APPLICATION NOTE DEM 7508 (DEM 7510)
APPLICATIONS
INPUT ATTENUATORS
To measure voltage greater than 200 mV
an input attenuator is needed.
The Full Scale Sensivity is given by
R1 + R2
VIN (F.S.) = 1.999 x VREF x
(V) VREF = 0.1
R2
R1 + R2 should be accurate and stable. Good metal film
resistor meet these requirement. The input attenuator
reduces the input resistance of the circuit from > 10M
ohms to R1 + R2. A practical upper limit is 9MW for R1.
CURRENT MEASUREMENTS
The use of a shunt resistor converts the current to a voltage.The following formula can be used to calculate the
shunt resistor.
VREF = 0.1V
Iin x Rs
Display Reading =
x 1000
Iin x Rs = 0.2V
VREF
(full scale)
A multirange current meter circuit is shown. Note that although the input current passes through the selector
switch, the voltage drop across the switch does not contribute to the measured voltage.
20 mV FULL SCALE
An operational amplifier is used to measure full scale voltages less than 199.9 mV. Note that the auto zero circuitry
within the module cannot compensate for the opamp offset or voltage drift.
MULTIRANGE MEASUREMENTS
In multirange application, it is necessary to also switch the
decimal point. This can easily be accomplished by
connecting the appropriate D1 to D3 terminal with a rotary
switch to VDD
(D1 = .000 D3 = 0.00 D3 = 00.0)
RESISTANCE MEASUREMENTS
The ratiometric technique is used for resistance measurements.The unknown resistance is placed in series with a
known resistor and current is passed through the pair.The
voltage developed across the unknown resistor applied to
the input (INHI, INLO).The voltage across the known resistor applied to reference inputs REH and REI. If the
unknown equals the standard, the display will read 1000.
Runknown
Displayed Reading =
x 1000
Rstandard
Due to the ratiometric technique, no accuratly defined
reference voltage is required. The module will overrangefor Runknown ³ 2 x Rstandard.
AC VOLTAGE MEASUREMENTS
The module can only measure DC. To measure AC voltage, an AC to DC converter needs to be used to condition
the waveform
1/2
APPLICATION NOTE DEM 7508 (DEM 7510)
APPLICATIONS
OVERRANGE-UNDERRANGE SIGNAL
The additional IC’s which are required to generate the
overrange and underrange signal.
The respective terminal are accessible to the module.
ZERO DISPLAY FOR NON-ZERO
INPUT VOLTAGE
If a zero display is required when the input voltage level if
not zero, the offset voltage should be connected between
INLO and COM while the input voltage is connected between COM and INHI.
TEMPERATURE MEASUREMENTS
SCALE FACTOR
ADJUST
ZERO
ADJUST
A simple temperature sensor is an ordinary silicon diode
or the base emitter of a silicon transistor.The forward voltage of a typical silicon pn junction change -2.1 mV/°C.
Since the sign of the voltage change is negative, the diode
voltage is applied to INLO to give correct polarity. R1 offsets the forward voltage drop of about 550 mV causing the
display to read 0.00 at 0°C R2 adjust the reference to
match the slope of the diode voltage versus temperature.
Idealy, the sensor diode should be drive by a current source.
Substituting a high value resistor will only add approximately 0.75°C of nonlinearity. The resistor also slightly compensates for the reference temperature drift and decreasing battery voltage. To calibrate the circle, the diode sensor must first be immersed in a stirred ice water bath (0°C)
adjust R1 to 0.00 reading. Then immerse the diode in boiling water and adjust R2 to 100.0 reading.
NON-FLOATING SUPPLY
figure a
figure b
figure c
START
ARE ANALOG
INPUTS FLOATING
WITH RESPECT TO
SUPPLY
YES
CONNECT
INHI/INLO
TO COMMON
NO
where: VDD-VSS<15V
Either INHI or INLO has to be connacted to
ground
VIN(MAX)(R2)
£ 200 mV
R1+R2+R3
USE A SPLIT
SUPPLY OR A
RESISTIVE DIVIDER
TO SHIFT INPUT
YES
DOES
INHI/INLO COMES
CLOSER THAN
1.5 V TO
VDD/VSS
NO
VIN(MAX)(R2+R3)
£ VDD-1.5V
R1+R2+R3
VIN(MIN)(R3)
³ VSS+1.5V
R1+R2+R3
A 9V battery is recommended for 9V operation. It is intended that the analog inputs (INHI/INLO) float with respect
to the 9V supply.
Please note, if a non-floating supply is required, some
care must be taken (Refer to the data sheet “200 mV digital voltmeter” circuit). In general, if the analog inputs do not
2/2
OK
float with respect to the supply, the analog inputs must be
no closer than 1.5V from either supply voltage
(VDD/VSS). This can be done by using a (a) split supply
(see figure a) or (b) resistive divider to shift the analog
inputs (see figure b). The flow chart in figure c will help to
guide you to the proper solution