TC818ACBU View Datasheet(PDF) - Microchip Technology
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TC818ACBU Datasheet PDF : 24 Pages
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TC818A
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
Function Select Pin
0 = Digital Ground
1 = Floating or Tied to VCC
OHM (Pin 2)
I (Pin 63)
0
0
0
1
1
0
1
1
Selected
Measurement
Voltage
Resistance
Current
Voltage
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,
LOΩ, 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 200Ω 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
Full-Scale
Range
200Ω
2000Ω
20kΩ
200kΩ
2,000kΩ
Standard
Resistance
163.85Ω (R1)
1638.5kΩ (R2)
16,385Ω(R3)
163,85Ω (R4)
1,638,500Ω (R5)
Low Power
Ohms Mode
NO
YES
YES
YES
YES
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 200Ω 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
Full Scale
Range
200k
2k
20k
200k
2M
Reference Resistor
163.85
1638.5
16385
163,850
1,638,500
Ω/Count
0.1
1
10
100
1000
DS21475B-page 8
© 2002 Microchip Technology Inc.
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