additional features as programmed by the user. The analog inputs can be programmed to enable the following extended features:

• Two sensor excitation current sources: all analog input pins (and REFP1 and REFN1 on the ADS114S06B)
• Sensor biasing voltage (VBIAS): pins AIN0, AIN1, AIN2, AIN3, AIN4, AIN5, AINCOM
• Four GPIO pins: AIN8, AIN9, AIN10, AIN11 (ADS114S08B only, the ADS114S06B has dedicated GPIOs)
• Sensor burn-out current sources: analog input pins selected for ADC input

Following the input multiplexer (MUX), the ADC features a high input-impedance, low-noise, programmable gain amplifier (PGA), eliminating the need for an external amplifier. The PGA gain is programmable from 1 to 128 in binary steps. The PGA can be bypassed to allow the input range to extend 50 mV below ground or above supply.

An inherently stable ΔΣ modulator measures the ratio of the input voltage to the reference voltage to provide the ADC result. The ADC operates with the internal 2.5-V reference, or with up to two external reference inputs. The external reference inputs can be continuously monitored for low voltage. The REFOUT pin provides the buffered 2.5-V internal voltage reference output that can be used to bias external circuitry.

The digital filter provides settled data with 50-Hz and 60-Hz line-cycle rejection at data rates of 2.5 SPS, 5 SPS, 10 SPS, and 20 SPS, 50-Hz rejection at data rates of 16.6 SPS and 50 SPS, and 60-Hz rejection at a data rate of 60 SPS.

Two programmable excitation current sources provide bias to resistive sensors [such as resistance temperature detectors (RTDs) or thermistors]. The ADC integrates several system monitors for read back, such as temperature sensor and supply monitors. Four GPIO pins are available as either dedicated pins (ADS114S06B) or combined with analog input pins (ADS114S08B).

The ADS114S0xB system clock is either provided by the internal low-drift, 4.096-MHz oscillator or an external clock source on the CLK input.

The SPI-compatible serial interface is used to read the conversion data and also to configure and control the ADC. The serial interface consists of four signals: CS, SCLK, DIN, and DOUT/DRDY. The dual function DOUT/DRDY output indicates when conversion data are ready and also provides the data output. The serial interface can be implemented with as little as three connections by tying CS low. Start ADC conversions with either the START/SYNC pin or with commands. The ADC can be programmed for a continuous conversion mode or to perform single-shot conversions.

The AVDD analog supply operates with bipolar supplies from ±1.5 V to ±2.625 V or with a unipolar supply from 2.7 V to 5.25 V. For unipolar-supply operation, use the VBIAS voltage to bias isolated (floating) sensors. The digital supplies operate with unipolar supplies only. The DVDD digital power supply operates from 2.7 V to 3.6 V and the IOVDD supply operates from DVDD to 5.25 V.