AD7709

–10–

REV. PrA

January 2001

PRELIMINARY TECHNICAL DATA

ADC CIRCUIT INFORMATION

Overview

The AD7709 incorporates an analog multiplexer with a Sigma-Delta ADC, on-chip programmable gain amplifier and

digital filtering intended for the measurement of wide dynamic range, low frequency signals such as those in weigh-scale,

strain-gauge, pressure transducer or temperature measurement applications. The AD7709 offers 16-bit resolution. The

AD7709 can be configured as 2 fully differential input channels or as 4 pseudo differential input channels referenced to

AINCOM. The channel is buffered and can be programmed for one of 8 input ranges from +20mV to +2.56V. Buffering

the input channel means that the part can handle significant source impedances on the analog input and that R, C

filtering (for noise rejection or RFI reduction) can be placed on the analog inputs if required. These input channels are

intended to convert signals directly from sensors without the need for external signal conditioning. Other functions

contained on-chip that augment the operation of the ADC include software configurable current sources, switchable

reference inputs and low side power switches.

The ADC employs a sigma-delta conversion technique to realize up to 16-bits of no missing codes performance. The

sigma-delta modulator converts the sampled input signal into a digital pulse train whose duty cycle contains the digital

information. A Sinc

give a valid data conversion result at programmable output rates from 5.35Hz (186.77mS) to 105.03Hz (9.52mS). A

Chopping scheme is also employed to minimize ADC channel offset errors. A block diagram of the ADC input channel

is shown in Figure 3 below.

1

8

SF

×

3

Digital

Output

A

in

+ V

os

A

in

os

chop

adc

1

2

---

SF)

×

×

XOR

- V

∑

∑

()

ff

(8

3

S-D

Analog

Input

Buffer

PGA

f

mod

Mux

M OD0

Figure 3. AD7709 ADC Channel Block Diagram