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Ilifferential input
buffer for oscilloscopes
This simple circuit will allow you to measure
mains waveforms on your oscilloscope and to
observe signals which cannot be connected to
earth. It has a differential input and it provides
7 .5kV isolation between the input & output by
means of linear optocouplers.
By JOHN CLARKE
Measuring mains voltages on most
general-purpose oscilloscopes is a
dangerous and inaccurate procedure.
Unfortunately, it's not simply a matter of connecting the CRO probe to the
mains, selecting the appropriate voltage division ratio and observing the
waveform on the screen. Instead, there
are at least two problems that must be
overcome.
First, you must connect the probe
earth clip to Neutral. Connecting the
probe the wrong way around, with
the earth clip to Active, will not only
blow a fuse but could also give you a
nasty shock. In addition, the Neutral
is not necessarily at earth potential
which means that a large current may
flow in the earth lead, even if the
probe is connected correctly.
The second problem you will probably run into is that the CRO lacks
sufficient voltage division ratios. Most
general purpose oscilloscopes can
only be switched to a maximum of
5V/division, which corresponds to
50V/division when the probe is set to
a 10:1 ratio. Because there are only
eight divisions on the screen, it follows that only waveforms up to 400V
p-p can be displayed in their entirety.
A 240V mains waveform is about
680V p-p which means that it is much
too big to fit on the screen.
Of course, the variable VOLT/DIV
control can be used to reduce the size
of the waveform so that it does fit.
However, the waveform then becomes
uncalibrated and no worthwhile voltage measurements can be made.
Difficulties also occur when measurements are to be made on Triac and
SCR phase control circuits, or where
you wish to monitor the current
through a mains appliance (via a suitable shunt).
Because the earth lead of the probe
can no longer be used in these cases,
the oscilloscope must be used in differential mode. This involves using
both inputs of the CRO to measure a
single waveform (ie, we must use a
dual-trace unit). However, the waveform will again be too large to fit on
the screen unless we are prepared to
settle for an uncalibrated display.
Differential input buffer
The Differential Input Buffer divides the input signal by 1000, which means that
you can now monitor calibrated mains waveforms. The unit also makes it
possible to measure signals which cannot be referenced to earth.
76
SILICON CHIP
This Oscilloscope Differential Input Buffer solves all these problems.
Because it has a differential input, no
earth lead is required on the probes
and so it is impossible to incorrectly
connect the earth to Active. In addition, the circuit divides the mains
voltage by 1000, so that the vertical
attenuator can be set to 1V/DIV to provide a fully calibrated waveform
which easily fits on the screen.
The input buffer also allows measur.ements of mains AC waveforms and
__
••
--"-'-
.,,~
The assembly of the unit is straightforward since most of the parts are mounted
on a single PC board. Power is derived from two internal 9V batteries & this,
together with the use of linear optoisolators in the signal paths, provides a
nominal 7.5kV of isolation between the input & output sockets.
other signal waveforms which cannot
be referenced to earth. These include
measurements at the outputs of bridged amplifier circuits and across motors driven by H-pack output circuits.
Fig.1 shows how the Differential
Input Buffer is used in a typical situation, in this case to monitor the output of a bridged audio amplifier. The
unit has two inputs, one inverting(-)
and the other non-inverting (+), and
these are simply connected across the
output'. The output of the input buffer
is then connected to the CRO input in
the conventional manner, leaving the
second input of a dual-trace CRO free
for other measurements.
To ensure operator safety, the metal
case of the input buffer is earthed
back to the oscilloscope case earth
and the rated isolation between the
inputs and the output is 7.5kV. This
isolation is provided by using optocouplers in the signal paths and by
running the circuit from 9V batteries.
Note that this figure is based on the
rated isolation of the optocouplers and
is a nominal value only. In practice,
the true isolation is likely to be somewhat less than this figure due to the
BNC input and output sockets.
The complete circuit is housed in a
metal case measuring 95 x 52 x
151mm. The front panel carries the
OSCILLOSCOPE
+
DIFFERENTIAL
INPUT
OSCILLOSCOPE
BUFFER
•
OUTPUT
FRAME
Fig.1: this diagram shows how the Differential Input Buffer can be used
to monitor the output of a bridged audio amplifier, where no earth can
be connected.
two BNC input sockets and an on/off
switch, while the rear panel carries
the BNC output socket. Also on the
back panel is an earth socket and this
is connected to the earth terminal on
the CRO via a banana plug lead (see
Fig.1).
Note that conventional CRO probes
are connected to the two BNC input
sockets and these should be set for 1:1
division. However, the earth clips on
the probes are no longer used. In fact,
depending on your application, it
would be better to remove the clips
altogether (or at least tape them up) to
avoid accidental contact with the
mains.
