7

FN3084.5

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 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.)

Analog COMMON

This pin is included primarily to set the common mode

voltage for battery operation 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 6.8V.

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 (<6.8V), the

COMMON voltage will have a low voltage coefficient

(0.001%/V), low output impedance (

≅15Ω), and a

The limitations of the on-chip reference should also be

recognized, however. The reference Temperature Coefficient

(TC), can cause some degradation in performance.

can give a scale factor error of a count or more. Also the

common voltage will have a poor voltage coefficient when the

total supply voltage is less than that which will cause the zener

to regulate (<7V). These problems are eliminated if an

external reference is used, as shown in Figure 5.

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 lC, analog COMMON is tied to an N channel FET

that can sink approximately 3mA of current to hold the volt-

age 2.8V below the positive supply (when a load is trying to

pull the common line positive). However, there is only 1

µA of

source current, so COMMON may easily be tied to a more

negative voltage thus overriding the internal reference.

FIGURE 5A.

FIGURE 5B.

FIGURE 5.

ICL7126

V+

REF LO

REF HI

V+

V-

6.8V

ZENER

ICL7126

V+

REF HI

REF LO

COMMON

V+

ICL8069

1.2V

REFERENCE

27k

Ω

200k

Ω

ICL7126