Analog Devices, Inc. introduced the AD7705 and AD7706 16-bit sigma delta analog-to-digital converters with a typical INL (integral non-linearity) of ý0.0003%. These ADCs offer programmability, low power consumption (1 mW max @ 3 V) and multiple channels for dc and low frequency ac measurement applications - such as process control, battery monitors, temperature and pressure transmitters, industrial and hand-held consumer instrumentation, and data acquisition. The two ADCs differ only in input structure.

The very low power consumption of these ADCs allows them to be loop-/battery-/locally-powered. The on-chip PGA (programmable gain amplifier) can accommodate both low-level and high-level analog inputs with no external signal conditioning hardware.

Programmability of these devices is achieved through a standard serial interface, enabling designers to configure the output update rate, PGA gain (1-128), signal polarity, channel selection, choice of calibration models and a MCLK OUT disable feature to further reduce power consumption when using an external clock. The programmable gain input also allows the AD7705 and AD7706 to accept input signals directly from a strain gage or transducer, removing a considerable amount of signal conditioning. Both converters have input signal buffers, which allow for high source impedance without adding errors. These buffers can be turned off via software to allow true rail-to-rail operation.

The three-wire serial interface of the AD7705 and AD7706 makes them suitable for use in microcontroller- or DSP-based systems by reducing the number of interconnect lines and opto-couplers required in isolated systems.

These converters operate from a single 2.7-3.3 V or 4.75-5.25 V supply. The AD7705 features two fully-differential analog input channels while the AD7706 has three pseudo-differential input channels. Both devices feature differential reference inputs allowing maximum design flexibility.

Input signal ranges of 0 to +20 mV through 0 to +2.5 V can be accommodated on both devices when operating with a V_DD of 5 V and a reference of 2.5 V. These ADCs can also handle bipolar input signal ranges of ý20 mV through ý2.5 V which are referenced to the A_IN(-) inputs on the AD7705 and to the COMMON input on the AD7706. The AD7705 and AD7706, with 3 V supply and a 1.225 V reference, can handle unipolar input signal ranges of 0 to +10 mV through 0 to +1.225 V. Their bipolar input signal ranges are ý10 mV through ý1.225 V. The converters thus perform all signal conditioning and conversion for a two- or three-channel system.

CMOS fabrication ensures very low power dissipation, and the power-down mode reduces the typical power consumption to 24 ýW at 3 V.

The flexibility of these 16-bit parts is amazing. With the programmability and the wide PGA range they should be suitable for a large range of applications. The low power consumption and the range of the input signals should make it possible to use them in some "almost-remote" applications. But beware! These parts in the hands of a poor designer could create disastrous results, and I would hate to be the ADI applications engineers taking those calls. I'm not sure I care for the pseudo-differential inputs on the AD7706 and I think I would avoid them until someone convinced me there were no untoward effects that are not obvious.

Used in the right way these converters offer the possibility of reducing external signal conditioning dramatically, or completely, and will be a boon to the good design engineer. I particularly like the choice of an input buffer -- for error-free high impedance -- or rail-to-rail input operation, and they may go some way to the prevention of designs using high-resolution parts just to avoid correct signal conditioning at the inputs. It is difficult to forsee where the major design wins will be with these products because the possible range of applications is so immense. The AD7705 and AD7706 are available in a 16-pin 0.3 in. plastic DIP, a 16-lead SOIC and also a 16-lead TSSOP. They are priced at $4.12 in 1000-piece lots.