5-19

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 to 40

µA give good results with a

nominal of 20

µA. The exact value may be chosen by:

NOTE: If gain is used in the buffer amplifier then

Integrating Capacitor

The product of integrating resistor and capacitor is selected

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

between possibly saturating the integrator (at +14 volts) due

to tolerance build-up 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

A very important characteristic of the integrating capacitor is

that it have 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 read half scale (100...000)

and any deviation is probably due to dielectric absorption.

Polypropylene capacitors give undetectable errors at reason-

able cost. Polystyrene and polycarbonate capacitors may

also 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, a large capacitor giving less noise.

The reference capacitor should be large enough such that

stray capacitance to ground from its nodes is negligible.

NOTE: 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.

Reference Voltage

The analog input required to generate a full scale output is

The stability of the reference voltage is a major factor in the

overall absolute accuracy of the converter. The resolution of

the ICL7104 at 16 bits is one part in 65536, or 15.26ppm.

Thus, if the reference has a temperature coefficient of

50ppm/C (on board reference) a temperature change of

1/3C will introduce a one-bit absolute error. For this reason,

it is recommended that an external high quality reference be

used where the ambient temperature is not controlled or

where high-accuracy absolute measurements are being

made.

R

INT

full scale voltage (see note)

20

µA

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

=

R

INT

Buffe rGain

() (full scale voltage)

20

µA

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

=

C

INT

(32768 for - 16)

(8192 for -14)



 20µA

×

clock period

×

Integrator Output Voltage Swing

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

=

COUNTS

PHASE I

PHASE II

PHASE III

-16

32768

32768

65536

-14

8192

8192

16384

FIGURE 8. CONVERSION TIMING

POLARITY

DETECTED

ZERO CROSSING

OCCURS

ZERO CROSSING

DETECTED

DEINT PHASE III

INT PHASE II

AZ PHASE I

AZ

INTEGRATOR

OUTPUT

INTERNAL CLOCK

INTERNAL LATCH

STATUS OUTPUT

NUMBER OF COUNTS TO ZERO CROSSING

AFTER ZERO CROSSING,

ANALOG SECTION WILL

BE IN AUTOZERO

CONFIGURATION

ICL8052/ICL7104, ICL8068/ICL7104