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DMS-40PC Datasheet PDF : 6 Pages
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APPLICATIONS
2. Differential Input Conï¬gurations: 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.
DMS-40PC-1-GS
DP1
11
R1
(+) IN HI
1k
R2
12
1k
(–) IN LO
R3
1
3
1k
+5V SUP
5V RET
120 VAC
DATEL
UPA-5/500
AC to DC Converter
Figure 3. Differential Input Conï¬guration
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 conï¬gured 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.
DMS-40PC Series
4½ Digit, LED Display Digital Panel Voltmeters
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.
LM2931T-5
+
4.5 - 18Vdc
–
IN OUT
GND
+
22μF
10V
(+) IN HI
11
1
+5V SUP
3
5V RET
DMS-40PC-2-RL
5
DP2
12
(–) IN LO
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
ampliï¬er. 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 ability to
make differential measurements as described in a previous section.
1.5V
CELL
–
11
(+) IN HI
10
ANA COMM
12
(–) IN LO
DMS-40PC-1-GS
1
3
6
+5V SUP
5V RET
DP1
120 VAC
DATEL
UPA-5/500
AC to DC Converter
Figure 5. Floating Input Measurements
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MPM_DMS-40PC.C03 Page 4 of 6
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