220 Input Signal Conditioning

The input signal (voltage, current, or resistance) must be scaled or conditioned to a form that can be measured by the a/d converter. High dc voltage levels must be attenuated. Resistances, currents, and ac voltages must be converted to a representative dc voltage. DC-type measurements (dc volts, dc current, ohms, and diode test) are then filtered by the active filter. AC measurements (ac volts and ac current) are passively filtered after being converted to a dc voltage. In the medium and fast measurement rates, the a/d converter uses one of two ranges: ±300 mV and ±3 V full scale. In the slow rate, the a/d converter uses one of two additional ranges (±100 mV and ±1000 mV full scale), for a total of four ranges.

2-21. Relays

Latching relays A1K1, A1K2, and A1K3 route and connect the input signal to the functional blocks required by the selected function. The Analog Measurement Processor (A1U1) function control outputs, which are 8-ms positive pulses (0 V to VDD) at FC0 to FC5 (pins 28 to 33), control the relay driver, A1U2. The A1U2 drivers are NPN Darlington pairs. Relay functions are defined in Table 2-2.

Table 2-2. Relay Operation

Function

Relays

A1K

A1K2

A1K3

DC mV, 3 V

Reset

Set

Set

DC 10 V, 30 V, 100 V, 300 V, 1000 V

Set

Set

Set

ACV

Set

Set

Reset

Ohms & k ohms

Reset

Reset

Set

M ohms

Reset

Set

Set

Diode Test

Reset

Reset

Set

mA DC and ADC

Set

Set

Reset

mA AC and AAC

Set

Set

Set

Frequency

Set

Set

Reset

Figure 2-3. DC Volts 300 V Range Simplified Schematic

2-22. DC Volts

For the mV and 3 V ranges, the input signal is applied to A1U1 through A1R6, A1R7, and A1K1. The signal is routed directly through A1U1 to the active filter and the a/d converter without attenuation. Higher voltages are attenuated by the dc input divider A1Z1.

For the 300 V range, switches S3 and S9 in A1U1 connect the A1Z1 10-MW resistor to the A1Z1 10.01-kW resistor, forming a 1000:1 divider. See Figure 2-3. The S10 switch in A1U1 routes the signal to the active filter. Pin 3 of A1Z1 serves as divider common, which is also the a/d converter low signal sensed through A1R9.

2-23. Ohms

Resistance measurements are made using the ratio ohms technique. See Figure 2-4. A voltage source is connected in series with the reference resistor (A1Z1) and the unknown resistor. Since the same current then flows through both resistors, the unknown resistance is determined from the ratio of the voltage drops across the reference and unknown resistors.

Depending on the range, S3, S6, S9, S13, or S15 connects the Analog Measurement Processor ohms voltage source to a reference resistor. The resulting current passes through the reference resistor, A1K2, the protection resistors A1RT1 and A1R5, and the unknown resistance. The a/d converter integrates with the voltage across the unknown through the OVS input (pin 23). For DE- integrate reference, the a/d converter uses the voltage across the reference resistor through RRS (pin 13) and Ohms Reference High through S5 (pin 21), S14 (pin 19), S17 (pin 16), S11 (pin 15) or S8 (pin 14). Reference resistances are identified in Table 2-3.

Figure 2-4. Ohms Simplified Schematic

Table 2-3. Reference Resistance

Range

Voltage Source

A1Z1 Reference Resistor

100 W /300 W

3 V

1 kW

1000 W/3 kW

1.3 V

10.01 kW

10 kW/30 kW

1.3 V

100.5 kW

100 kW/300 kW

1.3 V

1 MW

1000 kW/3 MW/10 MW

1.3 V

10 MW

30 MW

3 V

10 MW

100 MW/300 MW

3 V

10 MW

Diode Test

3 V

1 kW

A1R5 & V A1RT1 A1C1

A1R5 & V A1RT1 A1C1

Figure 2-5. AC Buffer Simplified Schematic Table 2-4. AC Volts Input Signal Dividers qb05f.eps

