3 V/5 V, 1 mW, 2-/3-Channel,
16-Bit, Sigma-Delta ADCs
AD7705/AD7706
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
GENERAL DESCRIPTION
FUNCTIONAL BLOCK DIAGRAM
VDD
REF IN(–)
REF IN(+)
AD7705/AD7706
CHARGE
BALANCING
A/D CONVERTER
ANALOG
INPUT
CHANNELS
MAX
BUFFER
Σ -Δ
MODULATOR
PGA
A = 1 ≈ 128
DIGITAL FILTER
SERIAL INTERFACE
REGISTER BANK
MCLK IN
MCLK OUT
SCLK
CLOCK
GENERATION
CS
DIN
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.
DOUT
GND
DRDY
RESET
01166-001
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.
Figure 1.
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).
Rev. C
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