External reference voltages can range from 1.0

volt minimum to 3.6 volts maximum. The com-

mon mode v oltag e range of the external

reference can allow the reference to lie at any

voltage between the VA+ and VA- supply rails.

Figures 5 and 6 illustrate how the CS5505/6/7/8

converters are connected for external and for in-

ternal voltage reference use, respectively.

Analog Input Range

The analog input range is set by the magnitude

of the voltage between the VREF+ and VREF-

pins. In unipolar mode the input range will equal

the magnitude of the voltage reference. In bipo-

lar mode the input voltage range will equate to

plus and minus the magnitude of the voltage ref-

erence.

While the voltage reference can be as

great as 3.6 volts, its common mode voltage can

be any value as long as the reference inputs

VREF+ and VREF- stay within the supply volt-

ages for the A/D. The differential input voltage

can also have any common mode value as long

as the maximum signal magnitude stays within

the supply voltages.

The A/D converter is intended to measure dc or

low frequency inputs. It is designed to yield ac-

curate conversions even with noise exceeding

the input voltage range as long as the spectral

components of this noise will be filtered out by

the digital filter. For example, with a 3.0 volt

reference in unipolar mode, the converter will

accurately convert an input dc signal up to

3.0 volts with up to 15% overrange for 60 Hz

noise. A 3.0 volt dc signal could have a 60 Hz

component which is 0.5 volts above the maxi-

mum input of 3.0 (3.5 volts peak; 3.0 volts dc

plus 0.5 volts peak noise) and still accurately

convert the input signal (XIN = 32.768 kHz).

This assumes that the signal plus noise ampli-

tude stays within the supply voltages.

The CS5505/6/7/8 converters output data in bi-

nary format when converting unipolar signals

and in offset binary format when converting bi-

polar signals. Table 2 outlines the output coding

for the 16-bit CS5505/7 and the 20-bit CS5506/8

in both unipolar and bipolar measurement

modes.

CS5505 and CS5507 (16 Bit)

CS5506 and CS5508 (20 Bit)

Unipolar Input

Voltage

Output

Codes

Bipolar Input

Voltage

Unipolar Input

Voltage

Output

Codes

Bipolar Input

Voltage

>(VREF - 1.5 LSB)

FFFF

>(VREF - 1.5 LSB)

>(VREF - 1.5 LSB)

FFFFF

>(VREF - 1.5 LSB)

VREF - 1.5 LSB

FFFF

FFFE

VREF - 1.5 LSB

VREF - 1.5 LSB

FFFFF

FFFFE

VREF - 1.5 LSB

VREF/2 - 0.5 LSB

8000

7FFF

-0.5 LSB

VREF/2 - 0.5 LSB

80000

7FFFF

-0.5 LSB

+0.5 LSB

0001

0000

-VREF + 0.5 LSB

+0.5 LSB

00001

00000

-VREF + 0.5 LSB

<(+0.5 LSB)

0000

<(-VREF + 0.5 LSB)

<(+0.5 LSB)

00000

<(-VREF + 0.5 LSB)

Note: VREF = (VREF+) - (VREF-); Table excludes common mode voltage on the signal and reference inputs.

Table 2. Output Coding

-VA

+VA

0.1

µF

CS5505/6/7/8

VA+

VREF+

VREF-

VREFOUT

VA-

Figure 6. Internal Reference Connections

CS5505/6/7/8

16

DS59F4

CS5505/6/7/8

16

DS59F5