Figure 2-5. AC Buffer Simplified Schematic Table 2-4. AC Volts Input Signal Dividers

Range (Drive Signal)

A1Z2 Feedback Resistor

Overall Gain

100/300 mV (ACR1)

111.1 kW

2.5

1000 mV/3 V (ACR2)

12.25 kW || 111.1 kW

.25

10/30 V (ACR3)

1.013 kW || 111.1 kW

.025

100/300/750 V (ACR4)

2.776 kW

.0025

2-24. 100 MW and 300 MW Ranges

The 100 MW and 300 MW ranges perform a conductance reading; the mathematical reciprocal of this reading is used as a display in ohms. The reference resistor (A1Z1, 10 MW) is integrated first, then the unknown resistance is used for DE- integrate reference.

2-25. AC Volts

AC voltage and ac current inputs are scaled by the ac buffer, then converted to a representative dc voltage by the true rms ac-to-dc converter.

Refer to Figure 2-5. JFETs A1Q2 to A1Q8 switch the ranges of the buffer amplifier A1AR1. The JFET drive signals, ACR1 to ACR4 (pins 5 to 8) turn the JFETs either on at 0 V or off at -VAC. The ratio of the feedback resistor to the 1.111-MW input resistor divides the input signal by 10, 100, or 1000. These arrangements are summarized in Table 2-4. This signal is then amplified by 25 using the 2.776-kW and 115.7Wbuffer output divider resistors. The A1Z2 111.1-kW feedback resistor is left in parallel with the higher range feedback resistors. For the 300/750 V range, the 2.776-kWresistor becomes the feedback element. A1R15 and A1C2 compensate the 300-mV range of the ac buffer.

The ac signal is then routed to the rms converter by Analog Measurement Processor switch S38. Capacitors A1C1, A1C7, A1A1C2, and A1A1C3 function as dc blocking capacitors. A1A1R1 provides input bias current for the rms converter buffer, and A1A1C1 is the converter's averaging capacitor.

The rms converter output is divided down by 2.5 by A1Z4; A1R19 and A1C10 form the passive filter for ac volts. Analog Measurement Processor switch S80 shorts A1R19 both during ranging and in the fast measurement rate. Components A1R16, A1R17, A1C3,

A1C4, A1C5, and A1C6 provide a filtered power supply for the ac buffer, the ac buffer switching JFETs, and the rms converter.

2-26. DCmA

Current through A1R2 develops a voltage that is proportional to the input. This dc voltage is routed through A1R4 to the active filter, then to the a/d converter. The 100 mA current range uses the 3 V range of the a/d converter. See Figure 2-6.

2-27. ACmA

In ACmA, relay A1K3 connects the ac voltage developed across A1R2 to the ac buffer. The signal is then conditioned as described for ac volts.

2-28. Amps

The dc voltage output of the 10 amp shunt (A1R3) is routed directly to the a/d converter through the Analog Measurement Processor OVS input (pin 23). For ac amps, the ac voltage output of the shunt is routed to the rms converter through Analog Measurement Processor switches S35 and S37.

2-29. Diode/Continuity Test

In Diode Test, the meter front end is in the 300 ohm range configuration. The a/d converter measures the dc voltage at the binding posts through the OVS input (pin 23).

For the continuity function, the frequency/continuity comparator senses the signal through Analog Measurement Processor switches S35 and S41. The comparator toggles when the input goes below about 20 mV.

2-30. Frequency

The frequency/continuity comparator uses the ac volts/ac mA output of the ac buffer as its input at the Analog Measurement Processor ACBO input (pin 2). In the 10 A ac range, the signal is routed to the comparator through Analog Measurement Processor switches

S35 and S41.

Diagramas Tipo Escalera

Figure 2-6. DC mA and Amps Simplified Schematic qb06f.eps

Figure 2-6. DC mA and Amps Simplified Schematic

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