AD7711A:  CMOS, 24-Bit Sigma-Delta, Signal Conditioning ADC with RTD Current Source

The AD7711A is a complete analog front end for low-frequency measurement applications. The device accepts low level signals directly from a transducer and outputs a serial digital word. It employs a sigma-delta conversion technique to realize up to 24 bits of no missing codes performance. The 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 the on-chip control register allowing adjustment of the filter cutoff and settling time.

The part features two differential analog inputs and a differential reference input. Normally, one of the channels will be used as the main channel with the second channel used as an auxiliary input to periodically measure a second voltage. It can be operated from a single supply (by tying the V_SS pin to AGND) provided that the input signals on the analog inputs are more positive than -30 mV. By taking the V_SS pin negative, the part can convert signals down to -V_REF on its inputs. The part also provides a 400 µA current source that can be used to provide excitation for RTD transducers. The AD7711A thus performs all signal conditioning and conversion for a single or dual channel system.

The AD7711A is ideal for use in smart, microcontroller based systems. Input channel selection, gain settings and signal polarity can be configured in software using the bidirectional serial port. The AD7711A contains self-calibration, system calibration and background calibration options and also allows the user to read and write the on-chip calibration registers.

CMOS construction ensures low power dissipation, and a software programmable power-down mode reduces the standby power consumption to only 7 mW typical. The part is available in a 24-pin, 0.3 inch-wide, hermetic dual-in-line package (Cerdip) as well as a 24-lead small outline (SOIC) package.