ICL7126 View Datasheet(PDF) - Renesas Electronics

Part Name

Description

Manufacturer

ICL7126

3 1/2 Digit, Low Power, Single Chip A/D Converter

Renesas Electronics 

ICL7216

V+

REF HI

REF LO

ICL7126

V+

6.8V

ZENER

IZ

V+

V+

ICL7126

200k

27k

REF HI

REF LO

ICL8069

1.2V

REFERENCE

V-

FIGURE 5A.

COMMON

FIGURE 5B.

FIGURE 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

(15), and a temperature coefficient typically less than

80ppm/oC.

The limitations of the on-chip reference should also be

recognized, however. The reference Temperature Coefficient

(TC), can cause some degradation in performance.

Temperature changes of 2oC to 8oC, typical for instruments, 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 voltage

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

common line positive). However, there is only 1A of source

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

voltage thus overriding the internal reference.

FN3084 Rev.5.00

Oct 25, 2004

Page 7 of 15

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