ꢀꢁ ꢂꢃ ꢄ ꢅ ꢆ ꢁꢇ ꢈꢂ ꢁꢃ ꢄꢅ ꢆꢁ
ꢉ ꢄ ꢊꢋ ꢌꢍꢀ ꢎ ꢀꢈ ꢏ ꢐꢑ ꢁ ꢀꢒ ꢀ ꢓꢔ
ꢕꢔꢕ ꢂꢓ ꢎ ꢌꢈꢓ ꢌꢍꢀꢎ ꢀ ꢈꢕꢂ ꢁꢓ ꢔꢖ ꢑꢐ ꢈꢑ ꢐ ꢒ
SLAS074D − DECEMBER 1986 − REVISED SEPTEMBER 2003
Integrating Capacitor
The product of the integrating resistor and capacitor should be selected to give the maximum voltage swing
without causing the integrating amplifier output to saturate and get too close to the power supply voltages. When
the amplifier output is within 0.3 V of either supply, saturation occurs. With 5-V supplies and ANLG COMMON
connected to ground, the designer should design for a 3.5-V to 4-V integrating amplifier swing. A nominal
capacitor value is 0.47 µF. The equation for determining the value of the integrating capacitor (C ) is:
INT
10, 000 Clock Period I
INT
C
+
INT
Integrator Output Voltage Swing
where
I
is nominally 20 µA.
INT
Capacitors with large tolerances and high dielectric absorption can induce conversion inaccuracies. A capacitor
that is too small could cause the integrating amplifier to saturate. High dielectric absorption causes the effective
capacitor value to be different during the signal integrate and deintegrate phases. Polypropylene capacitors
have very low dielectric absorption. Polystyrene and polycarbonate capacitors have higher dielectric
absorption, but also work well.
Auto-Zero and Reference Capacitor
Large capacitors tend to reduce noise in the system. Dielectric absorption is unimportant except during power
up or overload recovery. Typical values are 1 µF.
Reference Voltage
For high-accuracy absolute measurements, a high quality reference should be used.
Rollover Resistor and Diode
The ICL7135C and TLC7135C have a small rollover error; however, it can be corrected. The correction is to
connect the cathode of any silicon diode to INT OUT and the anode to a resistor. The other end of the resistor
is connected to ANLG COMMON or ground. For the recommended operating conditions, the resistor value is
100 kΩ. This value may be changed to correct any rollover error that has not been corrected. In many noncritical
applications the resistor and diode are not needed.
Maximum Clock Frequency
For most dual-slope A/D converters, the maximum conversion rate is limited by the frequency response of the
comparator. In this circuit, the comparator follows the integrator ramp with a 3-µs delay. Therefore, with a
160-kHz clock frequency (6-µs period), half of the first reference integrate clock period is lost in delay. Hence,
the meter reading changes from 0 to 1 with a 50-µV input, 1 to 2 with a 150-µV input, 2 to 3 with a 250-µV input,
etc. This transition at midpoint is desirable; however, when the clock frequency is increased appreciably above
160 kHz, the instrument flashes 1 on noise peaks even when the input is shorted. The above transition points
assume a 2-V input range is equivalent to 20,000 clock cycles.
When the input signal is always of one polarity, comparator delay need not be a limitation. Clock rates of 1 MHz
are possible since nonlinearity and noise do not increase substantially with frequency. For a fixed clock
frequency, the extra count or counts caused by comparator delay are a constant and can be subtracted out
digitally.
For signals with both polarities, the clock frequency can be extended above 160 kHz without error by using a
low value resistor in series with the integrating capacitor. This resistor causes the integrator to jump slightly
towards the zero-crossing level at the beginning of the deintegrate phase, and thus compensates for the
comparator delay. This series resistor should be 10 Ω to 50 Ω. This approach allows clock frequencies up to
480 kHz.
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