10

GND, the normal integrator output swing at full scale is

±4V.

Since the integrator output can go to 0.3V from either supply

without significantly affecting linearity, a 4V integrator output

swing allows 0.7V for variations in output swing due to com-

ponent value and oscillator tolerances. With

±5V supplies

and a common mode range of

±1V required, the component

values should be selected to provide

±3V integrator output

swing. Noise and roll-over will be slightly worse than in the

±4V case. For larger common mode voltage ranges, the inte-

grator output swing must be reduced further. This will

increase both noise and roll-over errors. To improve the per-

formance, supplies of

±6V may be used.

Integrating Resistor

Both the buffer amplifier and the integrator have a class A output

stage with 100

µA of quiescent current. They supply 20µA 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 409.6mV full-

scale, 200k

Ω is near optimum and similarly a 20kΩ for a

should be chosen by the relation :

Integrating Capacitor

maximum voltage swing that ensures tolerance build-up will

not saturate the integrator swing (approximately. 0.3V from

either supply). For the ICL7109 with

±5V supplies and analog

common connected to GND, a

±3.5V to ±4V integrator output

clock frequency) as provided by the crystal oscillator, nominal

tively. If different clock frequencies are used, these values

should be changed to maintain the integrator output swing. In

.

An additional requirement of the integrating capacitor is that

it have low dielectric absorption to prevent roll-over errors.

While other types of capacitors are adequate for this applica-

tion, polypropylene capacitors give undetectable errors at

The integrating capacitor should have a low dielectric

absorption to prevent roll-over errors. While other types may

be adequate for this application, polypropylene capacitors

Teflon™ capacitors are recommended for the military tem-

perature range. While their dielectric absorption characteris-

tics vary somewhat from unit to unit, selected devices should

give less than 0.5 count of error due to dielectric absorption.

Auto-Zero Capacitor

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

the noise of the system: a smaller physical size and a larger

capacitance value lower the overall system noise. However,

with the integrating capacitor forms an R-C time constant

that determines the speed of recovery from overloads and

the error that exists at the end of an auto-zero cycle. For

409.6mV full scale where noise is very important and the

mum. Similarly for 4.096V full scale where recovery is more

recommended.

should be connected to the R-C summing junction and the

connected to pin 32 and the inner foil to the R-C summing

junction. Teflon, or equivalent, capacitors are recommended

Reference Capacitor

A 1

µF capacitor gives good results in most applications.

However, where a large reference common mode voltage

exists (i.e., the reference low is not at analog common) and

a 409.6mV scale is used, a large value is required to prevent

roll-over error. Generally 10

µF will hold the roll-over error to

0.5 count in this instance. Again, Teflon, or equivalent

their low leakage characteristics.

full scale voltage

20

µA

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

·

.

=

2048 clock period

×

() 20µA

()

integrator output voltage swing

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

=

ICL7109