How it works
We've used a very interesting new
device in this circuit - an 11300 linear
optoisolator from Siemens. In fact,
the circuit uses two such devices, one
for each input. In addition, there are
five op amps, two transistors and a
handful of minor parts - see Fig.2.
Before we get fully immersed in the
circuit description though, we'll first
take a closer look at the 11300. There's
a very interesting twist to this device,
as we shall explain.
In the past, optocouplers have been
used mainly to isolate digital control
circuits from Triac circuits operating
APRIL
1992
77
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V+
TP5
INVERTING'
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T
0.1
TP4
VR3, VR4
CALIBRATE V-
4.7k
0.1T
LINEAR
OPTOISOLATOR
IC2
IL300
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GAIN
VR3
10k
8.2k
6"
1.1k
TO
OSCILLOSCOPE
INPUT
5-
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+1000'
VR1'
2oon-
v-
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IC5
LM334
CURRENT
SOURCE
0.1!
10k
V1-
680Q
V+
V-
V+
TP7
0.1T
l
NONINVERTINf
INPUT
"j_
V-
TP6
VR3, VR4
CALIBRATE
10k
V+
B1
9V.
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1.1k
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5
1/2 SUPPLY AMPLIFIER
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+1000
VR2.
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*PHILIPS VR37 0.5W 1500VAC
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8
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9V
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16VW
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DIFFERENTIAL INPUT BUFFER FOR OSCILLOSCOPES
Fig.2: the differential input signals are first divided by 1000 & then applied to
buffer stages ICla & IClb. These op amps in turn drive linear optoisolators IC2
& IC3 via current amplifiers Ql & Q2. Finally, the outputs of the optoisolators
drive differential amplifier IC6a which produces the output signal.
at mains potential. These optocouplers usually contained an internal
LED and a photodiode detector. To
activate the device, the LED was simply driven at a level sufficient to fully
saturate the detector, which meant
that the device was either on or off.
We cannot use this type of optocoupler in an analog circuit however
- at least, not if we expect good performance. That's because their output
characteristics are far from linear and
are subject to wide variations with
changes in temperature. We cannot
apply feedback either, since that
would defeat the purpose of the
optoisolator.
And so we come to the 11300. Here's
the twist: it overcomes the above problem because it has not one photodiode
but two. One photodiode (on the iso78
SILICON CHIP
lated side) is used to provide the output as before, while the second is
used to provide the feedback to the
LED driver circuit that's so necessary
to ensure good linearity. In fact, the
output sensor can be driven with a
linearity of .01 %.
Now let's get down to the circuit
details. Op amps ICla and IClb are
used to buffer the differential input
signals. In each case, the incoming
signal is.attenuated by a factor of 1000
using a voltage divider network consisting of a 1.ZMQ input resistor, a
1. lkQ resistor and a ZO0Q calibration
trimpot.
ICla and IClb drive optocouplers
ICZ and IC3 via buffer transistors Ql
and QZ. These transistors are wired as
emitter followers and ensure sufficient current drive for the optocou-
1/2 SUPPLY AMPLIFIER
pler LEDs via series ZZ0Q resistors.
Feedback from the optocouplers is
derived from the non-isolated detectors and is applied to the inverting
input of each driver op amp. Since we
want each optocoupler to operate with
an AC signal, it is necessary to bias
them so that their internal LEDs are at
about "half-brightness" when no signal is applied to the inputs. This bias
is derived from a constant current
sink (IC5) which pulls 50µA via each
of the 10kQ resistors connected to the
inverting inputs of ICla and IClb.
Essentially, ICla and Ql operate as
a class-A amplifier, with the emitter
of Ql sitting at close to half-supply
(ie, 0V) under no-signal conditions.
As the signal increases, Ql turns on
harder and increases the drive to the
LED. Conversely, as the signal decreases, Ql throttles back and the LED
output decreases.
IClb and QZ operate in exactly the
same fashion.
This CRO screen
photograph shows the
waveform across a 'Ii:iac
'in a typical phase control
circuit. The vertical
attenuator was set to 0.lV/
division and this,
combined with the 1000:1
attenuation of the
Differential Input Buffer,
gave a display calibrated
to lO0V/division.
IC5 (the constant current sink) is an
LM334 3-terminal device. Its operating current is set by the 680Q resistor
between its "R" input and V-. In this
circuit, the current is set at lO0µA
and is equally divided between the
inputs of ICla & IClb.
IC4 is used to derive a half-supply
rail from 9V battery BATT1. This op
amp is wired as a voltage follower
and derives its input from a voltage
divider consisting of two 10kQ resistors. The resulting mid-point voltage
is then decoupled by a l0µF capacitor
and buffered by IC4, the output of
which connects to the signal ground.
