Xp-750 Digital Meters - Elenco Electronics XP-650 Instruction Manual

XP-750 Digital Meters

The Model XP-750 uses two digital meters, one for output voltage and one for output current. The input to
the voltmeter comes from a 1000:1 voltage divider, R101 and R102 on the power supply output. Thus, the
voltmeter input will be 1mV for a 1V output voltage. Since the meter sensitivity is 10 counts per mV, the
meter will read 010. The decimal point is fixed by a jumper wire on the meter so that the output appears as
01.0 volts.
The ammeter input comes from a .01 ohm shunt resistor in series with the output current. The input is thus
10mV (100 counts) for a 1A output current. The ammeter decimal point is set to the 2nd place so that the
meter reads 1.00 amps.
Meter Operation
METERS - Meter operation centers around the 7107 integrated circuit (IC). This chip contains a dual slope
A/D (analog to digital) converter, display latches, seven segment decoder, and display drivers.
The input of the 7107 IC is fed to an A/D converter. Here, the DC voltage is changed to a digital format. The
resulting signals are processed in the decoders to light the appropriate LED segments.
Timing for the overall operation of the A/D converter is derived from a 40kHz external oscillator. The IC
divides this frequency by four and the resulting clock pulses are used to drive its decade counters. It is
then further divided to form three convert-cycle phases. The final readout is clocked at about 2.5 readings
per second.
The digitized data is presented to the display as four decoded digits (seven segments) plus polarity. The
decimal point position on the display is determined by a jumper wire on each meter PC board.
A/D CONVERTER - Any given measurement cycle performed by the A/D converter can be divided into three
consecutive time periods, autozero (AZ), integrate (INTEG) and read. A counter determines the length of
the time periods. The integrate period is fixed at 1000 clock pulses. The read period is a variable time that
is proportional to the unknown input voltage. It can vary from zero counts for zero input voltage to 2,000
counts for a full scale input voltage. The autozero period varies from 1,000 to 3,000 counts. For an input
voltage less than full scale autozero gets the unused portion of the read period.
During the autozero cycle, the accumulated offset voltage errors in the converter are measured and stored
as a voltage on the external autozero capacitor. This voltage is used to correct for the offset voltage errors
during the read cycle.
During the INTEG cycle, the INTEG capacitor is charged up for 1,000 clock pulses (100ms.), see Figure 3.
The charging rate is determined by the unknown input voltage. At the end of the integrate cycle the volt-
age on the capacitor is proportional to the unknown input voltage.
During the read cycle, the INTEG capacitor is discharged at a constant rate. The time required for the dis-
charge is therefore proportional to the unknown input voltage. This time is converted to a digital format by
counting the number of clock pulses that occur during the discharge.
Counter Output
Figure 3 Dual Slope A/D Converter
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