show up as unstable display readings. When an input signal, assumed to
be exactly zero volts, has a 1mV (0.001V) ground-loop induced offset, it
is displayed as “0010” on a ±2V input-range meter! DATEL’s new Digital
Panel Meter Catalog contains an application note describing power sup-
ply wiring and ground loop avoidance techniques.
The DMS-40PC’s simplified schematic, shown in Figure 1, can be very
useful when debugging a malfunctioning circuit, especially if the user
has some knowledge of operational amplifiers (op amps). The meter’s
high-impedance input consists of an op amp powered from a ±5Vdc
power supply (the –5V is internally generated). Figure 1 shows that input
signals applied to (–) INPUT LO and (+) INPUT HI must to be kept within
the ±5V power-supply rails. Also note that only pin 11 has a current-
limiting 909kΩ series resistor. High input voltages that have a common
ground connection with pin 3 (5V RETURN) should only be applied to pin
11 ((+) INPUT HI) and never to pin 12. In these high-voltage cases, pin 12
should always be tied to pin 3 (5V RETURN).
One of the simplified schematic’s more noteworthy features is that it
shows some DMS-40PC internal voltage values and also that pin 3 is
the meter’s zero-volt reference point — regardless of the type of power
or signal source used. This is an important point to keep in mind when
using digital multimeters to perform system-level measurements. The
multimeter’s negative lead (usually the black one) must always be con-
nected to pin 3 (5V RETURN). This is the only way accurate readings can
be made, particularly if the meter is configured for a differential input and
concerns arise regarding common-mode voltages.
1. Single-Ended Input Configurations: The DMS-40PC can measure
single-ended signals with either positive or negative polarities. True
single-ended inputs always have one of their two terminals at the
same potential as the DMS-40PC’s 5V RETURN (pin 3). Single-ended
inputs are usually derived from power supplies that have a common
ground with the meter’s +5V supply. Figure 2 shows the recom-
mended connections to be used with this type of input. Pin 12, (–)
INPUT LO, is shown directly tied to ground. This connection to ground
must be a separate wire or pc-board trace originating at V
IN’s negative
terminal. This hook-up will normally eliminate display instabilities and
errors caused by ground-loop currents which can occur if
(–) INPUT LO is tied to ground at, or near, pin 3.
Hard wiring is preferable, however, you can exercise dynamic control
over the location of the decimal point by employing drive circuits that
are capable of sinking a minimum of 20mA at voltages less than +0.4
Volts.
6. Gain Adjust: There is a gain-adjust potentiometer on the back of
each meter. It has approximately ±150 counts of adjustment range.
Since these meters essentially have no zero/offset errors, a gain
adjustment is effectively an overall accuracy adjustment. Though
they may be performed at any point (except zero), accuracy adjust-
ments are most effective when performed with higher level input
signals.
7. Soldering Methods: All models in the DMS-40PC Series easily
withstand most common wave soldering operations. We recom-
mend, however, that you evaluate the effects your particular soldering
techniques may have on the meter’s plastic case and high-precision
electrical performance. We recommend the use of no-clean solders.
8. Suggested Mating Connectors:
DMS-40PC Series meters are high-precision versatile devices that
can be used in many applications requiring a 0 to 19,999 count digital
display. The application circuits chosen for this section are ones that
have historically received many inquiries. Every attempt has been made
to ensure technical accuracy, and all of the following circuits have been
prototyped and tested to ensure functionality. Please keep in mind,
however, that real-world applications are seldom as straightforward as
the approaches presented here.
All inputs applied to DMS-40PC meters must be steady, dc values,
otherwise the input itself may cause display instabilities. Due to their 4½
digit resolution, DMS-40PC meters must be wired with greater care than
their 3½ digit counterparts. Correct power-supply and input-signal wiring
— an absolute must! — helps eliminate ground-loop induced errors that
Figure 2. Single-Ended Input Configuration
4. DISPLAY HOLD (Pin 9): For normal operation, this pin should be left
open. To hold the meter’s last reading and display it continuously, tie
pin 9 to ground (pin 3, 5V RETURN).
5. Decimal Point Placement: The location of the decimal point is user-
selectable, and the decimal point control pins (DP1-DP4) are active
low functions. Select the appropriate decimal point by tying pin 4,
5, 6 or 8 to pin 3 (5V RETURN). Unused decimal point location pins
should be left open.
Panel mounted:
Connector housing DATEL P/N 4320-01069-0
Terminal type DATEL P/N 4400-01032-0
Crimping tool DATEL P/N 39-2099000
Wire size 22 to 26 AWG
Insulation diameter 0.062” (1.57mm) maximum
Stripping length 0.100 to 0.125” (2.54 to 3.17mm)
Board mounted:
Socket DATEL P/N 4320-01074-0
1
12
3
+5V SUP
(–) IN LO
5V RET
120 VAC
11
(+) IN HI
6
DP1
+
–
AC to DC Converter
V
IN
DMS-40PC-1-GS
www.murata-ps.com/suppor t
DMS-40PC Series
4½ Digit, LED Display Digital Panel Voltmeters
09/27/11 MPM_DMS-40PC.C06 Page 3 of 6
APPLICATIONS
