TC820CPL datasheet(19/28 Pages) MICROCHIP

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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

µF 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.

polypropylene

capacitor

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

µA 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 100kΩ.

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

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

µV

500mV

250

µV

2V (Notes 3, 4)

500

µV

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

TC820

22k

Ω

VDD

VSS

VREF+

VREF-

Analog

Common

SET VREF = 1/2 VFULL SCALE

(a) Internal Reference

(b) External Reference

VREF+

VREF-

Analog

Common

V+

Ω

VREF

VIN

VOUT

VSS

MCP1525

µF

Displayed Reading =

RUNKNOWN

RSTANDARD

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