When using other, higher-power, DMS-40PC models in combina-

tion with three-terminal regulators, be sure to consult the regulator

manufacturer’s data sheet to ensure the device is being utilized safely

and correctly.

Figure 4. 4.5-18V Power Supply Monitor

4.  Floating Signal Source Measurements: A floating input is a signal

that, before it is applied to the DMS-40PC’s inputs, has no galvanic

connection (direct current path) to the meter or the meter’s power

supply. The circuit shown in Figure 5 illustrates the necessary con-

nections for measuring floating inputs. The 1.5V battery represents a

true floating input signal since it initially has no connection whatso-

ever in common with the meter. Real-world floating inputs typically

originate from power supplies which are transformer isolated from

the DMS-40PC’s +5V supply.

The connection of pin 12 ((–) INPUT LO) to pin 10 (ANALOG COMMON)

is required in order to provide a bias return for the meter’s input

amplifier. This is because neither pin 11 nor pin 12 are tied to any

reference voltage inside the DMS-40PC (see Figure 1). These connec-

tions are not made internally in order to give the meter the abil

ity to

make differential measurements as described in a previous section.

Figure 5. Floating Input Measurements

3.  Power Supply Monitoring: One of the most widely used digital panel

voltmeter applications is monitoring the output voltage of a system

power supply — often the same supply that also powers the meter.

The low-power, red LED DMS-40PC-2-RL, with its excellent 0.001Vdc

resolution, can be configured to monitor power supplies with outputs

in the range of 4.5-18Vdc. The circuit in Figure 4 uses a low-drop-

out, three-terminal regulator (LM-2931T-5, in a T0-220 package,

available from National Semiconductor) to provide regulated 5V

power to the meter.

The LM-2931 was chosen because it has the following on-chip pro-

tection features: reverse polarity, short circuit and thermal runaway.

The DMS-40PC-3-RL can monitor voltages up to ±200Vdc, provided

a separate +5V power source is used since many three-terminal

regulators cannot operate with supply voltages greater than 24V. Red,

low-power LED models, with their very low self-heating, are recom-

mended for applications in which low calibration drift is desirable.

Figure 3. Differential Input Configuration

2.  Differential Input Configurations: Differential inputs can also be

measured with DMS-40PC meters as shown in the circuit of Figure

3. Differential inputs must also originate from power supplies that

have a common ground with the meter’s 5V RETURN (pin 3). However,

differential inputs usually have both terminals above and/or below

5V RETURN. Figure 3, though not necessarily a typical real-world

application, does serve the purpose of illustrating the concept of a

differential signal.

The voltages developed across R1, R2 and R3 are equal to each other

and measure approximately +1.6666Vdc or 1/3 of the +5V power

source. More importantly, while the signal across R3 is single-ended,

both ends of R1 and R2 are well above ground and are described

here as being differential. Please note that while the DMS-40PC can

measure the voltages across either R2 or R3, it cannot measure

the +1.6666 volts across R1! The voltage at the lower end of R1 is

approximately 3.333V and this exceeds the common mode voltage

limit of ±2V.

1

12

3

+5V SUP

(–) IN LO

5V RET

120 VAC

11

(+) IN HI

DP1

AC to DC Converter

R2

R1

R3

1k

1k

1k

DMS-40PC-1-GS

1

3

+5V SUP

5V RET

120 VAC

6

DP1

AC to DC Converter

DMS-40PC-1-GS

12

(–) IN LO

11

(+) IN HI

1.5V

CELL

–

10

ANA COMM

DMS-40PC-2-RL

5

12

+

–

11

1

4.5 - 18Vdc

LM2931T-5

GND

IN

OUT

+5V SUP

3

5V RET

DP2

(+) IN HI

(–) IN LO

22µF

10V

+

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DMS-40PC Series

4½ Digit, LED Display Digital Panel Voltmeters

MPM_DMS-40PC.D04  Page 4 of 6

APPLICATIONS