5
3 ½
DIGIT ,
LCD
DISPLA Y DIGIT A L P ANEL
VOL TMETERS
DMS-20LCD
To calibrate the circuit of Figure 7, perform the following:
1. With 4mA applied, adjust the 50k
Ω potentiometer (R2) to
display a reading of "000" (assuming that is the desired
reading).
2. With 20mA applied, adjust the gain-adjust potentiometer on
the back of the meter to display a reading of "1999". For different
full scale readings, alter the value of RShunt accordingly.
RShunt = R1 = VFsr /IFsr
Where:
VFsr = Full scale reading (in Volts)
IFsr = Relative full scale current (in Amps)
Example: For a meter with a 2V full scale input (1.999 full
scale reading) and a desired full scale display reading of
1000 (with an input of 20mA), VFsr = 1.000 Volts
RShunt = 1.000V/(0.020 – 0.004)A
RShunt = 1.000V/0.016A = 62.5 Ohms
function of the scaling requirements of the particular application and
can be calculated using the following equation:
Figure 7. 4-to-20mA Current Loop Operation
(5V-Powered Models)
R2
(R1 + R2 + R3)
= 0.1
6. Power Supply Monitoring: A popular application for DATEL's
low-power LCD meters is monitoring the supply voltage in battery-
operated portable equipment. Figure 8 demonstrates how a 9V-
powered DMS-20LCD can be used to monitor its own supply. The
meter used is the DMS-20LCD-1-9. A three-resistor voltage
divider is used to attenuate the battery voltage and also to satisfy
the requirement that the input voltages applied to pins 12 and 11
be at least 1.5 Volts above and below the battery voltage applied
to pins 1 (+BATTERY) and 3 (–BATTERY). The divider should be
designed so that 1/10th the battery voltage falls across the inputs
to the meter:
Therefore, the 9V battery voltage appears to the meter inputs
as 0.9V. With the decimal point moved to its DP2 position
(pin 5 tied to pin 3), the meter reads 9.00 Volts.
The circuit can be calibrated by first measuring the actual
battery voltage with another meter and then adjusting the gain-
adjust potentiometer on the back of the DMS-20LCD until a
similar reading is obtained. If possible, the resistors in the
divider should be ±1% metal-film types with TCR's less than
50ppm/°C.
7. External Gain Adjustment: Connect REFERENCE OUT
(pin 8) to REFERENCE IN (pin 7) for normal, factory calibrated,
operation. Use the circuit shown in Figure 9 for applications
needing external gain adjustment. Calibration is performed with
a precise, near-full-scale, input voltage.
Figure 9. External Gain Adjustment
Figure 8. Power Supply Monitor
(9V-Powered Models)
Applications
1
3
+ 5 V S U P
5 V R E T
A C t o D C C o n v e r t e r
1 2
( ) I N L O
1 0
A N A C O M M
1 1
( + ) I N H I
R 1
4 - 2 0 m A
5 0 k
R 2
8
7
R E F O U T
R E F I N
D M S - 2 0 L C D - 1 - 5
D A T E L
D M S - P S 1 - C M
8 5 - 2 6 4 V a c
1
12
3
+BAT
(–) IN LO
–BAT
11
(+) IN HI
5
DP2
DMS-20LCD-1-9
R2
R1
R3
45.3k
10.1k
45.3k
9V
BATTERY
–
8
7
REF OUT
REF IN
12
VOLTAGE
CALIBRATOR
OUT
10
(+) IN HI
(–) IN LO
ANA
COMM
= 10 to 20 Turns
7
REF
IN
11
732, 1%
8.06k, 1%
5V RET/
–BAT IN
Connections
for ±2V, ±20V
and ±200V models
Connections
for ±200mV
models
COM
1
+5V IN/
+BAT IN
17.4k, 1%
2k
24.3k, 1%
3
12
DMS-20LCD
For +5V models, tie (–) IN LO to pin 3.
For +9V models, tie (–) IN LO to pin 10
8
NC
200
Q
Q
Q
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