TC815 datasheet(7/26 Pages) MICROCHIP

7 / 26 page

DS21474B-page 7

TC815

3.0

DETAILED DESCRIPTION

3.1

Resistance, Voltage, Current

Measurement Selection

The TC815 is designed to measure voltage, current,

and resistance. Auto-ranging is available for resistance

and voltage measurements. The OHMS (Pin 2) and

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

VCC. By tying these pins to Digital Ground (Pin 58), the

TC815 is configured internally to measure resistance,

voltage, or current. The required signal combinations

are shown in Table 3-1.

TABLE 3-1:

MEASUREMENT SELECTION

LOGIC

Note

1: 0=DigitalGround

2: 1 = Floating or Tied to VCC

3: OHM and I are normally pulled internally high to

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

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

3.2

Resistance Measurements

(Ohms and Low Power Ohms)

The TC815 can be configured to reliably measure in-

circuit resistances shunted by semiconductor junc-

tions. The TC815 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/LOWΩ

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

ment mode. For low power ohms measurements,

Ω/LOWΩ (Pin 62) is momentarily brought low to digital

ground potential. The TC815 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,

LOW

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

Ohms mode is not active.

If the Manual mode has been selected, toggling

Ω/LOWΩ will reset the TC815 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 100

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 in Figure 3-1, provide input voltage pro-

tection during ohms measurements.

TABLE 3-2:

OHMS RANGE LADDER

NETWORK

3.3

Ratiometric Resistance

Measurements

The TC815 measures resistance ratiometrically. Accu-

racy is set by the external standard resistors connected

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 semicon-

ductor junctions. Full auto-ranging is provided. External

precision standard resistors are automatically switched

to provide the proper range.

Figure 3-1 shows a detailed block diagram of the

TC815 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 3-2 shows the

conversion accuracy relies on the accuracy of the

external standard resistors only.

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

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

1638.5 k

Ω (R2)

YES

20k

Ω

16,385

Ω (R3)

YES

200k

Ω

16385

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

163850

100

1638500

1000

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