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| 型号： | TC820CLW |
| 元件分类： | ADC |
| 英文描述： | 1-CH DUAL-SLOPE ADC, PQCC44 |
| 封装： | PLASTIC, LCC-44 |
| 文件页数： | 11/28页 |
| 文件大小： | 616K |
| 代理商： | TC820CLW |
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2002 Microchip Technology Inc.
DS21476B-page 19
TC820
5.6.3
INTEGRATING CAPACITOR - CINT
CINT should be selected to maximize integrator output
voltage swing without causing output saturation. Analog
common will normally supply the differential voltage ref-
erence. For this case, a ±2V integrator output swing is
optimum when the analog input is near full scale. For 2.5
readings/second (FOSC = 40kHz) and VFS = 400mV, a
0.22
F value is suggested. If a different oscillator fre-
quency is used, CINT must be changed in inverse pro-
portion to maintain the nominal ±2V integrator swing.
An exact expression for CINT is:
EQUATION 5-2:
CINT must have low dielectric absorption to minimize
rollover
error.
A
polypropylene
capacitor
is
recommended.
5.6.4
INTEGRATING RESISTOR - RINT
The input buffer amplifier and integrator are designed
with class A output stages. The integrator and buffer
can supply 40
A drive currents with negligible linearity
errors. RINT is chosen to remain in the output stage lin-
ear drive region, but not so large that printed circuit
board leakage currents induce errors. For a 400mV full
scale, RINT should be about 100k.
5.7
Reference Voltage Selection
A full scale reading (4000 counts for TC820) requires
the input signal be twice the reference voltage. See
Reference Voltage Selection, Table 5-1 below.
Note
1:
TC820 in A/D Converter mode, RANGE/FREQ =
logic low.
2: Not recommended.
3: VFS > 2V may exceed the Input Common mode
range. See Section 3.2.7, "10:1 Range Change".
4: Full scale voltage values are not limited to the val-
ues shown. For example, TC820 VFS can be any
value from 400mV to 2V.
In some applications, a scale factor other than unity
may exist between a transducer output voltage and the
required digital reading. Assume, for example, that a
pressure transducer output is 800mV for 4000 lb/in2.
Rather than dividing the input voltage by two, the refer-
ence voltage should be set to 400mV. This permits the
transducer input to be used directly.
The internal voltage reference potential available at ana-
log common will normally be used to supply the con-
verter's reference voltage. This potential is stable
whenever the supply potential is greater than approxi-
mately 7V. The low battery detection circuit and analog
common operate from the same internal reference. This
ensures that the low battery annunciator will turn on at
the time the internal reference begins to lose regulation.
The TC820 can also operate with an external refer-
ence. Figure 5-7 shows internal and external reference
applications.
FIGURE 5-7:
REFERENCE VOLTAGE
CONNECTIONS
5.8
Ratiometric Resistance
Measurements
The TC820 true differential input and differential refer-
ence make ratiometric readings possible. In ratiometric
operation, an unknown resistance is measured with
respect to a known standard resistance. No accurately
defined reference voltage is needed.
The unknown resistance is put in series with a known
standard and a current is passed through the pair
(Figure 5-8). The voltage developed across the unknown
is applied to the input and voltages across the known
resistor applied to the reference input. If the unknown
equals the standard, the input voltage will equal the refer-
ence voltage and the display will read 2000. The displayed
reading can be determined from the following expression:
EQUATION 5-3:
The display will over range for values of RUNKNOWN ≥
2x RSTANDARD.
TABLE 5-1:
REFERENCE VOLTAGE
SELECTION
Full Scale Input Voltage
(VFS)(Note 1)
VREF
Resolution
200mV
(Note 2)
—
400mV
200mV
10
V
1V
500mV
250
V
2V (Notes 3, 4)
1V
500
V
CINT =
4000 VFS
VINT RINT FOSC
Where: FOSC = Clock Frequency
VFS = Full Scale Input Voltage
RINT = Integrating Resistor
VINT = Desired Full Scale Integrator
Output Swing
9V
TC820
+
22k
VDD
VSS
VREF+
VREF-
Analog
Common
SET VREF = 1/2 VFULL SCALE
(a) Internal Reference
(b) External Reference
VREF+
VREF-
Analog
Common
V+
2k
VREF
VIN
VOUT
VSS
MCP1525
1
F
Displayed Reading =
RUNKNOWN
RSTANDARD
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