±18-Bit ADC with Serial Interface
Table 1. Crystal Frequencies and
Integrator Capacitors for 50Hz to 60Hz
Operation
Conv/sec
Crystal
Freq.
(Hz)
CINT/60Hz CINT/50Hz Resistor
(pF)
(pF)
(kΩ)
16
32,768
4700
6800
602
32
65,536
2700
3300
602
48
98, 304
1800
2000
602
64
131,072
1200
1500
602
80
163,840
1000
1200
602
96
196,608
820
1000
602
Note: Capacitor values are for a 3.0V integrator swing.
Manufactures of miniature quartz resonators include:
Epson of America
C-2 (through-hole), MC-306 (SMD)
Phone: (310) 787-6300; Fax: (310) 782-5320
Integrator Resistor
The integrator resistor sets the maximum integrator out-
put current for the integrate phase. A 602kΩ low-noise,
metal-film integrator resistor is recommended for use
with reference voltages between 545mV and 655mV.
Best linearity is achieved when the integration current
(IINT) does not exceed 2.5µA. For other reference volt-
ages, select RINT as follows:
RINT
=
2.5µA
<
VREF
IINT <
0. 5µA
and
IINT =
VREF
RINT
Integrator Capacitor
The oscillator frequency, integrator resistor, and inte-
grator capacitor set the maximum integrator output volt-
age swing for full-scale reading. The integrator voltage
swing is about 3V and should not come within 2V of
either supply rail to avoid saturation. A 602kΩ integrator
resistor and a 4.7nF integrator capacitor are recom-
mended with a clock frequency of 32,768Hz. If different
clock frequencies are used, select CINT using the fol-
lowing equations:
tINT
=
545 , for 60Hz mode
fOSC
or
tINT
=
655 , for 50Hz mode
fOSC
The integrator capacitor’s dielectric absorption directly
affects integral nonlinearity. High-quality metal-film
capacitors are recommended in the following order of
preference: polypropylene, polystyrene, polycarbon-
ate, and polyester (Mylar). The polyester capacitor will
generate some integral nonlinearity.
To minimize noise, INT OUT should drive the outside
foil (negative end) of the capacitor. Manufacturers of
polypropylene capacitors include Sprague (715P),
Panasonic (ECQ-P), Roderstein (KP1835), Wima (FKP),
and CSF Thompson (PL/PS).
Reference Capacitor
The reference capacitor must be small enough to fully
charge from a discharged state on power-up in reason-
able time, and large enough so the charge does not
droop excessively during a conversion. The reference
capacitor is normally 0.1µF for all oscillator frequencies.
For applications that require a physically smaller capaci-
tor, the equation below will maintain CREF proportionality:
CREF
=
0.0033
fOSC
The reference capacitor must have low leakage, since
it stores the reference voltage while floating during the
deintegrate phase. Any leakage or charge loss during
this phase changes the scale factor and will cause an
error. Appropriate metal-film capacitors recommended
for their low-leakage characteristics1 are (in this order):
polypropylene (up to +105°C, large size), teflon (suit-
able for use up to +125°C, large size), polystyrene,
polycarbonate, and polyester.
At temperatures above +85°C, capacitor leakage may
affect accuracy. In such cases, increasing the value of
CREF up to 50% and more will help at the expense of
longer start-up time at power-on. The start-up time is
proportional to CREF and can be estimated by:
tSTART−UP = CREF(µF) x 10 x 100kΩ
CINT
=
(VIN(FS) ) (tINT) ,
(RINT) (VSWING)
where
1V <
VSWING < 3.5V;
and
1 Pease, R.A., “Understanding Capacitor Soakage to Optimize
Analog Systems,” EDN, October 13, 1982, p.125.
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