TC826

DS21477B-page 10

© 2002 Microchip Technology Inc.

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:

• 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:

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

4.4

Components Value Selection

4.4.1

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 1

µA drive current with

1

µA full scale current:

EQUATION 4-4:

4.4.2

The integrating capacitor should be selected to maxi-

mize integrator output swing. The integrator output will

The integrating capacitor is easily calculated:

EQUATION 4-5:

The integrating capacitor should be selected for low

dielectric absorption to prevent rollover errors. Polypro-

pylene capacitors are suggested.

4.4.3

capacitor. A polypropylene capacitor is suggested. Typ-

ical values from 0.14

µF to 0.068µFare satisfactory.

4.4.4

A1

µF 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.

Required Full Scale Voltage

20mV

10mV

2V

1V









32

TC826

REF IN

ANALOG

COMMON

2.50V

Reference

5

2

V+

8

V+

(b)

MCP1525

1

µF

Full Scale Voltage(V)

1x10–6

1x10 – 6

=









640