The symbolic diagram of the delta-sigma ADC is shown as above. It consists of an analog subtractor, an

integrator, a comparator, a 1-bit digital-to-analog converter (DAC), and a low-pass digital filter. The analog

signals are continuously sampled and are subtracted by the expected voltage. The difference of the signals is

fed into the integrator, and then the signal is compared with a reference voltage to find a digital output. This

digital output is converted by the 1-bit DAC to become an analog signal (+Vref or -Vref) and then negatively fed

back into the integrator. Due to the infinitive DC gain of the integrator, if the change of the input signal is much

slower than the sampling speed, the average voltage obtained by the delta-sigma modulator will be very close

to the input signal. In some resolution they can be treated to be the same, therefore, the 1-bit output data from

the comparator are equivalent to the  Vref analog signal values. The digital filter then decimates the 1-bit data

to get a very high resolution digital code.

ENAD(ADG<7>) is the enable control signal for the ADC. It is 1 to enable the ADC; it is 0 to turn off the ADC

and can save power.

11..3.1.

Gain Stage Setting

There are four different gain paths to the input of the FS511 ADC, and they are controlled by control

register ADG[3:0]. Two different gain paths control the input reference voltage, and they are controlled by

control register ADG[5:4]. The gains shown here are not accurate. The accurate gains can be found by careful

calibration.

0.5

0.5

0.25

0.1

Vin

(IN+, IN-)

ADG[0]

ADG[1]

ADG[2]

ADG[3]

To ADC

signal

input

1.0

0.25

Vref (VR+, VR-)

ADG[4]

ADG[5]

To ADC

reference

input

Diagram of FS511Gain Stage Setting

By proper selection of the gain paths, this ADC can be applied to the optimum dynamic range for all the

measuring applications. Table shows values of ADG[5:0] for three frequently used applications.

Table: FS511 ADC Typical Gain Setting

First Scale

Second Scale

Third Scale

ADG<5:0>

01_0011

11_0111

11_1000



1.0



1.25



1.25



1.0



1.25



0.1

The transfer function for each scale is as follows,

Equation 1

r

e f

x

R

E

F

i

SI

G

i

x

v

v

G

G

D





The gains for the reference voltages and input voltages shown in Table are approximate values. The

accurate gains for the reference voltages and input voltages can be found by careful calibration.