±18-Bit ADC with Serial Interface
MAX132
Table 1. Crystal Frequencies and
Integrator Capacitors for 50Hz to 60Hz
Operation
Conv/sec
16
32
48
64
80
96
Crystal
Freq.
(Hz)
32,768
65,536
98, 304
131,072
163,840
196,608
C
INT
/60Hz
(pF)
4700
2700
1800
1200
1000
820
C
INT
/50Hz
(pF)
6800
3300
2000
1500
1200
1000
Resistor
(kΩ)
602
602
602
602
602
602
t
INT
=
or
545
f
OSC
655
, for 60Hz mod e
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
, for 50Hz mod e
f
OSC
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).
t
INT
=
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
(I
INT
) does not exceed 2.5µA. For other reference volt-
ages, select R
INT
as follows:
R
INT
=
and
I
INT
=
V
REF
R
INT
V
REF
2.5
µ
A
<
I
INT
<
0.5
µ
A
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 C
REF
proportionality:
C
REF
=
0.0033
f
OSC
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 C
INT
using the fol-
lowing equations:
C
INT
=
and
(V
IN(FS)
) (t
INT
)
(R
INT
) (V
SWING
)
, where 1V
<
V
SWING
<
3.5V;
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 characteristics
1
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
C
REF
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 C
REF
and can be estimated by:
t
START
−
UP
=
C
REF
(
µ
F) x 10 x 100k
Ω
1
Pease, R.A., “Understanding Capacitor Soakage to Optimize
Analog Systems,”
EDN,
October 13, 1982, p.125.
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9
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