FS9721_LP3

Rev. 2.1

30/39

In TSTB connecting to VSS mode, the current signal enters IC from SA.

When doing the current measurement, The sampling resistance of μA mode is R16+R17+R28, the

sampling resistance of mA mode is R16+R28, and the sampling resistance of 10A mode is R28. They are

measured respectively through the mode switch. When measuring μA, J1 is open and J7 is close; when

measuring mA, J7 is open and J1 is close; the large current enters directly via 10A port.

The maximum reduced voltage for μA, mA and 10A modes is 4V.

These voltages are input voltage comparator to compare. If the voltage is smaller than 40mV, it will be sent

to ×10 times ampifier to amplify, then sent to A/D converter; if the voltage is larger than 40mV, the system will

work the auto-ranging signal, and jump up one mode under the control of microprocessor and send the current

signal directly to the A/D converter.

The accuracy of resistance R16, R17, R28, R7 and R19 affects the accuracy of the current measurement.

15.11

Resistance Measurement

J5

J3

Ohm

COM

TENM

ONEM

HUNK

TENK

ONEK

SMV

SGND

FS9721_LP3

AGND

PTC

1K

CRES1

C13 27nF

R19 900k

R22 1k

R23 10k

R24 101.01k

R26 10M

R25 1.111M

Rx

R20 100k

RL

R31 10k

Diagram 28

Resistance Measurement

Resistance measurement refers to the standard resistance, and then takes a comparison between measuring

resistance and standard resistance to get the measuring resistance value. The standard resistance of 40MΩ mode is

10MΩ (R26). The standard resistance of other modes are R26 to parallel respectively with R25, R24, R23 and R22

to get 1MΩ, 100kΩ, 10kΩ, 1kΩ resistance. When doing resistance measurement, internal IC will generate 0.4V

voltage (relative to AGND), and the voltage is output respectively through resistance R26, R25, R24, R23 and R22 to

the measuring resistance to do the comapring measurement.

R20 connects to RL, which is the negative input of the voltage reference that is gotten by the standard

resistance.

J3 and J5 are mode switches. When doing resistance measurement, J3 and J5 are close.

C13 is the wave filter capacitance of measuring point in resistance measurement.

15.12

Diode Test

TENM

ONEK

SMV

DT

SGND

FS9721_LP3

J5

J3

VD

Diode

COM

PTC

1k

R26

10M

R18

100k

R19

900k

R22

1k

R31

10k

Diagram 29

Diode Measurement

Diode test generates 1.5V voltage from internal IC and outputs it through R22, then adds it to the positive of diode

through PTC. The positive voltage reduction VD generated by diode is approx. 0.5V-0.7V. VD is divided by R20 and

R19 to be 1/10 VD, then is amplified 10 times by internal OP and is delivered to A/D before displaying the VD value. J3

and J5 are mode switches. When doing diode measurement, J3 and J5 are close.