10

FN3086.6

July 21, 2005

System Timing

Figure 7 shows the clocking arrangement used in the

ICL7136. Two basic clocking arrangements can be used:

1. Figure 9A, an external oscillator connected to DIP pin 40.

2. Figure 9B, an R-C oscillator using all three pins.

The oscillator frequency is divided by four before it clocks

the decade counters. It is then further divided to form the

three convert-cycle phases. These are signal integrate

(1000 counts), reference de-integrate (0 to 2000 counts) and

auto-zero (1000 to 3000 counts). For signals less than full

scale, auto-zero gets the unused portion of reference de-

integrate. This makes a complete measure cycle of 4,000

counts (16,000 clock pulses) independent of input voltage.

For three readings/second, an oscillator frequency of 48kHz

would be used.

To achieve maximum rejection of 60Hz pickup, the signal

integrate cycle should be a multiple of 60Hz. Oscillator

frequencies of 240kHz, 120kHz, 80kHz, 60kHz, 48kHz,

rejection, Oscillator frequencies of 200kHz, 100kHz,

40kHz (2.5 readings/sec.) will reject both 50Hz and 60Hz

(also 400Hz and 440Hz).

Component Value Selection

Integrating Resistor

Both the buffer amplifier and the integrator have a class A

output stage with 100

µA of quiescent current. They can

supply 1

µA of drive current with negligible nonlinearity. The

integrating resistor should be large enough to remain in this

very linear region over the input voltage range, but small

enough that undue leakage requirements are not placed on

the PC board. For 2V full scale, 1.8M

Ω is near optimum and

similarly a 180k

Ω for a 200mV scale.

Integrating Capacitor

The integrating capacitor should be selected to give the

maximum voltage swing that ensures tolerance buildup will

not saturate the integrator swing (approximately 0.3V from

either supply). In the ICL7136, when the analog COMMON

is used as a reference, a nominal +2V full-scale integrator

swing is fine. For three readings/second (48kHz clock)

respectively. Of course, if different oscillator frequencies are

used, these values should be changed in inverse proportion

to maintain the same output swing.

An additional requirement of the integrating capacitor is that

it must have a low dielectric absorption to prevent roll-over

errors. While other types of capacitors are adequate for this

application, polypropylene capacitors give undetectable

errors at reasonable cost.

Auto-Zero Capacitor

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

the noise of the system. For 200mV full scale where noise is

very important, a 0.47

µF capacitor is recommended. On the

2V scale, a 0.047

µF capacitor increases the speed of

recovery from overload and is adequate for noise on this

scale.

Reference Capacitor

A 0.1

µF capacitor gives good results in most applications.

However, where a large common mode voltage exists (i.e.,

the REF LO pin is not at analog COMMON) and a 200mV

scale is used, a larger value is required to prevent roll-over

error. Generally 1

µF will hold the roll-over error to 0.5 count

in this instance.

Oscillator Components

For all ranges of frequency a 180k

Ω resistor is

recommended and the capacitor is selected from the

equation:

CLOCK

INTERNAL TO PART

40

39

38

TEST

CLOCK

INTERNAL TO PART

40

39

38

R

C

FIGURE 7B. RC OSCILLATOR

FIGURE 7. CLOCK CIRCUITS

FIGURE 7A. EXTERNAL OSCILLATOR

f

0.45

RC

=

=

ICL7136