GENERAL DESCRIPTION
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 on chip 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).

FEATURES
  AD7705: 2 fully differential input channel ADCs
  AD7706: 3 pseudo 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