10

Component Value Selection

For optimum performance of the analog section, care must

be taken in the selection of values for the integrator capacitor

and resistor, auto-zero capacitor, reference voltage, and

conversion rate. These values must be chosen to suit the

particular application.

Integrating Resistor

The integrating resistor is determined by the full scale input

voltage and the output current of the buffer used to charge

the integrator capacitor. This current should be small

compared to the output short circuit current such that

thermal effects are kept to a minimum and linearity is not

affected. Values of 5

µAto 40µA give good results with a

nominal of 20

µA. The exact value may be chosen by:

NOTE: If gainisusedinthe buffer amplifier, then:

Integrating Capacitor

The product of integrating resistor and capacitor is selected

to give 9V swing for full scale inputs. This is a compromise

between possibly saturating the integrator (at +14V) due to

tolerance buildup between the resistor, capacitor and clock

and the errors a lower voltage swing could induce due to

offsets referred to the output of the comparator. In general,

A very important characteristic of the integrating capacitor is

that it has low dielectric absorption to prevent roll-over or

ratiometric errors. A good test for dielectric absorption is to

use the capacitor with the input tied to the reference.

This ratiometric condition should be read half scale 1.0000,

and any deviation is probably due to dielectric absorption.

Polypropylene capacitors give undetectable errors at

reasonable cost. Polystyrene and polycarbonate capacitors

may be used in less critical applications.

Auto-Zero and Reference Capacitor

The size of the auto-zero capacitor has some influence on

the noise of the system, with a larger value capacitor giving

less noise. The reference capacitor should be large enough

such that stray capacitance to ground from its nodes is

negligible.

When gain is used in the buffer amplifier the reference

capacitor should be substantially larger than the auto-zero

capacitor. As a rule of thumb, the reference capacitor should

be approximately the gain times the value of the auto-zero

capacitor. The dielectric absorption of the reference cap and

auto-zero cap are only important at power-on or when the

circuit is recovering from an overload. Thus, smaller or

cheaper caps can be used here if accurate readings are not

required for the first few seconds of recovery.

FIGURE 5. TIMING DIAGRAM FOR OUTPUTS

AUTO

COUNTS

10,001

INTEGRATOR

BUSY

SIGNAL

COUNTS

10,000

INTEG.

REFERENCE

COUNTS MAX

20,001 / 2,001

INTEGRATE

AUTO ZERO

DIGIT SCAN

FOR OVER-RANGE

SIGNAL

INTEGRATE

REFERENCE

INTEGRATE

ONE COUNT LONGER

STROBE

1000

DIGIT SCAN

FOR OVER-RANGE

EXPANDED SCALE BELOW

/1,000

OVER-RANGE

WHEN APPLICABLE

UNDER-RANGE

WHEN APPLICABLE

OUTPUT

/1,001

ZERO

FULL MEASUREMENT CYCLE

40,002/4,002 COUNTS

R

INT

Full Scale Voltage (See Note)

20

µA

-------------------------------------------------------------------------------

=

R

IN T

Bu fferGa in

() (Full Scale Voltage)

20

µA

--------------------------------------------------------------------------------------------

=

C

INT

10,000(4-1/2 Digit)

1000(3-1/2 Digit)

Clock Period

×

20

µA

()

×

Integrator Output Voltage Swing

-------------------------------------------------------------------------------------------------------------------------

=

ICL8052A/ICL71C03, ICL8068A/ICL71C03