ICL7135CSLASHD datasheet(8/13 Pages) MAXIM

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amount of dielectric absorption in the integrating capacitor.

A good integrating capacitor will result in a reading of

9999, and any deviation from this reading is probably due

to dielectric absorption. Polypropylene capacitors have

been found to be suitable, as have Teflon capacitors.

Polystyrene and polycarbonate capacitors may also be

used in less critical applications.

Auto-Zero Capacitor

The size of the auto-zero capacitor will have a significant

effect on the overall system noise, with larger auto-zero

capacitors resulting in a quieter system. The dielectric

absorption of the auto-zero capacitor affects only the

speed of settling at power-up or recovery from overload

and nearly any capacitor type can be used. The zero

integrator phase of the ICL7135 allows the use of large

auto-zero capacitors while avoiding the “over-range hang-

over” and hysteresis effects that occur in A/D converters

without the zero integrator phase.

Reference Capacitor

Like the auto-zero capacitor, the reference capacitor’s

dielectric absorption is rarely critical. Low dielectric

absorption reference capacitors are only required where

fast settling time is needed in systems with a rapidly

changing reference voltage such as ratiometric ohms

measurement in multimeters.

The reference capacitor DOES need to be a low leakage

capacitor since it must store the reference voltage while

floating during both the signal integrate and the reference

deintegrate phases. Any leakage or charge loss during

these two phases results in an effective change in the

scale factor of the ICL7135. Low cost film capacitors such

as polyester or polystyrene have been found to be suitable

in most applications.

In addition to leakage requirements, another effect that

sets a lower limit on the value of the reference capacitor

is the “charge suckout” caused by stray capacitance on

the reference capacitor terminals. For a negative polarity

input signal, the reference capacitor does not shift its

common mode voltage, but with a positive polarity input

signal it undergoes mode shift equal to the reference

voltage. If there are stray capacitances on the reference

capacitor terminals, some of the charge on the reference

capacitor will be used to charge these stray capacitances

as the reference capacitor makes this common mode

voltage shift. This loss of charge reduces the voltage

on the reference capacitor, and causes positive polarity

signals to have a higher measured result than a corre-

sponding negative voltage. This error can be reduced by

minimizing the stray capacitance on the reference capacitor

terminals, and by increasing the value of the reference

capacitor.

Reference Voltage

The full scale reading of 20,000 will occur when VIN = 2 x

VREF. Since the 20,000 count resolution of the ICL7135

is equivalent to a 50ppm resolution, a high stability

reference is recommended for high accuracy absolute

measurements. Figure 7 shows two suitable methods of

generating the reference voltage.

Rollover Resistor and Diode

The ICL7135 is tested for rollover using the circuit

of Figure 1, with the 100kΩ resistor and diode in the

circuit. The diode is noncritical, and is typically a low

cost 1N4148. The resistor value is dependent on many

factors including integrator swing, clock frequency, and

the amount of rollover error due to “charge suckout” on

the reference capacitor. 100kΩ is the optimum value for

most circuits and is the value used in testing the ICL7135.

Speedup Resistor

The 27Ω speedup resistor in series with the integrat-

ing capacitor adds a pedestal voltage on top of the

integrating capacitor voltage. This pedestal voltage

causes zero crossing to occur earlier than would occur

without the resistor. The effect of the earlier zero crossing

is to give the comparator an overdrive voltage, speeding

its response and reducing the conversion error due to

comparator delay. If the integrator current is changed, the

speedup resistor value should be changed so that the IINT

x RSPEEDUP = 500µV.

Clock Frequency

The clock source should be free of short-term phase and

frequency jitter during the conversion period, but long

term stability is not critical. The clock frequency is chosen

to obtain the desired conversion rate, and to maximize

the normal mode rejection of power line frequency inter-

ference. The conversion rate is directly proportional to

the clock frequency, with each conversion taking 40,002

clock cycles. For maximum normal mode rejection,

Figure 7. External Reference Voltage

ICL7135

41/2 Digit A/D Converter with

Multiplexed BCD Outputs

www.maximintegrated.com

Maxim Integrated │ 8

V-

REF HI

6.8 VOLT

ZENER

kΩ

6.8kΩ

V-

ANALOG

COMMON

ICL7135

V-

V+

REF HI

1.2 BANDGAP

REFERENCE

ANALOG

COMMON

ICL7135

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