概要

The AD7705/AD7706 are complete analog front ends for low frequency measurement applications. These 2-/3-channel devices can accept low level input signals directly from a transducer and produce serial digital output. The devices employ a Σ-Δ conversion technique to realize up to 16 bits of no missing codes performance. The selected input signal is applied to a proprietary, programmable-gain front end based around an analog modulator. The modulator output is processed by an onchip digital filter. The first notch of this digital filter can be programmed via an on-chip control register, allowing adjustment of the filter cutoff and output update rate. The AD7705/AD7706 devices operate from a single 2.7 V to 3.3 V or 4.75 V to 5.25 V supply. The AD7705 features two fully differential analog input channels; the AD7706 features three pseudo differential input channels. Both devices feature a differential reference input. Input signal ranges of 0 mV to 20 mV through 0 V to 2.5 V can be incorporated on both devices when operating with a VDD of 5 V and a reference of 2.5 V. They can also handle bipolar input signal ranges of ±20 mV through ±2.5 V, which are referenced to the AIN(−) inputs on the AD7705 and to the COMMON input on the AD7706.

The AD7705/AD7706 devices, with a 3 V supply and a 1.225 V reference, can handle unipolar input signal ranges of 0 mV to 10 mV through 0 V to 1.225 V. The devices can accept bipolar input ranges of ±10 mV through ±1.225 V. Therefore, the AD7705/AD7706 devices perform all signal conditioning and conversion for a 2-channel or 3-channel system. The AD7705/AD7706 are ideal for use in smart, microcontroller, or DSP-based systems. The devices feature a serial interface that can be configured for 3-wire operation. Gain settings, signal polarity, and update rate selection can be configured in software using the input serial port. The parts contains self-calibration and system calibration options to eliminate gain and offset errors on the part itself or in the system. CMOS construction ensures very low power dissipation, and the power-down mode reduces the standby power consumption to 20 μW typ.  These parts are available in a 16-lead, wide body (0.3 inch), plastic dual in-line package (DIP); a 16-lead, wide body (0.3 inch), standard small outline (SOIC) package; and a low profile, 16-lead, thin shrink small outline package (TSSOP).

 

特徴

AD7705: 2 fully differential input channel ADCs

16 bits no missing codes

0.003% nonlinearity

Programmable gain front end: gains from 1 to 128

3-wire serial interface

SPI®-, QSPI™-, MICROWIRE™-, and DSP-compatible

Schmitt-trigger input on SCLK

Ability to buffer the analog input

2.7 V to 3.3 V or 4.75 V to 5.25 V operation

Power dissipation 1 mW maximum @ 3 V

Standby current 8 μA maximum

16-lead PDIP, 16-lead SOIC, and 16-lead TSSOP packages

 

アプリケーション

The AD7705 provides a dual-channel, low cost, high resolution analog-to-digital function. Because the analog-to-digital function is provided by a Σ-Δ architecture, the part is more immune to noisy environments, thus making it ideal for use in industrial and process-control applications. It also provides a programmable gain amplifier, digital filter, and calibration options. Therefore, it provides far more system level functionality than off-the-shelf integrating ADCs, but without the disadvantage of needing to supply a high quality integrating capacitor. In addition, using the AD7705 in a system allows the designer to achieve a much higher level of resolution, because noise performance of the AD7705 is better than that of the integrating ADCs. The on-chip PGA allows the AD7705 to handle an analog input voltage range as low as 10 mV full scale with VREF = 1.25 V. The differential inputs of the part allow the absolute value of this analog input range to be between GND and VDD when the  part is operated in unbuffered mode. It allows the user to connect the transducer directly to the input of the AD7705. The programmable-gain front end on the AD7705 allows the part to handle unipolar analog input ranges from (0 mV to 20 mV) to (0 V to 2.5 V), and bipolar inputs of ±20 mV to ±2.5 V. Because the part operates from a single supply, these bipolar ranges are with respect to a biased-up differential input.