UTC7106

CMOS IC

7

QW-R502-018,B

case condition would be a large positive common mode voltage with a near full scale negative differential input

voltage. The negative input signal drives the integrator positive when most of its swing has been used up by the

positive common mode voltage. For these critical applications the integrator output swing can be reduced to less

than the recommended 2V full scale swing with little loss of accuracy. The integrator output can swing to within 0.3V

of either supply without loss of linearity.

DIFFERENTIAL REFERENCE

The reference voltage can be generated anywhere within the power supply voltage of the converter. The main

source of common mode error is a roll-over voltage caused by the reference capacitor losing or gaining charge to

stray capacity on its nodes. If there is a large common mode voltage, the reference capacitor can gain charge

(increase voltage) when called up to de-integrate a positive signal but lose charge (decrease voltage) when called

up to de-integrate a negative input signal. This difference in reference for positive or negative input voltage will give

a roll-over error. However, by selecting the reference capacitor such that it is large enough in comparison to the stray

capacitance, this error can be held to less than 0.5 count worst case. (See Component Value Selection)

COMMON

IN LO

30

32

31

V+

A-Z

INT

INT

A-Z AND DE(±)

V-

FIGURE 1. ANALOG SECTION

DE+

DE-

DE-

DE+

A-Z

A-Z

REF HI

REF LO

STRAY

34

36

35

33

28

1

2.8V

-

+

-

+

-

+

29

27

INT

A-Z

INTEGRATOR

V+

STRAY

TO

DIGITAL

SECTION

A-Z

COMPARATOR

INPUT

LOW

-

+

N

INPUT

HIGH

6.2V

10 A

IN HI

μ

BUFFER

ANALOG COMMON

This pin is included primarily to set the common mode voltage for battery operation (UTC 7106) or for any system

where the input signals are floating with respect to the power supply. The COMMON pin sets a voltage that is

approximately 2.8V more negative than the positive supply. This is selected to give a minimum end-of-life battery

voltage of about 6V. However, analog COMMON has some of the attributes of a reference voltage. When the total

supply voltage is large enough to cause the zener to regulate(>7V), the COMMON voltage will have a low voltage

coefficient (0.001%/V), low output impedance (≒15Ω), and a temperature coefficient typically less than 80ppm/℃.

The UTC 7106, with its negligible dissipation, suffers from none of these problems. In either case, an external

reference can easily be added, as shown in Figure 1.

Analog COMMON is also used as the input low return during auto-zero and de-integrate. If IN LO is different from

analog COMMON, a common mode voltage exists in the system and is taken care of by the excellent CMRR of the

converter. However, in some applications IN LO will be set at a fixed known voltage(power supply common for

instance).In this application, analog COMMON should be tied to the same point, thus removing the common mode

voltage from the converter. The same holds true for the reference voltage. If reference can be conveniently tied to

analog COMMON, it should be since this removes the common mode voltage from the reference system.

Within the IC, analog COMMON is tied to an N-Channel FET that can sink approximately 30mA of current to hold

the voltage 2.8V below the positive supply (when a load is trying to pull the common line positive). However, there is