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Tektronix DM502A
3½ digit TRMS digital multimeter
Tektronix DM502A

Produced from 1980 to 1988

(All manuals in PDF format unless noted otherwise)
Manuals – Specifications – Links – Pictures

The Tektronix DM502A is a 3½ digit TRMS digital multimeter plug-in for the TM500 system.

It superseded the DM502 predecessor in 1980.

Compared to the DM502, the DM502A adds autoranging, a True RMS converter, and offset circuits to read dB directly in all ranges. The temperature input is for a PT-100 temperature probe (P6601).

An "Option 2" instrument deletes the temperature measurement function, P6601 temperature probe, and all internal circuits associated with this function.

The rear interface of DM502A is limited compared to the DM502 in that it has no current input and no digital output, the latter having been obsoleted by TM5000 GPIB based instruments.

The 2 kΩ measurement range (+HI in DM502A) can be used for diode testing, since a current source of 1 mA is active and the reading corresponds to the (forward) voltage drop of the diode in Volts. The voltage can rise to a maximum of 10 Volts. A second voltmeter is needed to measure the voltage if 2 V are exceeded.

The DM502A has an Int/Ext switch that allows a voltage input via the rear interface to be selected.

Key Specifications

Voltage ranges 200 mV to 200 V in decade steps; 1 kV (DC) / 500 V (AC) top range
Input impedance 10 MΩ
Current ranges 200 μA to 2 A in decade steps
Resistance ranges 200 Ω to 20 MΩ in decade steps


Documents Referencing DM502A

Document Class Title Authors Year Links
Tekscope 1979 V11 N2.pdf Article New Products 1979
Tekscope 1979 V11 N3.pdf Article A New DMM Family for the TM 500 Series Bill Mark 1979
070-2088-04.pdf Book TM500 Series Rear Interface Data Book 1985


The DM502A circuit is built around a Siliconix LD110/111A DMM chipset. The entire circuit is supplied from one of the mainframe's 25 VAC windings through a main-frequency isolation transformer with a 25 V primary winding and 7815/7915/7805 regulators. The 25 VAC input is the only mainframe connection, so the DM502A could easily be used outside a TM500 mainframe as well.



Some Parts Used in the DM502A

Part Part Number(s) Class Description Used in
120-1243-00 120-1243-00 Discrete component transformer DM502A
AD536 156-1259-00 Monolithic integrated circuit RMS/DC converter DM501A DM502A Keithley 177 Keithley 179 Keithley 195A
Siliconix LD110/111A 156-0476-00 156-0476-00 156-1268-00 Monolithic integrated circuit 3½ digit A/D converter DM502 DM502A DM505