The positive terminal of the battery
thus becomes the +4.5V supply (V+),
while the negative terminal becomes
the -4.5V supply (V-).
These rails power all the circuitry
on the non-isolated side of the 11300
optocouplers.
The outputs from the isolated detectors in IC2 and IC3 are used to
drive differential amplifier IC6a. VR3
and its series 8.2kQ resistor set the
voltage between the inverting input
(pin 2) and the output (pin 1), while
VR4 and its series 8.2kQ resistor set
the voltage on the non-inverting input (pin 3).
In operation, IC6a acts as an inverting amplifier for signals from IC2 and
as a non-inverting amplifier for signals from IC3 . VR3 sets the overall
Specifications
Frequency response: DC to 100kHz
Input impedance: 1.2MQ
Input attenuation: 1000:1
Isolation: 7.SkV
Power supply: 2 x 9V batteries
PARTS LIST
gain of this stage, while VR4 is an
offset adjustment that's used to compensate for the extra gain in the noninverting path.
IC6 is powered from 9V battery
BATT2. In this case, voltage follower
stage IC6b and the 10kQ voltage divider resistors on its input are used to
derive a half-supply rail. This stage
works in exactly the same manner as
IC4.
The output of IC6b connects to the
signal ground and also to the case.
The positive terminal of BATT2 thus
becomes the +4.5V rail (Vl+) for IC6,
while the negative terminal becomes
the -4.5V supply rail (Vl-).
Double-pole switch S1 provides
power on/ off switching. S la switches
the BATT1 supply while Slb switches
the BATT2 supply.
Construction
The circuit is assembled on a PC
board coded SC04204921 and measuring 121 x 84mm. This fits neatly
into a standard metal case (see parts
list) that's also large enough to accommodate the various input/output terminals and the on/off switch.
Fig.3 shows the parts layout on the
PC board. Start construction by installing PC stakes at the six test points
and all external wiring points. Once
this has been done, install the passive
components (links, resistors, capacitors & trimpots), then the transistors
and ICs. The LM334 can also be installed at this stage. Make sure that
you don't confuse this device with
the two transistors.
The board assembly can now be
completed by installing the two battery holders. Secure them using 2mm
screws and nuts, then check the board
assembly carefully against Fig.3 be-
1 PC board, code SC04204921,
121 x84mm
1 Dynamark front panel label, 92
x51mm
1 K&W instrument case, 95 x 52
x 151mm
2 216 9V PCB-mount battery
holders
2 216 9V batteries
1 double pole rotary mains
switch (must be mains-rated)
1 knob for switch
3 BNC panel sockets
1 banana earth terminal
4 6mm standoffs
5 3mm x 9mm screws plus nuts
& shakeproof washers
6 2mm x 6mm screws plus nuts
1 solder lug
15 PC stakes
1 150mm-length 0.6mm tinned
copper wire
1 250mm-length blue mains wire
1 250mm-length brown mains
wire
1 100mm-length green/yellow
mains wire
2 10kQ miniature horizontal
trimpots (VR3,VR4)
2 220Q miniature horizontal
trimpots (VR1 ,VR2)
Accessory leads
2 oscilloscope probes
1 BNC to BNC plug lead
1 banana plug to banana plug
earth lead
Semiconductors
2 TL072 dual op amps (IC1 ,IC6)
2 IL300 linear optocouplers
(IC2,IC3 - Siemens)
1 TL071 op amp (IC4)
1 LM334 3-terminal current
source (IC5)
2 BC548 transistors (Q1 ,Q2)
Capacitors
2 10µF 16VW PC electrolytic
4 0.1µF monolithic
2 10OpF ceramic
Resistors (0.5W, 1%)
2 1.2MQ Philips VR37 0.5W
610kQ
21 .1kQ
2 8.2kQ
1 6800
2 4.7kQ
2 220Q
Note: do not substitute for the
Philips VR37 resistors as these
are necessary to ensure an adequate voltage rating.
APRIL 1992
79
SOLDER
LUG
FRONT
RTH
~TERMINAL
REAR
Fig.3: install the parts on the PC board as shown in this wiring diagram & be
sure to use the specified 1.2MQ input resistors (see parts list). All external
wiring from the board must be run using mains-rated cable.
fore moving on to the next stage.
Once you are satisfied with the
board assembly, the adhesive label
can be attached to the case and holes
drilled to accept the two BNC input
sockets and the on/off switch. This
job is best done by drilling small pilot
holes first and then carefully enlarging them to the correct size with a
tapered reamer. You will also have to
drill holes in the rear of the case to
accept the output BNC socket, earth
terminal and earth screw, plus four
mounting holes in the base for the PC
board.
