TC818 datasheet(8/24 Pages) MICROCHIP

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TC818A

DS21475B-page 8

3.2

Resistance, Voltage, Current

Measurement Selection

The TC818A is designed to measure voltage, current,

and resistance. Auto-ranging is available for resistance

and voltage measurements. The OHM (Pin 2) and

I (Pin 63) input controls are normally pulled internally to

VCC.

By tying these pins to DGND (Pin 58), the TC818A is

configured internally to measure resistance, voltage, or

current. The required signal combinations are shown in

Table 3-1.

TABLE 3-1:

TC818A MEASUREMENT

SELECTION LOGIC

Note

1: OHM and I are normally pulled internally high to

VCC (Pin 28). This is considered a logic “1”.

2: Logic “0” is the potential at digital ground (Pin 58).

3.3

Resistance Measurements

(Ohms and Low Power Ohms)

The TC818A can be configured to reliably measure in-

circuit resistances shunted by semiconductor junc-

tions. The TC818A Low Power Ohms Measurement

mode limits the probe open circuit voltage. This pre-

vents semiconductor junctions in the measured system

from turning on.

In the Resistance Measurement mode, the

Ω/LOΩ

(Pin 62) input selects the Low Power Ohms Measure-

ment mode. For low power ohms measurements,

Ω/LOΩ (Pin 62) is momentarily brought LOW to digital

ground potential. The TC818A sets up for a low power

ohms measurement, with a maximum open circuit

probe voltage of 0.35V above analog common. In the

Low Power Ohms mode, an LCD display annunciator,

Ω, will be activated. On power-up, the Low Power

Ohms mode is not active.

If the Manual Operating mode has been selected, tog-

gling

Ω/LOΩ resets the TC818A back to the Auto-

Range mode. In Manual mode, the decision to make a

normal or low power ohms measurement should be

made before selecting the desired range.

The low power ohms measurement is not available on

the 200

Ω full scale range. Open circuit voltage on this

range is below 2.8V. The standard resistance values

are listed in Table 3-2.

R8, a positive temperature coefficient resistor, and the

6.2V zener, Z1, provide input voltage protection during

ohms measurements.

TABLE 3-2:

OHMS RANGE LADDER

NETWORK

3.4

Ratiometric Resistance

Measurements

The TC818A measures resistance ratiometrically.

Accuracy is set by the external standard resistors con-

nected to Pin 35 through 39. A Low Power Ohms mode

may be selected on all but the 200

Ω full scale range.

The Low Power Ohms mode limits the voltage applied

to the measured system. This allows accurate “in-

circuit” measurements when a resistor is shunted by

semiconductor junctions.

Full auto-ranging is provided. External precision stan-

dard resistors are automatically switched to provide the

proper range.

Figure 3-2 shows a detailed block diagram of the

TC818A, configured for ratiometric resistance measure-

ments. During the signal integrate phase, the reference

capacitor charges to a voltage inversely proportional to

the measured resistance, RX. Figure 3-3 shows that the

conversion accuracy relies on the accuracy of the

external standard resistors.

Normally, the required accuracy of the standard resis-

tances will be dictated by the accuracy specifications of

the users end product. Table 3-3 gives the equivalent

ohms per count for various full scale ranges to allow

users to judge the required resistor for accuracy.

TABLE 3-3:

REFERENCE RESISTORS

Function Select Pin

0 = Digital Ground

1 = Floating or Tied to VCC

Selected

Measurement

OHM (Pin 2)

I (Pin 63)

Voltage

Resistance

Current

Voltage

Full-Scale

Range

Standard

Resistance

Low Power

Ohms Mode

200

Ω

163.85

Ω (R1)

2000

Ω

1638.5k

Ω (R2)

YES

20k

Ω

16,385

Ω(R3)

YES

200k

Ω

163,85

Ω (R4)

YES

2,000k

Ω

1,638,500

Ω (R5)

YES

Full Scale

Range

Reference Resistor

Ω/Count

200k

163.85

0.1

1638.5

20k

16385

200k

163,850

100

1,638,500

1000

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