TC826 データシートの表示（PDF） - Microchip Technology
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TC826 Datasheet PDF : 18 Pages
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TC826
4.2 System Timing
The oscillator frequency is divided by 32 prior to clock-
ing the internal counters. The three-phase measure-
ment cycle takes a total of 80 clock pulses. The 80
count cycle is independent of input signal magnitude.
Each phase of the measurement cycle has the follow-
ing length:
• Auto-Zero Phase: 19 to 59 Counts
For signals less than full scale, the auto-zero is
assigned the unused reference integrate time period.
• Signal Integrate: 20 Counts
This time period is fixed. The integration period is:
EQUATION 4-3:
TSI
=
20
32
FOSC
Where FOSC is the externally set clock frequency.
• Reference Integrate: 0 to 41 Counts
4.3 Reference Voltage Selection
A full scale reading requires the input signal be twice
the reference voltage. The reference potential is mea-
sured between REF IN (Pin 5) and ANALOG
COMMON (Pin 2).
TABLE 4-1:
Required Full Scale Voltage
20mV
2V
VREF
10mV
1V
The internal voltage reference potential available at
analog common will normally be used to supply the
converter’s reference. This potential is stable when-
ever the supply potential is greater than approximately
7V. In applications where an externally generated refer-
ence voltage is desired, refer to Figure 4-3.
The reference voltage is adjusted with a near full scale
input signal. Adjust for proper LCD display read out.
FIGURE 4-3:
EXTERNAL REFERENCE
V+
8
V+
TC826
5
REF IN
ANALOG 2
COMMON
(b)
MCP1525
1µF
2.50V
Reference
DS21477B-page 10
4.4 Components Value Selection
4.4.1
INTEGRATING RESISTOR (RINT)
The desired full scale input voltage and output current
capability of the input buffer and integrator amplifier set
the integration resistor value. The internal class A out-
put stage amplifiers will supply a 1µA drive current with
minimal linearity error. RINT is easily calculated for a
1µA full scale current:
EQUATION 4-4:
RINT =
Full Scale Voltage(V)
1 x 10 – 6
=
VFS
1 x 10 – 6
Where VFS = Full Scale Analog Input
4.4.2
INTEGRATING CAPACITOR (CINT)
The integrating capacitor should be selected to maxi-
mize integrator output swing. The integrator output will
swing to within 0.4V of VS+ or VS- without saturating.
The integrating capacitor is easily calculated:
EQUATION 4-5:
CINT =
VFS
RINT
640
FOSC x VINT
Where: VINT = Integrator Swing
FOSC = Oscillator Frequency
The integrating capacitor should be selected for low
dielectric absorption to prevent rollover errors. Polypro-
pylene capacitors are suggested.
4.4.3
AUTO-ZERO CAPACITOR (CAZ)
CAZ should be 2-3 times larger than the integration
capacitor. A polypropylene capacitor is suggested. Typ-
ical values from 0.14µF to 0.068µF are satisfactory.
4.4.4
REFERENCE CAPACITOR (CREF)
A 1µF capacitor is suggested. Low leakage capacitors,
such as polypropylene, are recommended.
Several capacitor/resistor combinations for common
full scale input conditions are given in Table 4-2.
© 2002 Microchip Technology Inc.
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