Position the PC board towards the
back of the case as shown in the photograph, to provide clearance for the
OSCILLOSCOPE DIFFERENTIAL INPUT BUFFER
·- .
~N=
·.·c_ __
+
(+)
OFF
+
+
(-)
on/ off switch on the front panel.
The various items can now all be
mounted in position and the wiring
completed as shown in Fig.3. Note
that the earth lug is secured to the
case using a screw, nut and shakeproof
washer. The PC board is mounted on
6mm spacers and secured using
screws, nuts and washers.
Use mains-rated cable for all the
internal wiring.
Checkout & calibration
Before applying power, doublecheck your work to make sure that
there are no wiring errors. When you
are satisfied that all is well, install the
two 9V batteries and apply power.
Use your multimeter to check that
there is 9V between pins 4 & 8 of both
IC1 & IC6 and between pins 7 & 4 of
ON
OUTPUT AT REAR 1V:1000V; 7.SkV ISOLATION
CAPACITOR CODES
o
Value
IEC Code
EIACode
D
D
0.1µF
100pF
100n
100p
104
101
Fig.4: this artwork can be used as a drilling template for the front panel.
RESISTOR COLOUR CODES
D
D
D
D
D
D
D
D
80
No.
Value
4-Band Code
5-Band Code
2
6
2
2
2
1
2
1.2MO
10kO
8.2kO
4.7kO
1.1kO
6800
2200
brown red green yellow
brown black orange brown
grey red red brown
yellow purple red brown
brown brown red brown
blue grey brown brown
red red brown brown
not applicable
brown black black red brown
grey red black brown brown
yellow purple black brown brown
brown brown black brown brown
blue grey black black brown
red red black black brown
SILICON CHIP
r.
\ I
-•
el
N
0,
0
N
•
0
u
en
•
0
Fig.5: check your PC board against this full-size pattern before
mounting any of the parts.
IC4. The output ofIC4 at pin 6 should
be at half-supply. You can check this
by measuring between the positive
terminal of BATTl and TP3, and between the negative terminal ofBATT1
and TP3. You should get readings of
+4.5V and -4.5V respectively.
The output of IC6b should also be
at half supply. This can be verified by
checking for +4.5V between the positive terminal of BATT2 and chassis
and for -4.5V between the negative
terminal of BATT2 and ground.
To calibrate the instrument, first
connect TP4 to TP5 so that a voltage
is applied to the input of IC1a. Measure this voltage by connecting your
multimeter between TP1 & TP3.
Now connect your multimeter
across the output BNC socket. Adjust
VR3 so that the reading is the same as
that just measured between TP1 &
TP3.
Next, disconnect TP4 from TP5 and
connect TP6 to TP7 so that a voltage
is applied to the input of IC1b. Check
this voltage by measuring between
TP2 & TP3, then connect your multimeter to the output BNC socket again.
Adjust VR4 until the output voltage
matches the previous reading (ie, the
voltage between TPZ & TP3).
VR1 and VR2 are now be adjusted
to provide an exact 1000:1 division
ratio for the input signals. These adjustments must be done with the
power switched off.
To make these adjustments, first use
your digital multimeter to check the
resistance of the 1.2MQ resistor between the inverting input and TP1.
Note the value. This done, connect
the multimeter between TP1 & TP3
and adjust VR1 for a reading which is
111000th the previous value.
For example, if the 1.2MQ resistor
measures 1.195MQ, adjust VR1 for a
reading of 1.195kQ.
This procedure is now repeated for
the non-inverting input. Measure the
value of the 1.2MQ resistor connected
to this input, then connect your multimeter between TP2 & TP3 and adjust VR2 to the correct value (ie, 1/
1000th the previous reading).
The circuit is now calibrated and
ready for use. As a final c}:ieck, connect the unit to an oscilloscope as
shown in Fig.1 and connect the test
probes to mains Active and mains
Neutral. Check that the mains waveform can now be displayed on the
oscilloscope screen with the vertical
attenuator switched to the 1V/DIV setting.
If you are not going to be checking
mains voltages, you can modify the
input attenuator circuit so that you
can observe low-level signals in differential mode. For example, to divide the input signal by 100, increase
the 1.1kQ resistors at TP1 & TP2 to
10kQ and increase VR1 & VR2 to 5kQ.
Finally, exercise great care when
measuring mains voltages. Make sure
that the input probes & leads are in
good condition and are rated for mains
operation. Remember - contact with
the mains can be fatal!
SC
.1.~~.r..1;ti:.~:
RCS Radio Pty Ltd is the only company which
manufactures and sells every PCB [, front panel
published in SILICON CHIP, ETI and EA.
651 Forest Road, Bexley, NSW 2207. Phone (02) 587 3491.
APRIL
1992
81
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